Description.
The circuit diagram shown here is of a simple DC power delay circuit that is based on an SCR. This circuit is a very handy one and can be employed in many applications. The working of this circuit is very simple. When the input power is applied the capacitor C2 charges through resistor R2 and when the voltage across the capacitor just exceeds the Zener diode D3’s breakdown voltage, it breaks down and the SCR H1 is triggered and the delayed power will be available at the delayed OUT terminal.
Circuit diagram.

Notes.

• The circuit must be assembled on a good quality PCB.
• The Zener diode must be rated half the input supply voltage.
• The current capacity of the circuit depends on the SCR and here it is 4A.

Description.

Here is the circuit diagram of a fuse that has an automatic status indicator. This circuit can be added to circuit that operates from 12V DC. As long as the fuse is intact, the LED D3 will glow continuously and when the fuse blows off the LED will start and continue blinking.
The first part of the circuit includes an astable multivibrator built around transistors Q1 and Q2.The output of the multivibrator is coupled to the base of Q3 via diode D2.When the fuse is intact the base of Q3 will be pulled to a positive voltage by the resistor R5. The transistor will be ON and the LED D3 remains glowing. When the fuse of blown off, the base of Q3 will no longer be pulled to positive voltage and now the only biasing available at the base of Q3 will be the output from the astable multivibrator. Now the transistor Q3 will start switching in the frequency of astable multivibrator and the LED will blink in accordance.

Circuit diagram with Parts list.

Notes.

• The circuit can be assembled on a Vero board.
• This fuse circuit can be incorporated with any circuit that operates from 12V DC.
• The type no of the transistors are not very significant and almost any NPN transistors will do the job.
• The fuse F1 can be a metal wire fuse with your required rating.

Description.
This fan regulator circuit will automatically control the speed of your fan according to the temperature. Two thermistors (R1 and R2) are used to sense the temperature. The circuit works almost like the published here previously. Here the potentiometer is replaced by the thermistors. When the temperature is increasing the resistance of NTC thermistor ( R1) decreases and at the same time the resistance of PTC thermistor (R2) increases. At the same time, rate of change of the resistance will be different for R1 and R2.This action is similar to a potentiometer used in a conventional Triac based fan regulator. When the resistance is varied the firing angle of the triac changes and so do the speed of the fan.

Circuit diagram.

Notes.

• The circuit can be assembled on a Vero board.
• L1 can be a 70uH inductor.
• C1 and C2 must be rated at least 200V.
• C3 must be rated at least 600V.
• Triac BT136 can safely handle up to 4A load current.
• The two thermistors must be mounted as close as possible.

Description.

The circuit given here can be used to control the brightness of low power incandescent lamps. The circuit is based on IC NE555 which is wired as an astable multivibrator with variable duty cycle. The output of IC is connected to the base of transistor Q1.The Q1 drives the lamp. The duty cycle of the multivibrator can be varied by varying the POT R4.As a result, the brightness of the lamp varies according to the position of the POT R4.The same circuit can be also used for speed control of small DC motors.

Circuit diagram with Parts list.

Notes.

• The lamp L1 can be a 6V / 200 mA lamp.
• The switch S1 can be SPST ON/OFF switch.
• The IC1 must be mounted on a holder.
• The circuit can be wired on a good quality PCB or common board.

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.