In case of a starting fire, an early warning can prevent serious accidents.
Usually fire alarms and smoke detectors are powered by batteries, which makes them unreliable after some time. But also power adapters have a limited life cycle.
This design is fully passive and does not require any power source. For the housing the enclosure of a power transformer could be used. But also a printed version is available.
The thermal switch (normally open) is based on bi-metal. A full range of switching temperatures is available. We use the 40C switch. When the temperature raises above 40C, an alarmbuzzer with flashing led (AD16-22SM) is powered. The switch as well as the buzzer are specified for 240 volts. Since there is no ageing of the two components, the alarm will still be reliable after 20, 30 years. Plug some units into the wall outlets near suspicious electrical equipment and forget about them.
The housing of the switch is connected to 240 volts. This high voltage is not harmless. That is why the switch is carefully protected against unintended contact. Even with a penetrating metal object (kids), it will be very difficult to touch the switch. There are ventilation openings to make sure the switch can pick up the elevated temperature quickly. Everybody should have a couple of these in their house.
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