CHAPTER 14 ■ VARIABLE RESISTORS
When dialed all the way to one extreme, the resistance between the middle pin and either the first
or the third pin should be 0 W. When dialed all the way to the other extreme, the resistance should be the
maximum value printed on the potentiometer’s case.
Linear Versus Logarithmic/Exponential
Turn the potentiometer dial to the middle. Is the measured resistance approximately half the maximum
value? If it is, then the potentiometer is probably a linear taper. This means that the resistance changes at a
steady rate as you turn the dial. That’s desirable for most robotic applications.
If the resistance value halfway through the dial is much more or much less than half the maximum
value, then the potentiometer is probably a logarithmic or exponential taper. That’s less desirable for most
robotic applications. Logarithmic taper potentiometers are often used in stereos to increase the volume in
multiples to reflect the perceptions of human hearing.
Variable Brightness LED Circuit
The schematic in Figure 14-7 is very similar to the LED power indicator schematic shown earlier in the book.
The battery (B1) and the LED (D1) stay the same.
Figure 14-7. Schematic of a simple circuit to vary the brightness of an LED
However, a potentiometer (R2) now appears between the fixed resistor (R1) and the LED (D1). Notice
that the schematic symbol for a potentiometer looks somewhat like a fixed resistor, except there’s an arrow
in the middle. You can imagine this arrow sliding back and forth on the resistor, jumping in at some value
in between.
Also noteworthy is that the letter “R” is assigned to the potentiometer (R2) because it is in the resistor
family, even though it has a variable resistance. Recall that “D” is assigned to an LED (D1) because it is in the
diode family, even though it is a light-emitting variation.
Since you’ve got a potentiometer whose resistance spans a broad range, why bother to include a fixed
resistor (R1)? After all, a 100 kW potentiometer provides all the usable values of 470 W and up.