This is a simple - easy to build - alarm. Build it yourself using stripboard or veroboard and a few cheap off-the-shelf components. Fit it to a fridge door - and it'll tell you when the door has been left open. It has a very low stand-by current so it's ideal for battery operation.

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Simple Fire-Door Alarm - Photograph

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This is a photograph of the Cmos 4011 based Fire-Door Alarm. This circuit will let you know when something that should be kept closed - has in fact been left open. Attach it to a Fire-Door and it will allow you to pass back and forth without reacting. However - if the door is open for more than 30-seconds or so - the warning buzzer will sound. The circuit has an extremely low standby current - and it's ideal for battery operation.
A Photograph Of Ron J's Cmos 4011 
Based Fridge Alarm - Circuit Board


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