This circuit disconnects the mains voltage upon detection of smoke and starts flashing a red light + an alarm horn.
Originally built for some tests with cheap PC power supplies and general supervision of various experimental set-ups.
Rubber, plastic and synthetic materials often tend to smoke quite a lot on the brink of burning. Immediately interrupting the supply of electric energy into that system with a simultaneous notification can prevent worse. I thought I’d share my solution and hopefully someone finds it useful and maybe wants to build something similar.
Schematic
For this matter I chose the normally open contact of the relay to be in the main current path because if the 12V PSU from Mean Well or the relay fails, or even just a loose wire occurs, the power will be shut down. If a normally closed contact would be used, this wouldn’t be the case and the node would run without being monitored by the smoke detector and possibly without noticing that malfunction.
Video
The siren starts at 0:16 and is very loud, please check the volume of your speakers before viewing. I don’t want to trigger anyone’s tinnitus.
The blue lamp acts as load in the video. As soon as the smoke alarm triggers, it gets switched off.
The relay should switch the phase wire and not the neutral wire. That depends on how you insert the plug into the wall or the power strip! I recommend using a phase checker or a voltmeter (measure the voltage against PE then) to check it. Then mark on which side the phase wire is, for example, with a sticker or draw a symbol on it.
Parts:
PSU: Mean Well HDR-15-12, 12V
Relay: Finder 22.239.012.4000, 230V/20A Relay with NO and NC contact
Alarm siren: Basetech 1546853, 12V with light, 112dB
Already owned: Some wooden plates, a metal rod, metal shelf, power strips, pliers, cable ties, electric wires, ferrules and a diode (any with rated DC voltage >16V)
The smoke detector has no inbuilt siren, therefore the external siren. If the smoke detector triggers, it switches a NO or NC contact (selectable) closing the circuit for the siren. It has no internal battery for backup and is only supplied by the 12V PSU, so breaking up the 230V power line path in case of failure of the 12V PSU is also important for the purpose of that whole thing.
Connecting the metal shelf to PE should also not be forgotten.
Impressions of the prototype
