Instructables
Sounds are coming from the kitchen late at night. One morning a slice of pie is mysteriously missing.

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.
 
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Step 1: How it works...

The circuit for the fridge alarm is shown in Fig. 1. It's a basic light-activated switch that activates a piezo tone generator. When cadmium sulfide photoresistor R1 is dark, its resistance is very high and NPN switching transistor Q1 is off. When light strikes the sensitive surface of R1, its resistance falls significantly. This causes the voltage divider formed by R1 and R2 to apply sufficient bias to Q1's base to switch Q1 on. This allows current to flow through Q1 to PZ, a piezoelectric buzzer.

Step 2: Parts You Will Need...

The following parts were used to make a prototype circuit on a solderless breadboard. Jameco part numbers are shown in parentheses.

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.
sharlston5 years ago
can u wire it up to the fridge door to give u a electric shock when u open the door lol?
iNate sharlston3 years ago
I know what I'm doing for April Fools ;)
LittleWolf5 years ago
Can you wire it up so a pie comes hurling out when the door is opened? Then the trail of pie would lead you to the culprit.
...........or the trail of somethin' else..........................??
The trail of a clown? =O
Gamer9175 years ago
make something that when the lite turns on a motor presses the shutter button on a camera to take a picture so you know who it i
Leosmama5 years ago
I love it! This could be a great weight-loss aid for those mentioned on the fridge magnet I saw once ("Fridge pickers wear bigger knickers"). Might help to reduce the eating in-between meals (whatever the time) urge, especially if you connect the metal handles/plate to a low voltage circuit as 'palooko' suggests. Great instructable!
Really cool. And now I understand how transistors work. Thanks.
bowmaster5 years ago
I'm to fast for this alarm to work. I can open the door and grab the light switch before it pops up. (fine, it sticks a little. But just a little!! You still only have ~ 1/8 of a second.
its designed for people who ARENT aware that its in there.
t.rohner5 years ago
Good idea But why not just drive the buzzer with the fridge light voltage? You won't need a battery and it wouldn't be easily removed. The sound of the piezo would be even nastier, modulated with 60Hz and if you don't like it, add a capacitor. Working day and night, you could save energy, because people would close the fridge faster to make the sound stop... But be aware, it's 120 or 230 Volts. If you don't know how to do it, stay with the 9 Volt circuit.
I've done that one before. Nastily noisy.
my mom was reading this to my dad. to stop me btw (lol). and my dad is blind. and he said you should change: and it provides a convenient way for a blind person to know when unnecessary lights have been left on to and it provides a convenient way for ""the visually impared"" to know when unnecessary lights have been left on
GIVE ME A BREAK! I'm visually impaired (I wear glasses) and I can tell when unnecessary lights are left on.
Yeah, and please change that part about the short guy to vertically challenged :)
tht would help immensly!! >:D
wow
palooko5 years ago
Very nice instructable. If you really wanted to be a jerk, you could affix metal plates to the handle which is attached to a low voltage circuit for a nice "shock" as soon as the light switches on but I'm sure the room mates wouldn't be too happy in the morning time. :o
dataphool5 years ago
Yeaa! Hooray! A proper and readable schematic. I get so tired (not to mention old) when I see a dashed off photo of a piece of paper (that has obviously spent a week in someone's back pocket) with odd little squiggles and blotches to indicate some major (or magic) component. This is a simple and elegant statement; that will suffice to build the circuit with nothing else.
foxy1paco5 years ago
This is an extremely good idea if you have a roommate in your apartment/house/dorm. No more midnight booze stealing!! Plus, that's "my pie!"
Bardouv5 years ago
Oooohhh....that first pic of a fridge toatlly brings me back to my childhood when we lived in a trailer . I better not show this to my dad; I am notorious for midnight meals.