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Experiment with Light Dependent Resistor CircuitIn the dark, the resistance of the LDR is very high, typically around 1M ohm. In bright light it is low, typically 1K ohm. An example of the peak spectral response of the LDR (VT936G from EG&G) is 550nm. The continuous power dissipation is 80mW and the maximum voltage which can be applied to it is 100V. The snake like track on the face of the LDR is a cadmium sulphide (CdS) film. On each side is a metal film which is connected to the terminal leads. Use a multimeter to measure its resistance when light is shine on it and when it is placed in a dark place. This will help to enhance your knowledge of the concept as to how the LDR works. Simple Light Dependent Resistor Description The Light Dependent Resistor and a trimpot form a voltage divider which is used to apply bias to a transistor. As the LDR changes resistance the change in potential is detected by the circuit and the relay is activated. The PCB-mounted switch just interchanges the trimpot & the LDR as far as the detection circuit is concerned. So a dark activated switch becomes a light activated switch or vice versa. An LED with current limiting resistor is in parallel to the relay to give a visual indication of when the relay is turned on. The relay (Use a 5A/250VAC) can be connected to a light bulb and power supply which will light up when the environment is bright or vice versa. 
This circuit is satisfactory if the changes in light level to be detected are large and the transition is quick - for example, a person walking past a doorway. An inherent problem of the circuit is chattering of the relay for slowly changing light levels just at the transistion point between turning on/odd and vice versa. This leads to the relay chattering as it rapidly turns on/off. This problem can be overcome in by having a hysteresis circuit using an op-amp or a Schmidt Trigger.
Parts List The parts list of the LDR circuit is as shown below.
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