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Use SPICE to analyze the circuit in Exercise 74 by doing the following. (a) Simulate the circuit for varying values of Rpot to balance the circuit at 500 lux, where Rlight = 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 Rpot, 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 Vout.
74. A light-sensing circuit is in Fig. 4.90, including a resistor that changes value under illumination (photoresistor Rlight) and a variable resistor (potentiometer Rpot). The circuit is in the Wheatstone bridge configuration such that a “balanced” condition results in Vout = 0 for a defined value of incident light and a corresponding value for Rlight. (a) Derive an algebraic expression for Vout in terms of RS, R1, R2, Rlight, and Rpot. (b) Using the numerical values given in the circuit, calculate the value of Rpot required to balance the circuit at 500 lux, where Rlight = 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 Vout = 150 mV?
■ FIGURE 4.90
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