CHAPTER 14 ■ VARIABLE RESISTORS
An easy way to identify a multiturn trimpot is to look for a metal screw offset to the side of the trimpot
(see Figure 14-4) instead of a plastic dial in the center (see Figure 14-2). Multiturn trimpots are more
complex internally, and therefore more expensive than single-turn trimpots.
You will want to make very fine adjustments to the line-follower’s sensor balance. Also, you don’t
want the value to change much due to vibrations as the robot moves. Therefore, a multiturn trimpot
(see Figure 14-3) would be a good choice for the line-following robot’s sensor balance.
Obtaining Assorted Trimpots
It’s worthwhile to have a variety of sizes and ranges of trimpots. Table 14-1 lists suppliers of assorted kits.
Table 14-1. Assorted Value Trimmer Potentiometers
Supplier Part Number Price Quantity Case Description
Electronix Express 32CSP2 $14.95 20 ¼ square
Digi-Key 3386C-KIT $65.95 75 Single turn (see Figure 14-2)
Digi-Key 3296W-KIT $139.00 75 Multiturn (see Figure 14-3)
Trimpots are also available individually or in bulk packets of the same resistance value. After
determining your favorite resistance values and dial style, consider stocking up. I like 500 W, 2 kW, 10 kW,
25 kW, 100 kW, and 1 MW.
If you only want a pair of 20 kilohm multiturn potentiometers used in the Sandwich robot, the part
numbers are: Solarbotics RT20k, Electronix Express 18MPT20K, Mouser Electronics 652-3296W-1-203, or
Digi-Key 490-2881.
Testing Potentiometers
The maximum resistance value of the potentiometer usually appears printed on the casing as a three-digit
code. The third digit is the number of zeros to add to the end of the first two digits. For example, “501” is 500 W.
“103” is 10,000 W. “254” is 250,000 W.
To measure a potentiometer, set up your multimeter the same way as you would to measure the
resistance of a normal fixed resistor. An important difference is a potentiometer has three leads, not two.
Measuring the Maximum Resistance of a Potentiometer
To determine the maximum resistance of any kind of potentiometer, connect the multimeter test probes to
the first and third leads (see Figure 14-5). This should be approximately the same as the value printed on the
potentiometer’s case.