fridge alarm - protects tomorrow's dinner

july/august 1984 00" A . . . protects tomorrow"s dinner As we all know, it is important that doors of fridges and freezers are nor- mally closed. An alarm to tell you that it isn"t is the subject of this article. It is based on a light- dependent resistor (LDR). As soon as the door of the fridge, or freezer, being guarded is opened, light falls onto the LDR: the circuit is then ac- tuated and a warning tone is sounded until the door is closed again. The circuit may also be used to monitor other doors (for instance, to prevent heat loss, or as a precaution against a fire spreading), but because of the ambient light it is of course impossible to use an LDR. This can therefore be replaced by a microswitch, in which case the alarm will sound when the switch is closed. Note that this requires a switch which closes when the door is opened. A delay of about 10 s between the opening of the door and the sound- ing of the alarm is provided by the time constant R3C4. If faster reac- tion of ...