What's going on? Who's sneaking into the fridge?
Build this simple alarm circuit to catch the midnight snacker in the act! When the refrigerator door is closed, the alarm is quiet. When the refrigerator door is open, the inside light switches on and activates the alarm, which emits a shrill tone. Even if the tone fails to wake you, it just might cause the fridge invader to quickly close the door and return to bed.
Step 1: How it works...
Step 2: Parts You Will Need...
Q1-- 2N2222A NPN transistor (38236)
PZ1-- Piezoelectric tone generator (335557)
R1-- Cadmium sulfide photoresistor (202454)
R2-- 10K trimmer pot
Miscellaneous: Perforated prototype board (616622),
9-volt battery (198791), battery connector clip (216427), double-sided tape, refrigerator with snacks and potential refrigerator robber(s).
Note: While these components were used for the prototype, substitutions can be easily made. For example, Q1 can be a general purpose NPN switching transistor. If you substitute PZ, be sure the current required by the substitute doesn't exceed Q1's specs.
The Fridge Alarm Kit is also available as a bundle at Jameco.