Description.

Here is a simple yet powerful circuit that can be used for flashing 12V lamps especially that is used on automobiles.The flashing circuit is based on transistor Q1(BC557) and MOSFET Q2 (IRF530) where the Q2 provides the necessary drive for the lamp.Any number of bulbs can be flashed using this circuit provided that the total load must not exceed 42 Watts.

Circuit diagram with Parts list.

Notes.

•  Assemble the  circuit on a good quality PCB or common board.
• The circuit can be powered form the car battery itself.
• The switch S1 can be used as the ON/OFF switch.
• All capacitors must be rated 25V.
• Slight variations in the flashing frequency is possible by varying the value of C1.

Description.

Here is a simple circuit based on two transistors that can be used to control the speed of a 12 V DC fan depending on the temperature.A thermistor (R1) is used to sense the temperature. When the temperature increases the base current of Q1 (BC 547) increases which in turn decreases the collector voltage of the same transistor. Since the collector of Q1 is coupled to the base of Q2 (BD 140), the decrease in collector voltage of Q1 forward biases the Q2 more and so do the speed of the motor. Also, the brightness of the LED will be proportional to the speed of the motor.

Circuit diagram with Parts list.

Notes.

• The R1 can be a 15K @ 20°C ,N.T.C  thermistor.
• The M1 can be a 12V,700mA fan motor.
• The capacitor C1 must be rated 25V.
• The circuit can be powered from a 12V PP3 battery or 12V DC power supply.
• Assemble the circuit on a good quality PCB or common board.

Description.

This timer circuit can be used to switch OFF a particular device after around 35 minutes. The circuit can be used to switch OFF devices like radio, TV, fan, pump etc after a preset time of 35 minutes. Such a circuit can surely save a lot of power.

The circuit is based on quad 2 input CMOS IC 4011 (U1).The resistor R1 and capacitor C1 produces the required long time delay. When pushbutton switch S2 is pressed, capacitor C1 discharges and input of the four NAND gates are pulled to zero. The four shorted outputs of U1 go high and activate the transistor Q1 to drive the relay. The appliance connected via the relay is switched ON. When S2 is released the C1 starts charging and when the voltage at its positive pin becomes equal to ½ the supply voltage the outputs of U1 becomes zero and the transistor is switched OFF. This makes the relay deactivated and the appliance connected via the relay is turned OFF. The timer can be made to stop when required by pressing switch S1.

Circuit diagram with Parts list.

Notes.

• Assemble the circuit on a good quality PCB or common board.
  
• The circuit can be powered from a 9V PP3 battery or 12V DC power supply.
  
• The time delay can be varied by varying the values of C1&R1.
  
• The push button switch S2 is for starting the timer and S1 for stopping the time.
  
• The appliance can be connected via contacts N1 & N2 of the relay RL1.
• The IC U1 is 2 input quad NAND gate 4011.

 Description.

Here is a simple circuit that can be used to pulse an LED or a low power filament lamp. The effect is such that the LED lights up from OFF state gradually and then dims gradually. Such a circuit is very useful in decoration purposes.

The circuit is nothing but a triangular wave generator based on dual opamp IC LM 358.The two opamps inside the IC is used for the purpose. The transistor Q1 BC337 is used to drive the load (LED or lamp).The resistor R4 and capacitor C1 are the timing components of the circuit. Here the values of R4 & C1 are so chosen that the timing is around 5S.

Circuit diagram with Parts list.

Notes. 

• Assemble the circuit on a good quality PCB or common board.
• The circuit can be powered from three 1.5V cells in series (4.5V)  or a 5V DC regulated power supply
• By using a 5M POT in place of R4, you can vary the time period of the circuit.
• You can  use a 3.2 V /200mA bulb instead of the LED.If bulb is used R6 can be omitted.

Description.

The circuit given below shows a negative reference voltage supply.The reference voltage can be set to precise values by R2. The circuit operates in the same manner as the positive reference voltage supply circuit shown previously.The circuit is nothing but  an inverting DC amplifier based on IC uA741.The circuit operates on a negative power supply and produces a negative reference voltage at the output.

Circuit diagram with Parts list.

Notes. 

• Assemble the circuit on a good qualtiy PCB or common board.
• Mount the IC on a  holder.
• The POT R2 can be used for precisely setting the output voltage.
• The formula for determining the output voltage is ,Vout= -Vz [(R1+R2)(R1) ]; where Vz is the break down voltage of zener diode.