Browsing "Older Posts"

LED or Lamp Pulsar Circuit

By Circuit Projects → 31 Oct 2011
Astonishing effect 4.5V supply

This circuit operates a LED in pulsing mode, i.e. the LED goes from off state, lights up gradually, then dims gradually, etc. This operation mode is obtained by a triangular wave generator formed by two op-amps contained in a very cheap 8 pin DIL case IC. Q1 ensures current buffering, in order to obtain a better load drive. R4 & C1 are the timing components: using the values shown in the parts list, the total period is about 4 seconds.
Circuit diagram:

LED or Lamp Pulsar Circuit Diagram

R1 = 4.7K
R2 = 4.7K
R3 = 22K
R4 = 2.2M
R5 = 10K
R6 = 47R
C1 = 1µF-63V
Q1 = BC337
D1 = Red Led
IC1 = LM358

  • The most satisfying results are obtained adopting for R4 a value ranging from 220K to 4M7.
  • Adopting for R4 a value below 220K, the pulsing effect will be indistinguishable from a normal blinking effect.
  • The LED can be any type and color.
  • You can use a filament lamp bulb instead of the LED, provided it is rated in the range 3.2 to 6V, 200mA max.
  • Using a bulb as a load, R6 must be omitted.
  • Voltage supply range can be 4 to 6V: 4.5V is the best compromise.
  • Do not supply the circuit with voltages exceeding 6V: it will work less good and Q1 could be damaged when a bulb will be used as the load.
  • At 6V supply, increase R6 value to 100 Ohm.

Simple Hybrid Audio Amplifier

By Circuit Projects →
The debate still goes on as to which are better, valves or transistors. We don’t intend to get involved in that argument here. But if you can’t make your mind up, you should try out this simple amplifier. This amplifier uses a valve as a pre-amplifier and a MOSFET in the output stage. The strong negative feedback makes the frequency response as flat as a pancake. In the prototype of the amplifier we’ve also tried a few alternative components.

For example, the BUZ11 can be replaced by an IRFZ34N and an ECC83 can be used instead of the ECC88. In that case the anode voltage should be reduced slightly to 155 V. The ECC83 (or its US equivalent the 12AX7) requires 2 x 6.3 V for the filament supply and there is no screen between the two triodes, normally connected to pin 9. This pin is now connected to the common of the two filaments.

The filaments are connected to ground via R5. If you’re keeping an eye on the quality, you should at least use MKT types for coupling capacitors C1, C4 and C7. Better still are MKP capacitors. For C8 you should have a look at Panasonic’s range of audio grade electrolytics. P1 is used to set the amount of negative feedback. The larger the negative feedback is, the flatter the frequency response will be, but the smaller the overall gain becomes.

Circuit diagram:
Simple Hybrid Audio Amplifier Circuit Diagram

With P2 you can set the quiescent current through T2. We have chosen a fairly high current of 1.3 A, making the output stage work in Class A mode. This does generate a relatively large amount of heat, so you should use a large heatsink for T2 with a thermal coefficient of 1 K/W or better. For L1 we connected two secondary windings in series from a 2x18V/225 VA toroidal transformer.

The resulting inductance of 150 mH was quite a bit more than the recommended 50 mH. However, with an output power of 1 W the amplifier had difficulty reproducing signals below 160 Hz. The distortion rose to as much as 9% for a signal of 20 Hz at 100 mW. To properly reproduce low-frequency signals the amplifier needs a much larger coil with an iron core and an air gap. This prevents the core from saturating when a large DC current flows through the coil.

Parts layout:

Such a core may be found in obsolete equipment, such as old video recorders. A suitable core consists of welded E and I sections. These transformers can be converted to the required inductor as follows: cut through the welding, remove the windings, add 250 to 300 windings of 0.8 mm enamelled copper wire, firmly fix the E and I sections back together with a piece of paper in between as isolation.

The concepts used in this circuit lend themselves very well to some experimentation. The number of supply voltages can be a bit of a problem to start with. For this reason we have designed a power supply especially for use with this amplifier (Quad power supply for hybrid amp). This can of course just as easily be used with other amplifiers. The supply uses a cascade stage to output an unstabilised voltage of 170 V for the SRPP (single rail push pull) stage (V1).

PCB layout:

During initial measurements we found that the ripple on this supply was responsible for a severe hum at the output of the amplifier. To get round this problem we designed a separate voltage regulator (High-voltage regulator with short circuit protection), which can cope with these high voltages. If you use a separate transformer for the filament supply you can try and see if the circuit works without R5. During the testing we used a DC voltage for the filament supply. Although you may not suspect it from the test measurements (see table), this amplifier doesn’t sound bad.

In fact, it is easily better than many consumer amplifiers. The output power is fairly limited, but is still enough to let your neighbours enjoy the music as well. It is possible to make the amplifier more powerful, in which case we recommend that you use more than one MOSFET in the output stage. The inductor also needs to be made beefier. Since this is a Class A amplifier, the supply needs to be able to output the required current, which becomes much greater at higher output powers. The efficiency of the amplifier is a bit over 30%.
Author: Frans Janssens - Copyright: Elektor Electronics

IrDA Interface

By Circuit Projects →
Many modern motherboards are equipped with an infrared data interface compliant with the IrDA standard, but this interface not very often used. However, it is not difficult to build a data transmission module and connect it to the corresponding header. As can readily be seen from the schematic diagram, this doesn’t exactly involve a large array of ICs. This is because transceiver ICs are available for the IrDA standard, so only a few passive components have to be added to obtain an operational circuit. The author has successfully built this circuit many times using the TFDU5102 from Vishay Semiconductors (formerly Telefunken). If this IrDA transceiver is no longer available (it has been officially discontinued), the largely pin- and function-compatible TFDU6102 can be used without any problems.

IrDA Interface Circuit Diagram

This IC is faster and meets the latest IrDA specification. The TFDU6102 low-power receiver IC supports IrDA at data rates up to 4 Mbit/s (FIR), HP-SIR, Sharp ASK, and carrier-based remote control modes up to 2 MHz. The IC contains a photodiode, an infrared emitter and CMOS control logic. The IC also has internal protection against electromagnetic immissions and emissions, so no external screening is necessary. The IC works with a supply voltage of 2.7–5.5 V, so it is suitable for use in desktop PCs, notebooks, palmtops, and PDAs. It is also used in digital still and video cameras, printers, fax machines, copiers, projectors, and many other types of equipment.

The author has designed a printed circuit board for the IrDA module that is only 20 × 20 mm square. Of course, this means that all of the components are SMD types. The TFDU6102 in the ‘babyface’ package is available in upright and flat versions. Here the upright version (suffix ‘TR3’) is used. Thanks to its small size, the assembled circuit board can easily be placed behind a drive bay cover or the like. It is connected to the motherboard by a five-way flat cable. The pin assignments for header X1 must match the mating connector on the motherboard. After you have fitted the module, you may have to edit the BIOS settings to activate the UART for IrDA operation. These settings enable the (Windows) operating system to boot the new device and automatically install it. You may have to briefly insert the Windows CD to modify the settings. There is an abundance of free programs on the Internet that use the IrDA interface.

R1 = 7Ω5 (shape 1210)
R2 = 47 Ω (shape 1206)
R3 = 100 k (shape 1206)
C1 = 100nF (shape 1206)
C2 = 4µF7 (shape 1210)
IC1 = TFDU6102TR3 (Vishay) (Farnell)
X1 = 5-way SIL pinheader
Author: A. Bitzer
Copyright: Elektor Electronics

Linear RF Power Meter

By Circuit Projects →
The National Semiconductor LMV225 is a linear RF power meter IC in an SMD package. It can be used over the frequency range of 450 MHz to 2000 MHz and requires only four external components. The input coupling capacitor isolates the DC voltage of the IC from the input signal. The 10-k? resistor enables or disables the IC according to the DC voltage present at the input pin. If it is higher than 1.8 V, the detector is enabled and draws a current of around 5–8 mA. If the voltage on pin A1 is less than 0.8 V, the IC enters the shutdown mode and draws a current of only a few microampères. The LMV225 can be switched between the active and shutdown states using a logic-level signal if the signal is connected to the signal via the 10-kR resistor.

Circuit diagram:

Linear RF Power Meter Circuit Diagram

The supply voltage, which can lie between +2.7 V und +5.5 V, is filtered by a 100nF capacitor that diverts residual RF signals to ground. Finally, there is an output capacitor that forms a low-pass filter in combination with the internal circuitry of the LMV225. If this capacitor has a value of 1 nF, the corner frequency of this low-pass filter is approximately 8 kHz. The corner frequency can be calculated using the formula fc = 1 ÷ (2 p COUT Ro) where Ro is the internal output impedance (19.8 k?). The output low-pass filter determines which AM modulation components are passed by the detector.

The output, which has a relatively high impedance, provides an output voltage that is proportional to the signal power, with a slope of 40 mV/dB. The output is 2.0 V at 9 dBm and 0.4 V at –40 dBm. A level of 0 dBm corresponds to a power of 1 mW in 50 R. For a sinusoidal wave-form, this is equivalent to an effective voltage of 224 mV. For modulated signals, the relationship between power and voltage is generally different. The table shows several examples of power levels and voltages for sinusoidal signals. The input impedance of the LMV225 detector is around 50 R to provide a good match to the characteristic impedance commonly used in RF circuits.

The data sheet for the LMV225 shows how the 40-dB measurement range can be shifted to a higher power level using a series input resistor. The LMV225 was originally designed for use in mobile telephones, so it comes in a tiny SMD package with dimensions of only around 1 × 1 mm with four solder bumps (similar to a ball-grid array package). The connections are labelled A1, A2, B1 and B1, like the elements of a matrix. The corner next to A1 is bevelled.
Author: Gregor Kleine
Copyright: Elektor Electronics

ESR & Low Resistance Test Meter

By Circuit Projects →
As electrolytic capacitors age, their internal resistance, also known as "equivalent series resistance" (ESR), gradually increases. This can eventually lead to equipment failure. Using this design, you can measure the ESR of suspect capacitors as well as other small resistances. Basically, the circuit generates a low-voltage 100kHz test signal, which is applied to the capacitor via a pair of probes. An op amp then amplifies the voltage dropped across the capacitor’s series resistance and this can be displayed on a standard multimeter. In more detail, inverter IC1d is configured as a 200kHz oscillator.

Its output drives a 4027 J-K flipflop, which divides the oscillator signal in half to ensure an equal mark/space ratio. Two elements of a 4066 quad bilateral switch (IC3c & IC3d) are alternately switched on by the complementary outputs of the J-K flipflop. One switch input (pin 11) is connected to +5V, whereas the other (pin 8) is connected to -5V. The outputs (pins 9 & 10) of these two switches are connected together, with the result being a ±5V 100kHz square wave. Series resistance is included to current-limit the signal before it is applied to the capacitor under test via a pair of test probes. Diodes D1 and D2 limit the signal swing and protect the 4066 outputs in case the capacitor is charged.

Circuit diagram:

ESR & Low Resistance Test Meter Circuit Diagram
A second pair of leads sense the signal developed across the probe tips. Once again, the signal is limited by diodes (D3 & D4) before begin applied to the remaining two inputs of the 4066 switch (pins 2 & 3 of IC3a & IC3b). These switches direct alternate half cycles to two 1μF capacitors, removing most of the AC component of the signal and providing a simple "sample and hold" mechanism. The 1μF capacitors charge to a DC level that is proportional to the test capacitor’s ESR. This is differentially amplified by op amp IC4 so that it can be displayed on a digital multimeter – 10Ω will be represented by 100mV, 1Ω by 10mV, etc. To calibrate the circuit, first adjust VR1 to obtain 100kHz at TP3.

Next, momentarily short the test probes together and adjust VR4 for 0mV at pin 6 of IC4. That done, set your meter to read milliamps and connect it between TP4 and the negative (-) DMM output. Apply -5V to TP2 and note the current flow, which should be around 2.1mA. Transfer the -5V from TP2 to TP1 and adjust VR2 until the same current (ignore sign) is obtained. Remove the -5V from TP1. Again, set to your meter to read volts and connect it to the DMM outputs. Apply the probes to a 10W resistor and adjust VR3 for a reading of 100mV. Finally, ensure that all capacitors to be tested are always fully discharged before connecting the probes.
Author: Len Cox - Copyright: Silicon Chip Electronics

Hard Disk Switch

By Circuit Projects →
In these times with viruses and other threats from the Internet it would be nice to have reassurance that the PC cannot be infected. That is why this circuit was designed. It makes it possible to install multiple hard disks inside the case of a PC, which are separated in such a way that viruses cannot move from one disk to another. In this case there are three drives installed, one for use of the Internet via ADSL, one for working with email and one for other applications.

If data from the Internet never arrives on the third disk, it is effectively protected against viruses. The solution outlined here has been in satisfactory use for a couple of years. There is an additional benefit: if there are ever any problems with the operation of the computer, then it is very easy to change to another hard disk to check if the problem manifests itself there as well. In this case, fault finding can be made much easier. The circuit operates by only switching over the power supply voltages (5 V and 12 V) of the hard disks. The hard disk is out of service without a power supply. This works without a problem with S-ATA disks.

Circuit diagram:
Hard Disk Switch Circuit Diagram

With IDE disks this only works with modern drives. There may only be a combination of hard disks on the relevant port and no CD-ROM, DVD-drive, CD-burner or something similar. The selection of the desired hard disk is done with a rotary switch. This has to be set to the correct position before the computer is switched on. When the power supply is turned on, one of three relays is driven via diode D1, D2 or D3. The relays are provided with a hold circuit via a second diode (D4, D5 and D6). In this way the selected relay remains energised as long as the power supply voltage is present.

After switching on, electrolytic capacitor C1 is charged via R1, so that the common contact of the rotary switch is quickly at 0 V. This prevents an accidental change of hard disk while the computer is in operation. The ADSL modem is powered from the PC. This power supply voltage is only present if hard disk number 2 is selected. This prevents the use of the Internet if one of the other disks is selected.
Author: Uwe Kardel - Copyright: Elektor Electronics Magazine

Computer Off Switch

By Circuit Projects →
How often does it happen that you close down Windows and then forget to turn off the computer? This circuit does that automatically. After Windows is shut down there is a ‘click’ a second later and the PC is disconnected from the mains. Surprisingly enough, this switch fits in some older computer cases. If the circuit doesn’t fit then it will have to be housed in a separate enclosure. That is why a supply voltage of 5 V was selected. This voltage can be obtained from a USB port when the circuit has to be on the outside of the PC case.

It is best to solder the mains wires straight onto the switch and to insulate them with heat shrink sleeving. C8 is charged via D1. This is how the power supply voltage for IC1 is obtained. A square wave oscillator is built around IC1a, R1 and C9, which drives inverters IC1c to f. The frequency is about 50 kHz. The four inverters in parallel power the voltage multiplier, which has a multiplication of 3, and is built from C1 to C3 and D2 to D5. This is used to charge C5 to C7 to a voltage of about 9 V.

The generated voltage is clearly lower than the theoretical 3x4.8=14.4 V, because some voltage is lost across the PN-junctions of the diodes. C5 to C7 form the buffer that powers the coil of the switch when switching off. The capacitors charge up in about two seconds after switching on. The circuit is now ready for use. When Windows is closed down, the 5-V power supply voltage disappears. C4 is discharged via R2 and this results in a ‘0’ at the input of inverter IC1b. The output then becomes a ‘1’, which causes T1 to turn on.

Circuit diagram:
Computer Off Switch Circuit Diagram

A voltage is now applied to the coil in the mains switch and the power supply of the PC is turned off. T1 is a type BSS295 because the resistance of the coil is only 24R. When the PC is switched on, the circuit draws a peak current of about 200 mA, after which the current consumption drops to about 300 µA. The current when switching on could be higher because this is strongly dependent on the characteristics of the 5-V power supply and the supply rails in the PC. There isn’t much to say about the construction of the circuit itself.

The only things to take care with are the mains wires to the switch. The mains voltage may not appear at the connections to the coil. That is why there has to be a distance of at least 6 mm between the conductors that are connected to the mains and the conductors that are connected to the low-voltage part of the circuit.
Author: Uwe Kardel - Copyright: Elektor Electronics Magazine

Mobile Phone and iPod Battery Charger Circuit

By Circuit Projects →

Charge your iPod without connecting it to a computer!

Using the USB port on your computer to charge your player’s batteries is not always practical. What if you do not have a computer available at the time or if you do not want to power up a computer just for charging? Or what if you are traveling? Chargers for Mobile Phones iPods and MP3 players are available but they are expensive and you need separate models for charging at home and in the car.

This charger can be used virtually anywhere. While we call the unit a charger, it really is nothing more than a 5V supply that has a USB outlet. The actual charging circuit is incorporated within the iPOD or MP3 player itself, which only requires a 5V supply. As well as charging, this supply can run USB-powered accessories such as reading lights, fans and chargers, particularly for mobile phones.

The supply is housed in a small plastic case with a DC input socket at one end and a USB type "A" outlet at the other end, for connecting to Mobile Phone, an iPod or MP3 player when charging. A LED shows when power is available at the USB socket. Maximum current output is 660mA, more than adequate to run any USB-powered accessory.

Pictures, PCB and Circuit Diagram:

Front View Of Mobile Phone and iPod Battery Charger Circuit

Bottom View Of Mobile Phone and iPod Battery Charger Circuit

PCB Layout Of Mobile Phone and iPod Battery Charger Circuit

Mobile Phone and iPod Battery Charger Circuit Diagram

P1 = 1K
R1 = 1R-0.5W
R2 = 1R-0.5W
R3 = 1R-0.5W
R4 = 1K
R5 = 560R
R6 = 10R-0.5W
R7 = 470R
C1 = 470uF-25V
C2 = 100nF-63V
C3 = 470pF
C4 = 100uF-25V
D1 = 1N5404
D2 = 1N4001
D3 = 1N5819
D4 = 5.1V-1W Zener Diode
D5 = 5mm. Red LED
L1 = 220uH
S1 = USB 'A' Type Socket
SW1 = On/Off Switch
IC1 = MC34063A

Output voltage ----------------------5V
Output current ---------------------660mA maximum for 5V out
Input voltage range ------------------9.5V to 15V DC
Input current requirement ----------500mA for 9V in, 350mA for >12V input
Input current with output shorted--- 120mA at 9V in, 80mA at 15V in
Output ripple ------------------------14mV (from no load to 660mA)
Load regulation ----------------------25mV (from no load to 660mA)
Line regulation ----------------------20mV change at full load from 9 to 18V input
No load input current ----------------20mA

(The specification for the computer USB 2.0 port requires the USB port to deliver up to 500mA at an output voltage between 5.25V and 4.375V).

The circuit is based around an MC34063 switch mode regulator. This has high efficiency so that there is very little heat produced inside the box, even when delivering its maximum output current. The circuit is more complicated than if we used a 7805 3-terminal regulator but since the input voltage could be 15V DC or more, the voltage dissipation in such a regulator could be 5W or more at 500mA. and 5W is far too much for a 7805, even with quite a large heatsink. Credit for this circuit goes to SiliconChip, A wonderful electronics magazine.

Keyboard/Mouse Switch Unit

By Circuit Projects →
Unplugging or re-connecting equipment to the serial COM or PS2 connector always gives problems if the PC is running. Even if you only need to swap a mouse or changeover from a graphics keyboard to a standard keyboard. The chances are that the connected equipment will not communicate with the PC, it will always be necessary to re-boot. If you are really unlucky you may have damaged the PC or the peripheral device. In order to switch equipment successfully it is necessary to follow a sequence. The clock and data lines need to be disconnected from the device before the power line is removed. And likewise the power line must be connected first to the new device before the clock and data lines are re-connected.

This sequence is also used by the USB connector but achieved rather more simply by using different length pins in the connector. The circuit shown here in Figure 1 performs the switching sequence electronically. The clock and data lines from the PC are connected via the N.C. contacts of relay RE2 through the bistable relay RE1 to connector K3. Pressing push-button S1 will activate relay RE2 thereby disconnecting the data and clock lines also while S1 is held down the semiconductor switch IC1B will be opened, allowing the voltage on C4 to charge up through R4. After approximately 0.2 s the voltage level on C4 will be high enough to switch on IC1A, this in turn will switch on T1 energizing one of the coils of the bistable relay RE1 and routing the clock, data and power to connector K2.

When S1 is released relay RE2 will switch the data and clock lines through to the PC via connector K1. It should be noted that the push-button must be pressed for about 0.5s otherwise the circuit will not operate correctly. Switching back over to connector K3 is achieved similarly by pressing S2. The current required to switch the relays is relatively large for the serial interface to cope with so the energy necessary is stored in two relatively large capacitors (C2 and C3) and these are charged through resistors R1 and R6 respectively. The disadvantage is that the circuit needs approximately 0.5 minute between switch-overs to ensure these capacitors have sufficient charge.

The current consumption of the entire circuit however is reduced to just a few milliamps. The PCB is designed to accept PS2 style connectors but if you are using an older PC that needs 9 pin sub D connectors then these will need to be connected to the PCB via flying leads. In this case the mouse driver software configures pin 9 as the clock, pin 1 as the data, pin 8 (CTS) as the voltage supply pin and pin 5 as earth.

R1 = 2kΩ2
R2 = 47kΩ
R3 = 10kΩ
R4 = 4kΩ7
R5 = 1kΩ
R6 = 1kΩ2
C1 = 10µF 10V radial
C2 = 1000µF 10V radial
C3 = 2200µF 10V radial
C4 = 2µF2 10V radial
D1-D5 = 1N4148
T1 = BC547
IC1 = 4066 or 74HCT4066
RE1 = bistable relay 4 c/o contacts
RE2 = monostable relay 2 c/o contacts
K1,K2,K3 = 6-way Mini-DIN socket (pins at 240°, PCB mount
S1,S2 = push-button (ITTD6-R)

USB Operated Home Appliances

By Circuit Projects →
When turning a computer on and off, various peripherals (such as printers, screen, scanner, etc.) often have to be turned on and off as well. By using the 5-V supply voltage from the USB interface on the PC, all these peripherals can easily be switched on and off at the same time as the PC. This principle can also be used with other appliances that have a USB interface (such as modern TVs and radios). This so-called ‘USB-standby-killer’ can be realized with just 5 components. The USB output voltage provides for the activation of the triac opto-driver (MOC3043) which has zero-crossing detection. This, in turn, drives the TRIAC, type BT126.

The circuit shown is used by the author for switching loads with a total power of about 150 W and is protected with a 1-A fuse. The circuit can easily handle much larger loads however. In that case and/or when using a very inductive load a so-called snubber network is required across the triac. The value of the fuse will also need to be changed as appropriate. The circuit can easily be built into a mains multi-way powerboard. Make sure you have good isolation between the USB and mains sections.

Circuit diagram:
USB Operated Home Appliances Circuit Diagram

Please don't make this circuit if you are not an expert!

Courtesy Light

By Circuit Projects → 30 Oct 2011
15 seconds delayed switch-off, A good idea for bedroom lamps 

This circuit is intended to let the user turn off a lamp by means of a switch placed far from bed, allowing him enough time to lie down before the lamp really switches off.
Obviously, users will be able to find different applications for this circuit in order to suit their needs.

Circuit diagram :

Parts :

R1 = 470R 1/2W
R2 = 100K
R3 = 1M5
R4 = 1K
C1 = 330nF-400V
C2 = 100µF-25V
C3 = 10nF-63V
C4 = 10µF-25V
C5 = 10nF-63V
D1 = 1N4007
D2 = 1N4007
D3 = BZX79C10
D4 = TIC206M
Q1 = BC557
IC1 = 7555 or TS555CN CMos Timer IC
SW1 = SPST Mains suited Switch

Circuit operation:

Due to the low current drawing, the circuit can be supplied from 230Vac mains without a transformer. Supply voltage is reduced to 10Vdc by means of C1 reactance, a two diode rectifier cell D1 & D2 and Zener diode D3. IC1 is a CMos 555 timer wired as a monostable, providing 15 seconds on-time set by R3 & C4. When SW1 is closed, IC1 output (pin 3) is permanently on, driving Triac D4 which in turn feeds the lamp. Opening SW1 operates the monostable and, after 15 seconds, pin 3 of IC1 goes low switching off the lamp.

  • The circuit is wired permanently to the mains supply but current drain is negligible.
  • Due to transformerless design there is no heat generation.
  • The delay time can be varied changing R3 and/or C4 values.
  • Taking C4 = 10µF, R3 increases timing with about 100K per second ratio. I.e. R3 = 1M Time = 10 seconds, R3 = 1M8 Time = 18 seconds.
  • Low Gate-current Triacs are recommended.
  • Use a well insulated mains-type switch for SW1.
  • Warning! The circuit is connected to 230Vac mains, then some parts in the circuit board are subjected to lethal potential!. Avoid touching the circuit when plugged and enclose it in a plastic box.
 Source :

IC Controlled Emergency Light With Charger Circuit

By Circuit Projects →
Here is the circuit diagram of IC Controlled Emergancy Light With Charger or simply 12V to 220V AC inverter circuit. The circuit shown here is that of the IC controlled emergency light. Its main features are: automatic switching-on of the light on mains failure and battery charger with over-charge protection. When mains is absent, relay RL2 is in de-energized state, feeding battery supply to inverter section via its N/C contacts and switch S1.

The inverter section comprises IC2 (NE555) which is used in stable mode to produce sharp pulses at the rate of 50 Hz for driving the MOSFETs. The output of IC2 is fed to gate of MOSFET (Q4) directly while it is applied to MOSFET (Q3) gate after inversion by transistor Q2. Thus the power amplifier built around MOSFETs Q3 and Q4 functions in push-pull mode. The output across secondary of transformer T2 can easily drive a 230-volt, 20-watt fluorescent tube. In case light is not required to be on during mains failure, simply flip switch S1 to off position. Battery overcharge preventer circuit is built around IC1 (LM308).

Its non-inverting pin is held at a reference voltage of approximately 6.9 volts which is obtained using diode D5 (1N4148) and 6.2-volt zener D6. The inverting pin of IC1 is connected to the positive terminal of battery. Thus when mains supply is present, IC1 comparator output is high, unless battery voltage exceeds 6.9 volts. So transistor Q1 is normally forward biased, which energises relay RL1. In this state the battery remains on charge via N/O contacts of relay RL1 and current limiting resistor R2. When battery voltage exceeds 6.9 volts (overcharged condition), IC1 output goes low and relay RL1 gets de-energised, and thus stops further charging of battery. MOSFETs Q and Q4 may be mounted on suitable heat sinks.

Circuit diagram:
Align Center
IC Controlled Emergency Light with Charger Circuit Diagram


R1 = 1K
R2 = 10R-1W
R3 = 820R
R4 = 1K
R5 = 10K
R6 = 1K
R7 = 100R
R8 = 1K

C1 = 1000uF-25V
C2 = 10uF-16V
C3 = 0.01uF

D1 = 1N4007
D2 = 1N4007
D3 = 1N4007
D4 = 1N4007
D5 = 1N4148
D6 = 6.2V Zener
D7 = 1N4007
D8 = 1N4148

Q1 = SL100
Q2 = 2N2222
Q3 = IRF840
Q4 = IRF840

Integrated Circuits
IC1 = LM308
IC2 = NE555

S1 = SPST Switch
B1 = 6V-4A Battery
B2 = 6V-4A Battery
TI = 220V AC Primary to 0V-6V 250mA Secondary Transformer
T2 = 4.5V-0V-4.5V 5A Primary To 230V AC Secondary Transformer


Car Boot Lamp Warning (ICM7556)

By Circuit Projects →
On many cars, the boot light will not go out until the lid is properly closed. It is all too easy when unloading the car, to leave the lid ajar. If you are unlucky and the car remains unused for some time, the next time you try to start it, the lamp will have drained the battery and you will no doubt utter a few appropriate words. The circuit described here will give a warning of just such a situation. A mercury tilt switch is mounted in the boot so that as the lid is closed, its contacts close before the lid is completely shut. The supply for the circuit comes from the switched 12 V to the boot lamp and through the mercury switch. When the lid is properly closed, the boot lamp will go out and the supply to the circuit will go to zero. If however the lid is left ajar, the lamp will be on and the mercury switch will close the circuit.

After 5 seconds, the alarm will start to sound, and unless the lid is shut, it will continue for 1 minute to remind you to close the boot properly. The 1-minute operating period will ensure that the alarm does not sound continuously if you are, for example, transporting bulky items and the boot will not fully close. The circuit consists of a dual CMOS timer type 7556 (the bipolar 556 version is unsuitable for this application). When power is applied to the circuit (i.e. the boot lid is ajar) tantalum capacitors C1 and C2 will ensure that the outputs of the timers are high. After approximately 5 seconds, when the voltage across C2 rises to 2/3 of the supply voltage, timer IC1b will be triggered and its output will go low thereby causing the alarm to sound.

Meanwhile the voltage across C1 is rising much more slowly and after approximately 1 minute, it will have reached 2/3 of the supply voltage. IC1a will now trigger and this will reset IC1b. The alarm will be turned off. IC1a will remain in this state until the boot lid is either closed or opened wider at which point C1 and C2 will be discharged through R6 and the circuit will be ready to start again. To calculate the period of the timers use the formula: t = 1.1RC Please note that the capacitor type used in the circuit should be tantalum or electrolytic with a solid electrolyte. The buzzer must be a type suitable for use at D.C. (i.e. one with a built in driver).

Light-Operated Light Switch

By Circuit Projects → 29 Oct 2011

Here is a light-operated, remote-controlled solidstate switch to operate a lamp. During darkness, the resistance of LDR shoots up to meg-ohm range. Thus, the triac does not get gate drive and hence it does not conduct.

Circuit diagram:

Light-Operated Light Switch-Circuit-Diagram

Light-Operated Light Switch Circuit Diagram

When LDR is illuminated by means of a torch-light beam, the resistance of LDR suddenly decreases (below 10-kilo-ohm). This causes the triac to conduct and switch  ‘on’ the lamp. Light received from the lamp (not from the torch) keeps LDR’s resistance low. So, the lamp re-mains continuously ‘on’. Once the lamp is‘on’, it can be switched  ‘off’ again by in-terrupting the light falling on LDR, by either waving hand in front of it or by interrupting power supply to the circuit for a moment.

RFC emplo-yed here can be made by winding about 15 turns of 18 SWG wire over an insulated ferrite rod.

Author : Pradeep G.  - Copyright :electronicsforu

Inverter Circuit For Soldering Iron

By Circuit Projects →
Here is a simple but inexpensive inverter for using a small soldering iron (25W, 35W, etc) In the absence of mains supply. It uses eight transistors and a few resistors and capacitors. Transistors Q1 and Q2 (each BC547) form an astable multivibrator that produces 50Hz signal. The complementary outputs from the collectors of transistors Q1 and Q2 are fed to pnp Darlington driver stages formed by transistor pairs Q3-Q5 and Q4-Q6 (utilising BC558 and BD140).

The outputs from the drivers are fed to transistors Q7 and Q8 (each 2N3055) connected for push-pull operation. Use suitable heat-sinks for transistors Q5 through Q8. A 230V AC primary to 12V-0-12V, 4.5A secondary transformer (T1) is used. The centre-tapped terminal of the secondary of the transformer is connected to the battery (12V, 7Ah), while the other two terminals of the secondary are connected to the collectors of power transistors T7 and T8, respectively.

When you power the circuit using switch S1, transformer X1 produces 230V AC at its primary terminal. This voltage can be used to heat your soldering iron. Assemble the circuit on a generalpurpose PCB and house in a suitable cabinet. Connect the battery and transformer with suitable current-carrying wires. On the front panel of the box, fit power switch S1 and a 3-pin socket for connecting the soldering iron. Note that the ratings of the battery, transistors T7 and T8, and transformer may vary as these all depend on the load (soldering iron).

Circuit diagram:
Inverter Circuit Diagram For Soldering Iron


P1-P2 = 47K
R1-R2 = 1K
R3-R4 = 270R
R5-R6 = 100R/1W
R7-R8 = 22R/5W
C1-C2 = 0.47uF
Q1-Q2 = BC547
Q3-Q4 = BC558
Q5-Q6 = BD140
Q7-Q8 = 2N3055
SW1 = On-Off Switch
T1 = 230V AC Primary 12-0-12V
4.5A Secondary Transformer
B1 = 12V 7Ah


Active High Pass Filter Using LM741

By Circuit Projects →
This is active high pass filter circuit for 327Hz frequency using LM741. It will use to build Harmonic at 3 of 130.81 frequency have the value at least. More than the frequency Fundamental 30 dB, for output be sawtooth wave form for use in sound of music way system Electronic design will use the circuit filters three rank frequency. By have 3 dB you slopes can use Op-amp IC number LM741 or number LF351it will meet the frequency well.

Circuit diagram:

Active High Pass Filter Circuit Diagram
Source: ElecCircuit

Dry Cell Battery Charger Using LM741

By Circuit Projects →
This is Dry Cell Battery Charger Circuit. That can use charger battery get that about 12 hour. When apply to power supply 9 volt the equipment that fix in the circuit use for size battery AA. If use the size C or D should devalue of Resistor RX down be 68ohm and should not lead battery come to serial while voltage in cell battery lower 1.6V. The Comparator Circuit with (IC741) control Gate output from Pulse Oscillator at use the integrated circuit CMOS 4011 change Transistor that do infront charger battery until voltage tall 1.6V Comparator Circuit more make LED Flasher warn know for protect Charger battery expire. The next time is if friends have Dry Cell Battery that use be finished already , don’t abandon , try apply new again yes.

Circuit diagram:

Dry Cell Battery Charger Circuit Diagram

Telephone Conversation recorder

By Circuit Projects → 28 Oct 2011

This circuit enables  automatic switching-on  of  the  tape  recorder  when  the  handset  is  lifted.  The tape recorder gets switched off when the handset is replaced. The signals are suit-ably  attenuated  to  a  level  at  which  they can be recorded using the 'MICIN' socket of the tape recorder. Points X and Y in the circuit are connected to the telephone lines. Resistors R1 and R2 act as a voltage divider.

The voltage appearing across R2 is fed to the 'MIC-IN' socket of the tape recorder. The values of R1 and R2 may be changed depending on the input impedance of the tape recorder's 'MIC-IN'  terminals.  Capacitor C1 is used for blocking the flow of DC. The second part of the circuit controls relay RL1, which is used to switch on/off the tape recorder.A  voltage  of  48  volts  appears across  the  telephone  lines  in on-hook  condition. This  voltage drops  to  about  9  volts  when  the handset  is  lifted.  Diodes  D1 through  D4  constitute  a  bridge rectifier/polarity  guard. 

Circuit diagram :

Telephone Conversation recorder Circuit Diagram

Telephone Conversation recorder Circuit Diagram

This ensures that transistor T1 gets voltage of proper polarity, irrespective of the polarity of the telephone lines.During on-hook condition, the output from the bridge (48V DC) passes through 12V zener D5 and is applied to the base of transistor T1 via the voltage divider comprising resistors R3 and R4. This switches on transistor T1 and its collector is pulled low. This, in turn, causes transistor T2 to cut off and relay RL1 is not energised. When the telephone handset is lifted, the voltage across points X and Y falls below 12 volts and so zener diode D5 does not conduct.

As a result, base of transistor  T1  is  pulled  to  ground  potential  via resistor R4 and thus is cut off. Thus, base of  transistor  T2  gets  forward  biased  via resistor R5, which results in the energisation  of  relay  RL1. The  tape  recorder  is switched 'on' and recording begins. The tape recorder should be kept loaded with a cassette and the record button of the tape recorder should remain pressed to enable it to record  the conversation as soon as the handset is lifted. Capacitor  C2  ensures  that  the  re-lay is not switched on-and-off repeatedly when a number is being dialled in pulse dialling mode.

Author : PRADEEP  VASUDEVA - Copyright : EFY

Dual Relay Driver Board Circuit Schematic

By Circuit Projects →
A simple and convenient way to interface 2 relays for switching application in your project. This relay driver boosts the input impedance with a regular BC547 NPN transistor (or equivalent). Very common driver. It can drive a variety of relays, including a reed-relay. Transistor Q1and Q2 are a simple common-emitter amplifier that increases the effective sensitivity of the 12 volt relay coil about a 100 times, or in other words, the current gain for this circuit is 100. Using this setup reduces the relay sensitivity to a few volts. R3 and R4 restricts the input current to Q1 and Q2 to a safe limit. Diodes D3 and D4 are EMF dampers and filter off any sparking when the relay

Picture of the project:
Front View Of Dual Channel Relay Board Driver

Circuit diagram:
2 Channel Relay Driver Circuit Diagram


R1-R2 = 1K
R3-R4 = 5.6K
C1-C2 = 100nF-63V
D1-D2 = Red LED
D3-D4 = 1N4001
L1-L2 = 12V Relay
Q1-Q2 = BC547

  • Input - 12 VDC @ 84 mA
  • Output - two SPDT relay
  • Relay specification - 5 A @ 230 VAC
  • Trigger level-2~5VDC
  • Berg pins for connecting power and trigger voltage
  • LED on each channel indicates relay status

Courtesy Light Extender

By Circuit Projects → 22 Oct 2011
In essence, this circuit is a 15 to 20-second courtesy light extender for cars. It is activated in the usual way by opening a door but it also samples the negative lock/unlock signals from a car alarm or central locking and does two more things. First, when an unlock signal is received, it turns on the courtesy light for 15-20 seconds before you open the door. Second, when a lock signal is received, it turns off the courtesy light immediately, with no fade-out. This is done to eliminate false triggering of the burglar alarm through current drain sensing. When a car door is open or the unlock relay is activated, the 33µF capacitor discharges through diode D1 and this keeps transistor Q1 turned off.

Circuit diagram:
Courtesy Light Extender Circuit Diagram

This allows Q2 and Q3 to turn on and the courtesy lamp is activated. When the door is closed, the courtesy lamps stay illuminated and the 33µF electrolytic capacitor starts charging through the associated 1MO resistor. As the voltages rises, Q1 turns on slowly, turning off Q2 and Q3 which gradually fades out the courtesy lamp. If a lock signal from the central locking system is received, relay 1 closes and charges the capacitor instantly, so the lamp turns off immediately. Relays were used to interface to the central locking/alarm system as a safety feature, to provide isolation in case something goes wrong.
Author: Matt Downey - Copyright: Silicon Chip Electronics

A Friendly Charger Schematic for Mobile Phones

By Circuit Projects →
Charges and protects your mobile phone from voltage spikes and short-circuit

Most mobile chargers do not have current/voltage regulation or short-circuit protection. These chargers provide raw 6-12V DC for charging the battery pack. Most of the mobile phone battery packs have a rating of 3.6V, 650mAh. For increasing the life of the battery, slow charging at low current is advisable. Six to ten hours of charging at 150-200mA current is a suitable option. This will prevent heating up of the battery and extend its life.

Circuit diagram:
A Friendly Charger Circuit Diagram For Mobile Phones



P1 = 10K LOG
R1 = 1K
R2 = 1K
R3 = 1K
R4 = 1K
R5 = 3.3K
R6 = 16R/2W
R7 = 220R
R8 = 3.3R
R9 = 1K


C1 = 470uF/25V
C2 = 10uF/25V
C3 = 1KuF/25V


D1 = Red LED
D2 = Green LED
Q1 = BC547
Q2 = BD677
ZD1 = 12V/1W
ZD2 = 5.6V/1W
IC1 = CA3130

The circuit described here, provides around 180mA current at 5.6V and protects the mobile phone from unexpected voltage fluctuations that develop on the mains line. So the charger can be left ‘on’ over night to replenish the battery charge. The circuit protects the mobile phone as well as the charger by immediately disconnecting the output when it senses a voltage surge or a short circuit in the battery pack or connector. It can be called a ‘middle man’ between the existing charger and the mobile phone.

It has features like voltage and current regulation, over-current protection, and high- and low-voltage cut-off. An added specialty of the circuit is that it incorporates a short delay of ten seconds to switch on when mains resumes following a power failure. This protects the mobile phone from instant voltage spikes. When short-circuit occurs at the battery terminal, resistor R8 senses the over-current, allowing Q1 to conduct and light up D1. Glowing of D2 indicates the charging mode, while D1 indicates short-circuit or over-current status.

The value of resistor R8 is important to get the desired current level to operate the cut-off. With the given value of R8 (3.3 ohms), it is 350 mA. Charging current can also be changed by increasing or decreasing the value of R7 using the ‘I=V/R’ rule. Construct the circuit on a common PCB and house in a small plastic case. Connect the circuit between the output lines of the charger and the input pins of the mobile phone with correct polarity.

Power Flip-Flop Using A Triac

By Circuit Projects → 21 Oct 2011
Modern electronics is indispensable for every large model railroad system, and it provides a solution to almost every problem. Although ready-made products are exorbitantly expensive, clever electronics hobbyists try to use a minimum number of components to achieve optimum results together with low costs. This approach can be demonstrated using the rather unusual semiconductor power flip-flop described here. A flip-flop is a toggling circuit with two stable switching states (bistable multivibrator). It maintains its output state even in the absence of an input pulse.

Flip-flops can easily be implemented using triacs if no DC voltage is available. Triacs are also so inexpensive that they are often used by model railway builders as semiconductor power switches. The decisive advantage of triacs is that they are bi-directional, which means they can be triggered during both the positive and the negative half-cycle by applying an AC voltage to the gate electrode (G). The polarity of the trigger voltage is thus irrelevant. Triggering with a DC current is also possible. Figure 1 shows the circuit diagram of such a power flop-flop. A permanent magnet is fitted to the model train, and when it travels from left to right, the magnet switches the flip-flop on and off via reed switches S1 and S2.

Circuit diagram:

In order for this to work in both directions of travel, another pair of reed switches (S3 and S4) is connected in parallel with S1 and S2. Briefly closing S1 or S3 triggers the triac. The RC network C1/R2, which acts as a phase shifter, maintains the trigger current. The current through R2, C1 and the gate electrode (G) reaches its maximum value when the voltage across the load passes through zero. This causes the triac to be triggered anew for each half-cycle, even though no pulse is present at the gate. It remains triggered until S2 or S4 is closed, which causes it to return to the blocking state.The load can be incandescent lamps in the station area (platform lighting) or a

solenoid-operated device, such as a crossing gate. The LED connected across the output (with a rectifier diode) indicates the state of the flip-flop. The circuit shown here is designed for use in a model railway system, but there is no reason why it could not be used for other applications. The reed switches can also be replaced by normal pushbutton switches. For the commonly used TIC206D triac, which has a maximum current rating of 4 A, no heat sink is necessary in this application unless a load current exceeding 1 A must be supplied continuously or for an extended period of time. If the switch-on or switch-off pulse proves to be inadequate, the value of electrolytic capacitor C1 must be increased slightly.
Author: R. Edlinger - Copyright: Elektor July-August 2004

Mains Voltage Monitor

By Circuit Projects →
Many electronics hobbyists will have experienced the following: you try to finish a project late at night, and the mains supply fails. Whether that is caused by the electricity board or your carelessness isn’t really important. In any case, at such times you may find yourself without a torch or with flat batteries. There is no need to panic, as this circuit provides an emergency light. When the mains fails, the mains voltage monitor turns on five super bright LEDs, which are fed from a 9 V battery (NiCd or NiMH) or 7 AA cells. A buzzer has also been included, which should wake you from your sleep when the mains fails.

You obviously wouldn’t want to oversleep because your clock radio had reset, would you? When the mains voltage is present, the battery is charged via relay Re1, diode D8 and resistor R10. D8 prevents the battery voltage from powering the relay, and makes sure that the relay switches off when the mains voltage disappears. R10 is chosen such that the charging current of the battery is only a few milliamps. This current is small enough to prevent over-charging the battery. D6 acts as a mains indicator. When the relay turns off, IC1 receives power from the battery. The JK flip-flops are set via R12 and C4.

Circuit diagram:
Mains Voltage Monitor Circuit Diagram

This causes T1 and T2 to conduct, which turns on D1-D5 and the buzzer. When the push button is pressed, a clock pulse appears on the CLK input of flip-flop IC1b. The output then toggles and the LEDs turn off. At the same time IC1a is reset, which silences the buzzer. If you press the button again, the LEDs will turn on since IC1b receives another clock pulse. The buzzer remains off because IC1a stays in its reset state. R11, R3 and C3 help to debounce the push button signal. In this way the circuit can also be used as a torch, especially if a separate mains adapter is used as the power supply.

As soon as the mains voltage is restored, the relay turns on, the LEDs turn off and the battery starts charging. The function of R13 is to discharge C4, preparing the circuit for the next mishap. If mains failures are a regular occurrence, we recommend that you connect pairs of LEDs in series. The series resistors should then have a value of 100 ?. This reduces the current consumption and therefore extends the battery life. This proves very useful when the battery hasn’t recharged fully after the last time. In any case, you should buy the brightest LEDs you can get hold of. If the LEDs you use have a maximum current of 20 mA, you should double the value of the series resistors! You could also consider using white LEDs.
Author: Goswin Visschers - Copyright: Elektor July-August 2004

Mains Slave Switcher

By Circuit Projects →
There are many situations where two or more pieces of equipment are used together and to avoid having to switch each item on separately or risk the possibility of leaving one of them on when switching the rest off, a slave switch is often used. Applications which spring to mind are a computer/printer/scanner etc or audio amplifier/record deck/tuner combinations or perhaps closest to every electronics enthusiast’s heart, the work bench where a bench power supply/oscilloscope/soldering iron etc are often required simultaneously.

The last is perhaps a particularly good example as the soldering iron, often having no power indicator, is invariably left on after all the other items have been switched off. Obviously the simplest solution is to plug all of the items into one extension socket and switch this on and off at the mains socket but this is not always very convenient as the switch may be difficult to reach often being behind or under the work bench. Slave switches normally sense the current drawn from the mains supply when the master unit is switched on by detecting the resulting voltage across a series resistor and switching on a relay to power the slave unit(s).

This means that the Live or Neutral feed must be broken to allow the resistor to be inserted. This circuit, which is intended for switching power to a work bench when the bench light is switched on, avoids resistors or any modifications to the lamp or slave appliances by sensing the electric field around the lamp cable when this is switched on. The lamp then also functions as a ‘power on’ indicator (albeit a very large one that cannot be ignored) that shows when all of the equipment on the bench is switched on.

The field, which appears around the lamp cable when the mains is connected, can be sensed by a short piece of insulated wire simply wrapped around it and this is amplified by the three stage amplifier which can be regarded as a single super-transistor with a very high gain. The extremely small a.c. base current results in an appreciable collector current which after smoothing (by C3) is used to switch on a relay to power the other sockets. Power for the relay is obtained from a capacitor ‘mains dropper’ that generates no heat and provides a d.c. supply of around 15 volts when the relay is off.

Circuit diagram:
Mains Slave Switcher Circuit Diagram

The output current of this supply is limited so that the voltage drops substantially when the relay pulls in but since relays require more current to operate them than they do to remain energized, this is not a problem. Since the transistor emitter is referenced to mains Neutral, it is the field around the mains Live which will be detected. Consequently, for correct operation the Live wire to the lamp must be switched and this will no doubt be the case in all lamps where the switch is factory fitted. In case of uncertainty, a double-pole switch to interrupt both the Live and Neutral should be used.

The sensitivity of the circuit can be increased or decreased as required by altering the value of the T2 emitter resistor. The sensing wire must of course be wrapped around a section of the lamp lead after the switch otherwise the relay will remain energized even when the lamp has been switched off. The drawing shows the general idea with the circuit built into the extension socket although, depending on the space available an auxiliary plastic box may need to be used.

The circuit itself is not isolated from the mains supply so that great care should be taken in its construction and testing. The sensor wire must also be adequately insulated and the circuit enclosed in a box to make it inaccessible to fingers etc. when it is in use.

12V Halogen Dimmer

By Circuit Projects →
use a 12V 20W halogen lamp (MR16) and a 4.2Ah SLA battery for my bike light system. The battery has only limited life at this power rating, so I designed this cheap light dimmer to reduce the battery drain and allow for longer rides at night. Based on a simple 555 timer circuit and Mosfet switch Q1, it works by pulse-width modulating the 12V supply to the lamp. The 555 (IC1) is wired as a free-running oscillator, with two different mark/space ratios selectable via a 2-pole, 5-position rotary switch (S1). The third switch position bypasses the electronic circuitry and connects the lamp directly to battery negative. This gives three power levels of about 7W, 13W and 20W. A logic-level IRL530N Mosfet with a drain-source "on" resistance of only 0.1Ω ensures low losses and eliminates the need for a heatsink. An STP30NE06L Mosfet (Jaycar Cat. ZT-2271) would also be suitable.

Circuit diagram:
12V Halogen Dimmer Circuit Diagram
Author: Mike Dennis
Copyright: Silicon Chip Electronics

Intelligent Electronic Lock

By Circuit Projects →

This intelligent electronic lock circuit is built using transistors only. To open this electronic lock, one has to press tactile switches S1 through S4 sequentially. For deception you may annotate these switches with different numbers on the control panel/keypad. For example, if you want to use ten switches on the keypad marked ‘0’ through ‘9’, use any four arbitrary numbers out of these for switches S1 through S4, and the remaining six numbers may be annotated on the leftover six switches, which may be wired in parallel to disable switch S6 (shown in the figure). When four password digits in ‘0’ through ‘9’ are mixed with the remaining six digits connected across disable switch terminals, energisation of relay RL1 by unauthorized person is prevented.For authorized persons, a 4-digit password number is easy to remember. To energies relay RL1, one has to press switches S1 through S4 sequentially within six seconds, making sure that each of the switch is kept depressed for a duration of 0.75 second to 1.25 seconds. The relay will not operate if ‘on’ time duration of each tactile switch (S1 through S4) is less than 0.75 second or more than 1.25 seconds.

This would amount to rejection of the code. A special feature of this circuit is that pressing of any switch wired across disable switch (S6) will lead to disabling of the whole electronic lock circuit for about one minute. Even if one enters the correct 4-digit password number within one minute after a ‘disable’ operation, relay RL1 won’t get energized. So if any unauthorized person keeps trying different permutations of numbers in quick successions for energisation of relay RL1, he is not likely to succeed. To that extent, this electronic lock circuit is fool-proof. This electronic lock circuit comprises disabling, sequential switching, and relay latch-up sections. The disabling section comprises zener diode ZD5 and transistors T1 and T2. Its function is to cut off positive supply to sequential switching and relay latch-up sections for one minute when disable switch S6 (or any other switch shunted across its terminal) is momentarily pressed.

Circuit diagram :

Intelligent Electronic Lock -Circuit-Diagram

Intelligent Electronic Lock Circuit Diagram

During idle state, capacitor C1 is in discharged condition and the voltage across it is less than 4.7 volts. Thus zener diode ZD5 and transistor T1 are in non-conduction state. As a result, the collector voltage of transistor T1 is sufficiently high to forward bias transistor T2. Consequently, +12V is extended to sequential switching and relay latch-up sections. When disable switch is momentarily depressed, capacitor C1 charges up through resistor R1 and the voltage available across C1 becomes greater than 4.7 volts. Thus zener diode ZD5 and transistor T1 start conducting and the collector voltage of transistor T1 is pulled low. As a result, transistor T2 stops conducting and thus cuts off positive supply voltage to sequential switching and relay latch-up sections. Thereafter, capacitor C1 starts discharging slowly through zener diode D1 and transistor T1. It takes approximately one minute to discharge to a sufficiently low level to cut-off transistor T1, and switch on transistor T2, for resuming supply to sequential switching and relay latch-up sections; and until then the circuit does not accept any code.

The sequential switching section comprises transistors T3 through T5, zener diodes ZD1 through ZD3, tactile switches S1 through S4, and timing capacitors C2 through C4. In this three-stage electronic switch, the three transistors are connected in series to extend positive voltage available at the emitter of transistor T2 to the relay latch-up circuit for energising relay RL1.  When tactile switches S1 through S3 are activated, timing capacitors C2, C3, and C4 are charged through resistors R3, R5, and R7, respectively. Timing capacitor C2 is discharged through resistor R4, zener diode ZD1, and transistor T3; timing capacitor C3 through resistor R6, zener diode ZD2, and transistor T4; and timing capacitor C4 through zener diode ZD3 and transistor T5 only. The individual timing capacitors are chosen in such a way that the time taken to discharge capacitor C2 below 4.7 volts is 6 seconds, 3 seconds for C3, and 1.5 seconds for C4. Thus while activating tactile switches S1 through S3 sequentially, transistor T3 will be in conduction for 6 seconds, transistor T4 for 3 seconds, and transistor T5 for 1.5 seconds.

The positive voltage from the emitter of transistor T2 is extended to tactile switch S4 only for 1.5 seconds. Thus one has to activate S4 tactile switch within 1.5 seconds to energise relay RL1. The minimum time required to keep switch S4 depressed is around 1 second. For sequential switching transistors T3 through T5, the minimum time for which the corresponding switches (S1 through S3) are to be kept depressed is 0.75 seconds to 1.25 seconds. If one operates these switches for less than 0.75 seconds, timing capacitors C2 through C4 may not get charged sufficiently. As a consequence, these capacitors will discharge earlier and any one of transistors T3 through T5 may fail to conduct before activating tactile switch S4.  Thus sequential switching of the three transistors will not be achieved and hence it will not be possible to energise relay RL1 in such a situation. A similar situation arises if one keeps each of the mentioned tactile switches de-pressed for more than 1.5 seconds.

When the total time taken to activate switches S1 through S4 is greater than six seconds, transistor T3 stops conducting due to time lapse. Sequential switching is thus not achieved and it is not possible to energise relay RL1. The latch-up relay circuit is built around transistors T6 through T8, zener diode ZD4, and capacitor C5. In idle state, with relay RL1 in de-energised condition, capacitor C5 is in discharged condition and zener diode ZD4 and transistors T7, T8, and T6 in non-conduction state. However, on correct operation of sequential switches S1 through S4, capacitor C5 is charged through resistor R9 and the voltage across it rises above 4.7 volts. Now zener diode ZD4 as well as transistors T7, T8, and T6 start conducting and relay RL1 is energised. Due to conduction of transistor T6, capacitor C5 remains in charged condition and the relay is in continuously energised condition. Now if you activate reset switch S5 momentarily, capacitor C5 is immediately discharged through resistor R8 and the voltage across it falls below 4.7 volts. Thus zener diode ZD4 and transistors T7, T8, and T6 stop conducting again and relay RL1 de-energises.

Author : K. UdHaya Kumaran - Copyright : Electronics for you April 2001

Mains Slave Switcher II

By Circuit Projects →
As a guide, a one-inch reed switch with 40 turns reliably switched on with the current flowing through a 150-watt lamp (approx. 625 mA) but larger reeds may require more turns. If the master appliance draws less current (which is unlikely with power tools) more turns will be required. The reed switch is used to switch on transistor T1 which in turn switches the relay RE1 and powers the slave appliance. Since reed switches have a low mechanical inertia, they have little difficulty in following the fluctuations of the magnetic field due to the alternating current in the coil and this means that they will switch on and off at 100 Hz.

Circuit diagram:
Mains Slave Switcher II Circuit Diagram

C3 is therefore fitted to slow down the transistor response and keep the relay energised during the mains zero crossings when the current drawn by the appliance falls to zero and the reed switch opens. C1 drops the mains voltage to about 15 V (determined by zener diode D1) and this is rectified and smoothed by D2 and C2 to provide a d.c. supply for the circuit. The relay contacts should be rated to switch the intended appliance (vacuum cleaner) and the coil should have a minimum coil resistance of 400 R as the simple d.c. supply can only provide a limited current. C1 drops virtually the full mains voltage and should therefore be a n X2-class component with a voltage rating of at least 250V a.c.

The circuit is by its nature connected directly to the mains supply. Great care should therefore be taken in its construction and the circuit should be enclosed in a plastic or earthed metal box with mains sockets fitted for the master and slave appliances.
Author: Elektor - Copyright: Elektor Electronics Magazine

Mains Operated LED Circuit Schematic

By Circuit Projects →
Small in size! Big in use

Here is a simple and powerful LED circuit that can be operated directly from the AC 100 volt to AC 230 Volts mains supply. The circuit can be used as mains power locator or night lamp etc.. The resistor R1,R2 and capacitor C1 provides necessary current limiting. The circuit is sufficiently immune against voltage spikes and surges.

Circuit's pictures:

Front View of 220 Volt AC Operated LED Circuit

Circuit diagram:

Mains Operated LED Circuit Diagram

D1 = 1N4007
D2 = 1N4007
D3 = 1N4007
D4 = 1N4007
R2 = 1M-1/2W
R1 = 470R-1/2W
C1 = 220nF-275vAC
D5 = 5mm. Blue LED

  1. Small in size!
  2. Blue LED operated on mains voltage
  3. Suited for mains indicator or other pilot lamps
  4. For safety guidance, stairs, corridors…
  5. Special X2 safety capacitor
  6. 100Vac to 240Vac 50Hz or 60Hz Operated
  7. Dimensions: 28x18mm / 1.10 x 0.71"

  • Only for use inside a cabinet
  • The capacitor C1 can be polyester type.
  • Also white LED can be used in this circuit.
  • Assemble the circuit on a general purpose PCB.

Safety and Hazard WARNINGS:

This circuit operates on a lethal power voltage. Mount the circuit in a protective cabinet prio to applying AC Power. Do not modify the circuit - Wait 10 minutes before touching the circuit after disconnecting the AC Power. This circuit is not intended for children.