PRESS ON OFF SWITCH CIRCUIT

Simple soft press switch using an NPN transistor, N mosfets, and either a P mosfets or a relay depending on your load.

The soft press switch in this circuit is similar to our computer, monitor and some phone switches. I like it most because of its durability and nice look when fixed.

SOFT PRESS SWITCH CIRCUIT DIAGRAM

PRESS SWITCH CIRCUIT OPERATION

IRF5210 can be replaced with a 12V relay to control heavy loads. The relay is connect to point A and B. Connecting a 100uf capacitor across the relay coil will ensure better operation.

When an input voltage is applied, the circuit stays off until the switch S1 is pressed because Q8 has turned on and connected the base of Q10 close to ground.

Because U23 is not charging, it’s shorted, hence, pressing the switch S1 shorts the base of Q8 and connects it to ground through R30 and U23. This actions turns Q8 off to remove its latch and allows Q10 to turn on and Q9 or the relay is turned on and latched until the switch is pressed again.

As Q10 turns on, U23 is charged through R47 and R46 at time constant 2RC. When U23 is charged and the switch is pressed again, the voltage stored in capacitor U23 is connected to the base of the NPN transistor through the 220 ohms resistor and the circuit turns off again and stays off until next press or infinity.

Capacitor U21 ensures that the circuit operation is not affected by electrical noise.

 To let the circuit turn on automatically at DC in or power restored, remove U24 from the circuit. Else leave it as it is for the function explained above.

The circuit is tested and is in use.