FORREST M. MIMS III: FRIDGE ALARM KIT | JAMECO PART NO. 2158717
VISIT WWW.JAMECO.COM/FRIDGEALARM FOR COMPLETE KIT BUILD
Experience Level: Beginner | Time Required: 30 min
Build this simple alarm circuit to catch midnight snackers 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.
How It Works
The circuit for the fridge alarm is shown in Fig. 1. It’s a basic lightactivated switch that activated a piezo tone generator. When the
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.
Prepare the Board and Install the Components
Compare your included parts with the parts list and make sure you have all of the components
before you begin.
1) Photoresistor
The photoresistor can be installed in either direction. Install it so the face of the photoresistor is
about ½” above the board so you can have room to adjust it. The sensitivity can be adjusted
with the potentiometer, so it doesn’t have to be perfect.
2) Potentiometer
Line up the pins with the pads on the board. The shape should also match the silkscreen on the
PCB.
3) Transistor
The transistor Q1 can only be installed in one direction. There is a tab on the case of the
transistor to indicate the location of pin 1. The tab should also match the silkscreen image for
the transistor.
4) Buzzer
There is a sticker covering the opening of the buzzer and it also has a plus sign (+) to indicate
the positive side. Install the buzzer on the PCB making sure the positive pin of the buzzer goes
through square pad on the PCB.
5) Battery Holder
Solder the red wire of the battery holder to the positive (+) pad on the PCB. Solder the black
wire to the negative (-) pad. Use a couple screws and nuts to fasten the battery holder to the
PCB.
Try It Out
You may find that right when you plug in the battery, the buzzer will start to squeal. To calibrate
it, you’ll want to completely cover the photocell to block all light going to it. You could also do
this in a dark room, but it makes it hard to see the tiny tuning screw on the potentiometer. Using
a small flat head screwdriver, turn the screw on the pot until the buzzer stops making noise.
Return the sensor to the light and the buzzer should sound again. The fridge is now safe.
VISIT WWW.JAMECO.COM/FRIDGEALARM FOR COMPLETE KIT BUILD