Building a Push-Button Latch Circuit

Make a momentary button 'stick' — press once to turn something on, and have it stay on after you let go, using a relay's own contacts to hold itself.

You'll need: a relay (from Lesson cb-11) with at least one spare normally-open contact, a momentary push-button, a separate normally-closed "reset" button, and a supply matched to the relay coil.

A plain momentary button only does something while you're physically holding it down — release it, and the circuit returns to off. A latch circuit changes that: press once, and the circuit stays on by itself until something explicitly turns it off again.

The trick uses the relay's own contacts to hold itself energized: wire the push-button in parallel with one of the relay's normally-open contacts, both feeding the relay's coil. Pressing the button energizes the coil, which closes that same contact — and once closed, the contact itself now supplies the coil with current, even after you release the button, because current is flowing through the contact instead of the button.

To break the latch, wire a normally-closed reset button in series with the whole coil path. Pressing that button interrupts the coil current entirely, the relay de-energizes, its contact opens, and the circuit returns fully to off — ready to be latched on again with the first button.