Forrest M. Mims III: Fridge Alarm Project

By Forrest M. Mims III

Fridge Alarm Assembly Time: 1 hour
Skill Level: Intermediate

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 electronics DIY projects 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.

Fridge Alarm Electronics Component Kit Includes:

Transistor, 2N2222A
Buzzer, Piezo, 3400Hz, 90dB
Potentiometer, 3/8" Sq. Cermet, 10kΩ
9V Battery
Battery Holder, 9V snaps
Book, Electronic Sensor Circuits & Projects

Electronics DIY Project

How The Alarm Refrigerator 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 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

Circuit Figure 1. Circuit for the refrigerator alarm. The sensitivity of the circuit can be altered by adjusting trimmer potentiometer R2.

The photoresistor can be installed in either direction. Install it so the face of the photoresistor is about 1/2" above the board so you 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.

Testing the Circuit

Connect a fresh 9-volt battery to the connector clip. The PZ may or may not emit a tone. Use a small screwdriver to adjust R2 until PZ emits a tone.

Place a finger over the sensitive surface of R1, and the tone should stop. If not, switch off any nearby lights and adjust R2 until the tone stops. Exposing R1 to light should cause PZ to sound. The volume of the sound will increase with light intensity.

When the circuit is properly working, hold the battery upright with the positive terminal on your left. Place some double-sided tape on the side of the battery facing you. Press the battery against the circuit board with its terminals facing away from the components. Reconnect the battery if you previously disconnected it.

Place the fridge alarm inside your refrigerator. PZ should be emitting a tone when illuminated by the light inside the fridge. For best results, conceal the circuit behind a food product placed near the refrigerator light and orient the circuit so R1 faces toward the light. If necessary, adjust trimmer R2 to switch off the tone when R1 is dark. Close the door, and PZ should stop squealing. Open the door, and the alarm should sound.

The ultimate test is to place an enticing food or beverage in the fridge in the presence of the potential bandit and see what happens!

The circuit is switched on and off by connecting and removing the battery from the clip. The circuit will consume 5 to 10 mA from a fresh 9-volt battery when R2 is illuminated by bright light and around 3.5 mA when R2 is dark. It will work at reduced volume when the battery voltage falls to 4.5 volts.

Going Further

This circuit has other applications. For example, it's a great daylight alarm clock.

Add a pushbutton switch between the positive battery connection and the circuit, and it provides a convenient way for a blind person to know when unnecessary lights have been left on--or should be switched on when company arrives.

As noted above, keep in mind that all the parts can be exchanged. Just be sure that the current required by PZ doesn't exceed the limitations of Q1.

About Forrest M. Mims III