May 7, 2014



 This circuit protects refrigerators as well as other appliances from over and under-voltage. Operational amplifier IC LM324 (IC2) is used here as a comparator. IC LM324 consists of four operational amplifiers, of which only two operational amplifiers (N1 and N2) are used in the circuit.

The unregulated power supply is connected to the series combination of resistors R1 and R2 and potmeter VR1. The same supply is also connected to a 6.8V zener diode (ZD1) through resistor R3.Preset VR1 is adjusted such that for the normal supply of 180V to 240V, the voltage at the non-inverting terminal (pin 3) of operational amplifier N1 is less than 6.8V. Hence the output of the operational amplifier is zero and transistor T1 remains off. The relay, which is connected to the collector of transistor T1, also remains de energised. As the AC supply to the electrical appliances is given through the normally closed (N/C) terminal of the relay, the supply is not disconnected during normal operation.

When the AC voltage increases beyond  240V, the voltage at the non-inverting terminal (pin 3) of operational amplifier N1 increases. The voltage at the inverting terminal is still 6.8V because of the zener diode. Thus now if the voltage at pin 3 of the operational amplifier is higher than 6.8V, the output of the operational amplifier goes high to drive transistor T1 and hence energise relay RL. Consequently, the AC supply is disconnected and electrical appliances turn off. Thus the appliances are protected against over-voltage. Thus the appliances are protected against over-voltage.

Now let’s consider the under-voltage condition. When the line voltage is below 180V, the voltage at the inverting terminal (pin 6) of operational amplifier N2 is less than the voltage at the non-inverting terminal (6V). Thus the output of operational amplifier N2 goes high and it energises the relay through transistor T1. The AC supply is disconnected and electrical appliances turn off. Thus the appliances are protected against under-voltage. IC1 is wired for a regulated 12V supply.

Thus the relay energises in two conditions: first, if the voltage at pin 3 of IC2 is above 6.8V, and second, if the voltage at pin 6 of IC2 is below 6V. Over-voltage and under-voltage levels can be adjusted using presets VR1 and VR2, respectively.

Apr 19, 2014


There are several types of voice recorder and playback systems available in the market but most of them are expensive and their circuits are also very complex to assemble. Here is a simple circuit for recording and playback of voice messages. You can leave a voice message for your family or friends whenever you go out, which they can hear by pressing the ‘play’ button.

The circuit is built around a recording and playback chip that supports voice recording for 16 to 30 seconds and reproduces it clearly. It can be used in different types of applications such as door bells, railway announcement systems and automatic telephone answering devices.


Fig. 1 shows the circuit of voice recorder and playback system. The circuit is built around voice recording and playback IC APR9301-V2 (IC1), voltage regulator 7806 (IC2), npn transistor BC547 (T1), 8-ohm, 0.5W speaker (LS1), electret microphone (MIC1) and a few other components.
IC APR9301-V2 is a high-quality voice recording and playback IC. The length of message recording depends on the value of external resistor R1 connected to its pin 7. The operation modes are described below.

Recording mode. When switch S1 is pressed, LED1 glows to indicate that recording has started. Now you can speak close to microphone MIC1 in order to record your message. You may have to vary VR1 to adjust for different microphones. IC1 remains in recording mode as long as switch S1 is pressed and pin 27 of IC1 is grounded. Recording stops after 20 seconds (selected by 52-kilo-ohm resistance in this case), pin 25 of IC1 becomes ‘high’ and LED1 stops glowing.

The recording time duration can be increased or decreased depending on the value of resistor R1 as follows:

1. 38 kilo-ohms for 16 seconds

2. 52 kilo-ohms for 20 seconds

3. 67 kilo-ohms for 24 seconds

4. 75 kilo-ohms for 30 seconds

Apr 18, 2014


Keep away intruders with this compact electrified window charger. The charger produces non-lethal shocks that are strong enough to threaten intruders.

The circuit uses IC CD4047 as a free-running astable multivibrator. Capacitor C1 and preset VR1 are timing components. The pulse repetition rate is determined by the value of 4.4C1×VR1. The frequency can be varied with the help of preset VR1.

The IC generates complementary squarewave signals at pins 10 and 11. Transistors T1 and T2 serve as drivers for the following push-pull amplfier stage. A high-voltage generator, realised using step-up transformer X1 and medium-power transistors T3 and T4, follows the astable multivibrator. The stepdown transformer is used for reverse function (step-up) and its output is rectified by diode D1, filtered by capacitor C3 and then given to window (made of metal frame).


Using this electrolytic capacitor tester you can detect leaky and dead (open) electrolytic capacitors. It operates based on the time constant (T) of the capacitor when it is charged up to 63 percent of the applied voltage via a known resistor. The time constant is calulated as follows:

Where ‘T’ is in seconds, ‘C’ is in microfarads, and ‘R’ is in mega-ohms.

Two NE555 timer ICs are used. IC1 is wired in the monostable mode. Initially, when the power is applied, the low output of IC1 causes LED1 to glow. When IC1 is triggered by pressing switch S3, the capacitor under test starts charging via the selected resistor (R1, R2, R3, or R4) and its output jumps to high state, causing LED1 to go off. It remains high for a time duration (in seconds) depending on the RC time constant and then returns to the original low state, which causes LED1 to glow again.

 The monostable time period (=1.1×R×C) can be measured by a stop-watch. By comparing this time period (delay time) with that of a good capacitor, we can find the value of the capacitor.

IC2 is connected in the astable mode. Two red LEDs (LED2 and LED3) are connected to its output pin 3. When the output of IC1 jumps to high state, LED1 goes off and the power is applied to pins 4 and 8 of IC2, causing LED2 and LED3 (connected to IC2) to start flashing. Using VR1 adjust the flashing rate of LED2 and LED3 to one flash per second. After the monostable time period is over, LED2 and LED3 stop flashing and LED1 glows again. The number of flashes counted is the time period in seconds.

Connect the capacitor under test at the indicated position with polarity as shown in the figure. Close switch S1 to apply power to the tester. LED1 glows immediately to indicate that power is applied to the tester. Set selection switch S2 to low-resistance range position. On pressing switch S3, LED1 goes off and LED2 and LED3 start flashing. Count the flashes carefully until the LED stops flashing.

Now connect a good capacitor of the same value to the tester and note its delay period. If the delay period of the capacitor under test is almost equal to that of the good capacitor, it is in good condition. In case LED2 and LED3 flash indefinitely without stopping or no flashing, the capacitor under test is leaking or dead short.

To calculate the approximate value of the capacitor under test, multiply the delay time by an arbitrary factor. The arbitrary factor is different for different resistance ranges (refer Table I).

Example 1: For a 10µF capacitor, delay time is 126 seconds in the 10-mega-ohm range. On multiplying 126 by 0.09, we get 11.34 µF as the measured value of the capacitor.

Example 2: For a 1000µF capacitor, detail time is 130 seconds in the 100-kilo-ohm range. On multiplying 130 by 9.0, we get 1170 µF as the measured value.

The delay times and measured values of the capacitor are given in Table II. 


This circuit lets you turn on/off a fan by just directing torchlight or other light toward its light-dependent resistor (LDR). The circuit is powered from a 5V power supply.

Preset VR1 and a light-dependent resistor (LDR) work as the potential divider. Normally, the LDR’s resistance is high (20 kilo-ohms) in darkness and low (2 kilo-ohms) in light. This value of high and low resistances varies for other LDRs. Preset VR1 is used for setting the intensity of light, while preset VR2 is used for setting the output time period of IC1.

When light falls on the LDR, the monostable (IC1) triggers at pin 2, making its output at pin 3 from low to high. This low-to-high transition forms a clock for D flip-flop. The D flip-flop is operated in toggle mode by connecting its Q output to D point. The flip-flop output goes to an inverter (N1). The inverter output is fed to the relay driver transistor.

When the inverter output is low, diode D1 conducts and the current is diverted into the inverter. Hence the relay does not energise. When the inverter output is high, diode D2 conducts and the current is diverted into transistor T. Hence the relay energises.

One terminal of the fan is connected to the normally-open (N/O) contact of the relay, while another terminal is connected to the neutral (N) of mains. The mains live (L) is connected to the pole of the relay. When the relay energises, the fan turns on. Otherwise, the fan remains off.

Switches S1 and S3 are for initial resetting of the monostable (IC1) and D flip-flop (IC2), respectively, and switch S2 is used for setting the D flip-flop. Paste a piece of paper on the face of the LDR so that it doesn’t get activated by ambient light. Use a torch to light the LDR.

After initial resetting of the monostable and D flip-flop, the inverter output goes high and the fan turns on via the relay. When light falls on the LDR, the fan goes off. If torchlight is again directed toward the LDR, the fan turns on. The sequence repeats.

Initially if switch S2 is used to set the D flip-flop, the fan is held ‘off’. The relay does not energise as the Q output of D flip-flop goes high to make the inverter output low. Directing the light towards the LDR at this moment turns the fan ‘on.’


A key chain with a built-in white LED comes in handy to help you at your front door or search your valuables in the dark. The intensity of white LED is 4000 to 5600 mcd (millicandela) at forward voltage of 3.6V and forward current of 20 mA.

Here’s such an LED light circuit for key chains. It comprises a toroidal transformer and two complementary transistors, and is powered by a single AAA cell. Transistors T1 (BC547) and T2 (BC558) form a relaxation oscillator with capacitor C2 (0.01 µF) in the feedback loop. The feedback is controlled by the time constant of timing components R1 and C2, which controls the frequency of operation.

The toroidal transformer steps up the oscillator output to a sufficient value to flash the white LED. The values of R1 and C1 need not be precise. Use of surface mount devices will make the unit more compact.

A single 1.5V AAA cell gives enough brightness. For more brightness, connect two such cells in series. A good-quality white LED from a reputed manufacturer is highly recommended.

Caution. The white LED beam, when viewed directly, can harm the eyes.

Apr 17, 2014


This handy mobile bug or cell phone detector, pocket-size mobile transmission detector or sniffer can sense the presence of an activated mobile cellphone from a distance of one and-a-half metres. So it can be used to prevent use of mobile phones in examination halls, confidential rooms, etc. It is also useful for detecting the use of mobile phone for spying and unauthorised video transmission.

The circuit can detect both the incoming and outgoing calls, SMS and video transmission even if the mobile phone is kept in the silent mode. The moment the bug detects RF transmission signal from an activated mobile phone, it starts sounding a beep alarm and the LED blinks. The alarm continues until the signal transmission ceases.
An ordinary RF detector using tuned LC circuits is not suitable for detecting signals in the GHz frequency band used in mobile phones. The transmission frequency of mobile phones ranges from 0.9 to 3 GHz with a wavelength of 3.3 to 10 cm. So a circuit detecting gigahertz signals is required for a mobile bug.
Here the circuit uses a 0.22μF disk capacitor (C3) to capture the RF signals from the mobile phone. The lead length of the capacitor is fixed as 18 mm with a spacing of 8 mm between the leads to get the desired frequency. The disk capacitor along with the leads acts as a small gigahertz loop antenna to collect the RF signals from the mobile phone.

Op-amp IC CA3130 (IC1) is used in the circuit as a current-to-voltage converter with capacitor C3 connected between its inverting and non-inverting inputs. It is a CMOS version using gate-protected p-channel MOSFET transistors in the input to provide very high input impedance, very low input current and very high speed of performance. The output CMOS transistor is capable of swinging the output voltage to within 10 mV of either supply voltage terminal.
Capacitor C3 in conjunction with the lead inductance acts as a transmission line that intercepts the signals from the mobile phone. This capacitor creates a field, stores energy and transfers the stored energy in the form of minute current to the inputs of IC1. This will upset the balanced input of IC1 and convert the current into the corresponding output voltage.
Capacitor C4 along with high-value resistor R1 keeps the non-inverting input stable for easy swing of the output to high state. Resistor R2 provides the discharge path for capacitor C4. Feedback resistor R3 makes the inverting input high when the output becomes high. Capacitor C5 (47pF) is connected across ‘strobe’ (pin 8) and ‘null’ inputs (pin 1) of IC1 for phase compensation and gain control to optimise the frequency response.
When the cell phone detector signal is detected by C3, the output of IC1 becomes high and low alternately according to the frequency of the signal as indicated by LED1. This triggers monostable timer IC2 through capacitor C7. Capacitor C6 maintains the base bias of transistor T1 for fast switching action. The low-value timing components R6 and C9 produce very short time delay to avoid audio nuisance.
Assemble the cell phone detector circuit on a general purpose PCB as compact as possible and enclose in a small box like junk mobile case. As mentioned earlier, capacitor C3 should have a lead length of 18 mm with lead spacing of 8 mm. Carefully solder the capacitor in standing position with equal spacing of the leads. The response can be optimised by trimming the lead length of C3 for the desired frequency. You may use a short telescopic type antenna.
Use the miniature 12V battery of a remote control and a small buzzer to make the gadget pocket-size. The unit will give the warning indication if someone uses mobile phone within a radius of 1.5 meters.


Here, a mechanism is presented by which fused fluorescent lamp can be enlighted: 
                In this model, only healthy filament of fused fluorescent lamp can emit the sufficient electrons and collected by other one to glow up. In the circuit a DC voltage is provided across two filaments with fixed polarity through a Full Wave Bridge Rectifier. In which one port inputs are two different tube-end-pins connection. And other’s port inputs are Supply Line and Choke input point connection. Choke is connected between supply line and bridge rectifier input terminal to control the current flow through back emf process. Starter is also connected across the tube to develop striking voltage. Here Starter and Choke functions same as in a healthy fluorescent lamp.


Here is a simple battery charger circuit diagram:
Click on the image to enlarge it:


You can see the values of the components used below:

R1: 56 giga ohms resistor
R2: 220 Mega ohms resistor
C1: 105 Kilo pico farad , 250 voltage capacitor
D1: IN 4007 Diode
D2: Light emitting diode indicator
D3 : IN 4007 Diode

If you use the values of the components stated above the circuit can recharge a 3 voltage rechargeable battery , You can change the value of R1 and C1 to get recharge battery of more voltage.


Nowadays mobiles can also be charged using the USB outlet of PC. The mobile charger circuit presented in this project can give 4.7V of synchronized voltage for charging the phone. As USB outlets can give 5V DC and 100mA of current. It is sufficient for slow charging of mobile phones so they can be used to charge the mobile phones. USB stands for Universal Serial Port. It is one of the latest methods to exchange information from PC to the real world. The USB port offers power to the external devices. +5V of power is available at pin1 and -5V of it is available at pin4.