CHAPTER 14 ■ VARIABLE RESISTORSUnfortunately, the user has the ability to turn the potentiometer all the way down to no resistance
(0 W). Without a minimum value enforced by the fixed resistor (R1), the LED would be connected to the full
strength of the battery and would be destroyed. Unless you’re designing a self-destruct button, you generally
want to guard against extreme ranges on adjustable controls.
■ Tip Whenever you install a potentiometer, ask yourself what would happen if it were dialed to zero
and what would happen if it were dialed to the maximum. Consider the voltages and currents under those
extremes, and then test the design with a multimeter to be sure they’re within the values that the surrounding
components can tolerate.
Building the Variable Brightness LED Circuit
Make sure power is disabled on the solderless breadboard. Never connect or disconnect parts to a live
(powered on) system.
The variable brightness LED circuit (see Figure 14-7) is easy to build on a breadboard. See Figure 14-8
for a photograph of an actual example.
Figure 14-8. Variable brightness LED circuit built on a solderless breadboard
Note that the fixed resistor (R1) goes into the same 5-position group as the center lead of the trimpot (R2).
The first lead of the trimpot (R2) is connected to the LED (D1), but the third lead is not connected to anything.