Simple Momentary Switch Latching Circuit

I had to cook up a quick circuit to allow a momentary switch to turn the power on and off for a circuit but draw no current when off. I came up with this. I am sure someone else has done something similar before, but it was fun figuring it out for myself.

It uses two transistors or a transistor and a PMOS FET. It can be tailored to work with a large range of voltages… and very low voltages when using the NPN/PNP version. The values shown work well from about 5 volts up to 12 volts. If the  voltage is higher than that, some of the resistors (R1 and R5) should be increased so that the on current used by the circuit is not excessive.

I noticed when you have highly reactive inductive or capacitive loads the circuit can fail to turn off. You can use either an isolated switching element or a blocking diode to solve this.

It works because in the initial state the PMOS is turned off and C1/R4 is charged up enough to turn on the NPN. When the switch is pressed, the charge on C1 biases on the NPN, which then turns on the PMOS. Once the PMOS is on, R3 biases on the NPN, latching the “on state” for the circuit. However now the collector of Q2 is a path to ground and C1  discharges to ground through R2(R4 also). Now, if the switch is pressed the NPN gets turned off long enough to shut the whole thing off again.

You can adjust the resistor values and the C1 value to adjust the response time, quiescent “on” current, etc. I advise building it as shown, using  a 9V battery  and a LED as a load, to get a feel for how it works before changing the values too much. The values are interrelated… so you can’t make extreme changes to one component value without adjusting others.

momentary latching circuit