While experimenting with the light sensors yesterday we have created an ultra sensitive light sensor switch circuit. This little gem can detect a very small amount of light in complete darkness from far away distance or even a small amount of moonlight. It is an ideal circuit to use when you
need more sensitivity than a normal light sensor circuit, and can be used for wide variety of applications.
The circuit has three sensitivity levels low, medium, and high from which you can select your desired level. A relay switch is used at the output of the circuit from which you can connect any AC or DC device to operate with this sensor.
The heart of the circuit is a CA3140 opamp IC that is used in comparator mode here. The three levels of sensitivity are built by using three resistors, which are 1M, 10M and 20M or two 10M resistors. A 22uF electrolytic capacitor is used to save the relay from chattering and a 1N4007 diode across the relay is used to protect the IC from the back EMF of relay. The operating voltage of the circuit is 9 to 12V DC.
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