PHOTO_ALARM

Published:2009/6/30 23:47:00 Author:May | From:SeekIC

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 R1. 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 R1, 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 2/3 VCC, the relay pulls in. The relay drops out when the light level increases and the drop across the photocell falls below 2/3 VCC. (The circuit can be modified by placing relay K1 and diode Dl between pin 3 and ground. In this case, the relay drops out when the voltage on pins 2 and 6 rises above 2A Vgg, and pulls in when it falls below 1/3 VCC . This modification 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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