Chapter 4, Problem 75E

Use SPICE to analyze the circuit in Exercise 74 by doing the following. (a) Simulate the circuit for varying values of R_pot to balance the circuit at 500 lux, where R_light = 200 Ω. It is helpful to use a parameter sweep by defining a variable such as {potentiometer} (including the curly brackets) in the value for R_pot, and a SPICE directive such as .step param potentiometer 150 250 2 to step the variable from 150 to 250 in steps of 2. (b) If the resistance of the photoresistor decreases by 2% for for a light increase to 600 lux, use SPICE to find the resulting output voltage V_out.

74. A light-sensing circuit is in Fig. 4.90, including a resistor that changes value under illumination (photoresistor R_light) and a variable resistor (potentiometer R_pot). The circuit is in the Wheatstone bridge configuration such that a “balanced” condition results in V_out = 0 for a defined value of incident light and a corresponding value for R_light. (a) Derive an algebraic expression for V_out in terms of R_S, R₁, R₂, R_light, and R_pot. (b) Using the numerical values given in the circuit, calculate the value of R_pot required to balance the circuit at 500 lux, where R_light = 200 Ω. (c) If the resistance of the photoresistor decreases by 2% for a light increase to 600 lux (and assuming the resistance change with light is linear), what will the light level be if you measure V_out = 150 mV?

■ FIGURE 4.90