Photo alarm

Posted on Feb 7, 2013

LDR1, 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 LDRl's resistance causes the voltage on pins 2 and 6 to rise. If the control is set so the voltage rises above 2A Vcc, the relay pulls in. The relay drops out when the light level increases and the drop across the photocell Ms below lA Vcc.

Photo alarm
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(The circuit can be modified by placing relay K1 and diodeD1 between pin 3 and ground. In this case, the relay drops out when the voltage on pins 2 and 6 rises above V% Vcc, and pulls in when it falls below Vi Vcc. This modification is valuable when the relay has single-throw contacts.) Opening and dosing of the relay contacts occurs at different illumination levels. This J/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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