How to Convert Light into Electricity with Simple Operational Circuits
Here’s your chance to convert light into electricity using simple operational circuits. You can apply a similar approach to develop instruments that measure other physical variables in the environment, such as temperature and pressure.
You use an input transducer to turn a physical variable into an electrical variable. A photoresistor is an input transducer that converts light energy into a change in resistance, resulting in a change in the current flowing in the circuit. The light is, in fact, an electrical signal.
Suppose you’re dealing with a photoresistor that has a resistance value between 20 MW in total darkness and 20 kW in bright light. If the photoresistor is a linear device, then doubling the amount of light doubles the amount of voltage. You can therefore model a photoresistor as a variable resistor that changes resistance according to the amount of light.
The following figure shows a photoresistor and a complete design of an operationalamplifier (opamp) circuit to produce an output voltage v_{O}.
Use circuit analysis to show that the opamp output voltage v_{O} is 0 volts in total darkness and 5 volts in bright light. In other words, show that the voltage range of the output varies from 0 to 5 volts. Here’s how:

Determine the output voltage v_{2} from the transducer.
To determine the range of the output v_{2} from the transducer (that is, the voltage across Terminals A and B), you can use the voltage divider equation. This equation sets the output voltage equal to the input voltage multiplied by the ratio of the resistance of the output device (R_{2}) to the total series resistance (R_{1} + R_{2}):

Determine the lower bound of v_{2}.
The lower bound of the output voltage v_{2} occurs in bright light, when the photoresistor’s resistance is at a minimum. When R_{2} = 20 kW, the lower voltage v_{2}_{L} is

Determine the upper bound of v_{2}.
The upper bound of the output voltage v_{2} occurs when the photoresistor’s resistance is highest. In total darkness, R_{2} = 20 MW = 20,000 kW , so the upper voltage v_{2}_{U} is
The voltage of v_{2 }ranges from 5 to 10 volts.

Simplify the transducer (the source circuit) using the Thévenin technique.
The Thévenin technique reduces a source circuit to one single resistor R_{T} and one single voltage source v_{T}. By using the Thévenin equivalent to simplify the transducer, you get a Thévenin voltage v_{T} that varies from 5 to 10 volts and a Thévenin resistance R_{T} that varies from 10 kW to 20 kW.

Analyze the opamp circuit (inverting summer).
The opamp circuit in the following figure is a typical configuration of an inverting opamp summer circuit. In this circuit, you have two inputs: one coming from the transducer and another coming from a voltage source of –10 volts. The equation for the inverting operational amplifier is
Because the voltage range of v_{2 }varies between 5 and 10 volts, the output voltage range of the inverting summer goes from 0 to 5 volts.