LDR I, a cadmium sulphide (CDS) photoresistive cell is used as the lower leg of a voltage divider between VCC and ground. The timer terminals 2 and 6 are connected to the junction of the photocell and SENSITIVITY control Rl. The resistance of the photoresistive cell varies inversely as the light intensity; resistance is high when the illumination level is low; low in bright light. (The Radio Shack CDS cell 276-116 has a typically wide resistance range-about 3 megohms in darkness and 100 ohms in bright light.).
When the light is interrupted or falls below a level set by SENSITIVITY control Rl, the rise in LDR1’s resistance causes the voltage on pins 2 and 6 to rise. If the control is set so the voltage rises above 2/3 VCC, the relay pulls in. The relay drops out when the light level increases and the drop across the photocell falls below %VCC. (The circuit can be modified by placing relay K1 and diode D1 between pin 3 and ground.In this case, the relay drops out when the voltage on pins 2 and 6 rises above 36 VCC. and pulls in when it falls below 1/3 VCC. This modincation is valuable when the relay has single-throw contacts.) Opening and closing of the relay contacts occurs at different illumination levels. This 1/3 VCC hysteresis is an advantage that prevents the circuit from hunting and the relay from chattering when there are very small changes in illumination.
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