|Circuit and board designed by Phil Townshend 1998
This fridge alarm works by monitoring the light inside the fridge. When the door opens, the LDR detects the light and activates the circuit. The remainder of the circuit is a basic 555 monostable which activates a warning device when the monostable finishes timing. So when the fridge is opened the warning device will activate after the time set by the monostable, letting you know the fridge has been left open for too long.
An alternative is to turn the timing to the shortest period (1 second), then the alarm will sound almost immediately letting you know someone is in the fridge.
Timing varies from 1 second to about 50 seconds with the values given.
Note the use of a PNP transistor to drive the siren. Any general purpose PNP transistor will do as long as its Ic is high enough and has adequate gain (hFE).
The circuit is designed to be left on for long periods of time and draws a very small current, typically around 1.5 uA. This means a PP3 battery should last several weeks at least.
A standard 555 timer IC can be used but for longer battery life, the CMOS version is a better option.
Download PCB and circuit details
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