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.

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.

Step 3: Prepare the Board and Install the Components...

The circuit was assembled on a solderless breadboard and tested. When the circuit was operating properly, the components were transferred to a perforated prototype board (Jameco 616622).

You can follow your own parts layout or copy the layout I used that is shown in Fig. 3.

1. You can trim the perforated board now or after the components are soldered in place. The prototype board was cut along row 49, and the cut edge was filed smooth.

2. You will need to make a hole in the board for the battery clip leads. The hole for the prototype was made at hole O52 by carefully twisting an X-Acto knife through the hole until its diameter was enlarged to 1/8-inch (3 mm). Of course you can also use a drill.

3. Insert PZ between holes F58 (+) and C53. The leads of the PZ are thick, so gentle pressure may be required.

4. Insert Q1 into holes L57 (collector), K58 (base) and J57 (emitter--indicated by a protruding tab). Bend the leads slightly outward to hold Q1 in place.

5. Insert R1 in either direction between O57 and O54. Bend the leads slightly outward to hold R1 in place flush against the board.

6. Insert R2 into holes L52, K54 (center terminal) and J52. Bend the leads slightly outward to keep R2 from falling from the board.

While your soldering iron is warming up, check your work to make sure the parts are properly arranged on the board.

Step 4: Solder the Connections...

After the components are installed, turn the board over and solder the component leads together according to the circuit diagram in Fig 1.

Or refer to Fig. 4 and follow these steps:

1. Bend the positive PZ pin (F58) and the emitter lead of Q1 (J57) toward and alongside one another. Heat the junction of the two wires for a few seconds and then apply solder.

2. Bend the collector lead of Q1 (L57) toward the nearest lead from R1 (O57). Wrap the Q1 lead half way around the lead from R1, press the end of the Q1 lead flat against the board and solder in place.

3. Connect a length of hookup or wrapping wire between the minus PZ pin (C53) and the closest terminal from R2 (J52). Wrapping wire is the simplest solution to this connection.

4. Solder the wire to the minus PZ pin.

5. Bend the center terminal from R2 (K54) toward the terminal just connected to PZ. Push both terminals against the board and bond them and the connection wire together with solder.

6. Push the second lead from R1 (O54) toward and around the third lead from R2 (L52) and then toward and around the base lead from Q1 (Q58). Solder both connections.

7. Turn the board over and insert the leads from the 9-volt battery clip through the enlarged hole that you made at O52.

8. Flip the board over to the back side and tie a knot in the wire leads (see Fig. 4), being sure there is ample space to connect the clip to the battery when the battery is placed on the board with its terminals opposite the circuit components. The knot will keep the battery clip's leads from breaking loose should the battery fall away from the circuit board.

9. Wrap the bare wire at the end of the black battery clip lead (-) around the minus pin of PX (C53) and solder in place and Wrap the bare wire at the end of the red battery clip lead (+) around the R1 lead at O57.

10. Protect your eyes (safety glasses are best) and clip off the excess lead lengths from the components.

Step 5: Test 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 frig 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.

Step 6: 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.

Another great from Forrest M Mims III www.forrestmims.com.