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This is the light sensor circuit being used in the SCADA 2 project which is designed to output 5 volts at maximum brightness and 0 volts at total darkness. At the heart of the circuit is the LDR, a resistor whose resistance varies in proportion to the amount of light falling on it. The LDR here has a resistance of about 2 kW at maximum brightness, and a resistance of about 2 MW at total darkness. The 10 kW resistor above and below the LDR merely serves to limit the maximum current that passes through the LDR assuming that shorts occur somewhere in the circuit.
The coarse zeroing potentiometer serves to minimize the change in voltage going into the non-inverting input of the op amp. That is, the difference in voltage at maximum brightness and the voltage at total darkness is as small as possible. The 4 kW resistor connected to the non-inverting input of the op amp is the zero compensation resistor, which gets rid of leakage current.
The span potentiometer fixes the range of the output of the op amp to around 5 volts. The fine zeroing potentiometer is used to either increase or decrease both the minimum and maximum output voltage. The two 5 kW resistors connected to the 10kW buffer serve to divide the output voltage.
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