A latch circuit is supposed to lock on and off a load (such as a relay coil or a led light), through a momentary input signal.

Basically it works like a bistable relay, but backs to the reset condition when the power supply gets off.

In this article I'm  going to talk about two latch circuits I designed and made for hobby purpose.

Both circuits use transistors. The first one latches a relay coil, whose contacts are also used to let the circuit work.

The second one uses just two transistors to latch a led light.