CHAPTER 14 ■ VARIABLE RESISTORSLike potentiometers, photoresistors are available in a variety of resistance ranges and physical sizes.
Larger photoresistors change resistance more slowly, but are generally more sensitive to the true lighting
conditions as they have a larger “eye” to experience more of the lightness or darkness.
Table 14-2 lists suppliers of assorted photoresistors. It’s much cheaper to buy an assorted bagful rather than
specific values. I highly recommend buying several Electronic Goldmine (http://www.goldmine-elec.com/)
bags, because of the variety and low price.
Table 14-2. Assorted Cadmium-Sulfide Photoresistors
Supplier Part Number Price Quantity Description
Electronic Goldmine G14025 $3.00 20 Assorted sizes, encapsulated and plain
Jameco 169578 $14.95 50 Small, plain only
Experiencing the Variable Resistance of a Photoresistor
Attach a photoresistor to your multimeter using the same method as measuring a fixed resistor
(see Figure 14-13). Watch the resistance change as you bring the photoresistor towards a light or cover it with
your hand. Really, this is worth trying.
Figure 14-13. Hook probes connected to a cadmium-sulfide photoresistor to measure varying values
Light-Controlled Circuit
Substitute a photoresistor for the potentiometer in the variable brightness LED circuit presented earlier in
this chapter (see Figure 14-7). See Figure 14-14 for a photograph of the revised circuit built on a breadboard.
As the room darkens, the LED also darkens. As the room brightens, the LED also brightens. Putting your
finger over the photoresistor also does the trick.