CHAPTER 14 ■ VARIABLE RESISTORS
Remember when you had to choose a resistor size to compromise between LED brightness and battery
life? With a photoresistor, the circuit can automatically brighten the LED so that it’s visible in daylight, but
conserve battery power in dim areas where the LED is perfectly noticeable.
Photoresistors are commonly found in automatic nightlights and streetlights. In these applications, the
photoresistors are hooked up to circuitry that turns on a bulb when the ambient lighting gets dark.
Balanced Brightness-Sensing Circuit
Sandwich, the line-following robot, uses four cadmium-sulfide photoresistors in pairs to detect the amount
of light at front of the robot. Figure 14-15 is an exact schematic of the robot’s brightness-sensing circuit.
Figure 14-14. Light-controlled variable-brightness LED circuit built on a solderless breadboard