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Universal Alarm

Smoke alarm and then some

Just add a few external components to a standard low-cost smoke alarm and turn it into a universal alarm sounder. You can easily hook up the outputs from any number of sensors or detectors to trigger the alarm and the smoke detector still functions as normal.

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Discussion (0 comments)

Donald Weissgerber 3 years ago

Where in the detector is terminal C ? The remainder ABD are clearly marked.

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Happy Harry 3 years ago

C is actually not on the board but is a break in the positive line from the battery to the board. Until you do the following, C is actually the same point as B. This allows a switch to be put between the board and the battery with its charging circuit. You could do this by either cutting the +ve lead from the battery to make B and C or unsoldering the lead from the board at B and adding in the wire to the switch from B with the other wire coming from the +ve battery lead.

As a general comment though, there is no way I would build this directly connected to the mains like this, no fuse and it's asking for trouble. Although the risk is probably low for the neon to fail to a short, it still could potentially leave live on the circuit. The more likely issue of D1 failing though would allow the voltage to potentially rise to mains, although the battery initially will keep it low. This will then cause D2 to fail as it's only rated to 30v and the capacitor will love having AC on it.