Here are the circuits of light dark sensors using CA3140 IC. CA3140 is an op amp IC that is used in voltage comparator mode in these circuits. In this mode the IC compares the voltage difference between pin2 and pin3 and give high and low output at its pin6 according to the voltage difference.
The figures below show two different circuits of light dark sensors built around the CA3140 IC. The figure 1 is a light sensor switch circuit that will activate the relay switch when light falls on the surface of the LDR and the figure 2 is a dark sensor switch circuit so it will activate the relay switch when there is small amount of light or no light on the surface of the LDR. Both the circuits are using 10K variable resistor to adjust the sensitivity.
You can connect your AC and DC appliances with the relay switch to operate them with these light and dark sensors. The operating voltages of the circuits are 9 to 12 volt DC.
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Figure 1.
Figure 2.
