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# Long description

This is a circuit drawn in Multisim Live. On the left, it shows a Wheatstone bridge with a light-dependent resistor (LDR). The resistances are as follows:

top left, R1 = 680 Ω

bottom left, R2 = 1000 Ω (this is the LDR)

top right, R3 = 100 kΩ

bottom right, R4 = 100 kΩ.

An op-amp is connected to the terminals of the Wheatstone bridge. The connection node with the LDR (between R1 and R2) says ‘4.5 V or more when dark’, while the connection node with resistors (between R3 and R4) says ‘4.5 V reference voltage’. So the voltage that will vary is on the branch containing the LDR. The op-amp is powered by two constant voltage sources of 9 V each. They are arranged as shown in Figure 16. The output of the op-amp is connected in series with an LED (which appears to be lit) and a resistor with label R5 = 470 Ω. The resistor is connected to ground.