Bistable Light Alarm With the 4093 IC (ART197E)

The circuit is versatile and will work with almost any light-dependent resistor (LDR) or cadmium sulfide photocell such as the Radio Shack 27 6-1657 , with face diameters ranging from 1/8 to 1 inch.

The LDR and R1 form a potential divider that supplies gate drive to IC1-a.

When the circuit turns on, IC1-d is activated as an oscillator until the flip-flop formed by IC1-b and c is reset.

IC1-d acts as an audio oscillator, driving a piezoelectric transducer. If you need a more powerful audio output, you can use a power output stage with one or two transistors driving a loudspeaker, as shown in many other projects in this site.

A schematic diagram of the Bistable Light Alarm is shown in Fig. 1.

Figure 1 – Schematic diagram of the alarm

Transducer X1 is a piezo element such as the Radio Shack 27 3-07 3 or equivalent. You can also use as transducer a crystal earphone, but these elements will give less audio output than the original.

R1 adjusts sensitivity, and you can both use a potentiometer or trimmer pot to fit the intended application. Sensitivity is so high that the device can be turned on with light levels too small to be detected by the human eye.

Directional action is obtained by mounting the LDR into a opaque cardboard tube with a convergent lens. The LDR’s sensitive surface should be near the lens, focus. Sensitivity is also proportional to the lens diameter.

Changing this device into a dark-activated alarm is very easy: you simply transpose the LDR and R1 positions. The frequency of the au- dio signal can be altered by changing both C1 and R3.