Using a house fire alarm to shut-off a power outlet. The circuit uses transistors in a latching fashion, such that anything can happen to the fire alarm, it can stop or burn, the circuit will remain latched and the power outlet will not be conducting. Until the user comes and reset the unit. The circuit uses a photo-isolator between the fire alarm and the circuit, 4 transistors and a 16A relay for the power outlet. The photo-isolator allows an alarm unit to provide a trigger at a voltage up to 30V and also ensures that a voltage spike from the fire alarm will not cause further harm to the circuit, in case the fire alarm short circuits or burns for example.

Jumpers JP1A and JP2B are available in case external indicators already contain their needed resistors. In such case, the jumper should be shorted.

Jumper JP3 is available in case another relay is used. Shorting JP3 will provide the full 12V at the relay bobbin. Keeping JP3 opened provides a voltage of ~9V at the relay bobbin and keeps the power dissipation at a minimum; since the relay is meant to be activated during normal operation.

In addition to the surface mounts components listed in the BOM, the project needs these through-hole components:

1 Relay: RZ03-1A4-D012 (works up to 264VAC 16A)

1 AC/DC Transformer: RAC03-12SK (produces 12VDC, accepts AC up to 264VAC)

1 "TE Connectivity" Male connector for the fire alarm, part number: 643488-1 (digikey: A25429-ND)

2 Terminals for the optional external indicators 3.5mm pitch, for example digikey 609-4725-ND

3 Terminals for high voltage wires, the PCB works with "Metz Connect": 360381 (digikey: 1849-1248-ND, will work up to 16A)

Note: The female counterpart connector for the fire alarm would be 1-480303-2 (digikey: A121323-ND) along with 3 contact sockets 61117-4 (digikey: A100716CT-ND)

3D CAD on grabcad

Fire alarm need to be a "hardwired" type (meaning not battery operated) with an interconnect. I used the "First Alert" model BRK-9120B

Video in action

Purchase assembled on Tindie