Dаvid Jones through his Youtube channel EEVblog described in detail how to design a cheap soft latch power switch circuit, using one push button switch to toggle your circuit power on and off with the following design requirements:
- Zero power when off.
- One on/off switch.
- Standalone(no MCU).
- General components only (Diodes, Transistors, ..ect).
- Minimal parts.
First Basic Circuit
A passing transistor is between the input and the output, with another latching transistor.
When we initially power on the circuit, it will be off because the passing transistor is off and so the latching transistor is off (like the egg and chicken). When we turn on the “ON” switch, the passing transistor is on, so the current flows from input to output and therefor the latching transistor is on. To turn it off, press the “OFF” button and the latching transistor is off, and the passing transistor is off.
First design uses 2 switches and we are looking forward a circuit with one switch only, so let us get a look to the next circuit design.
When we push the button, the transistor connected to the gate of passing MOSFET will be on, and therefor the passing transistor will be on, on the right side the BJT will be on driving the line down to ground. So next time when you press the the button the transistor connected to gate will be off.
The capacitor connected to base of right side transistor is used to prevent the oscillating on/off while the pressing of the button needs milliseconds and the transistors works much faster.
To see an experiment of the design and to learn more details, check David’s video bellow: