CHAPTER 17 ■ DC MOTORS
However, you can measure even tens of thousands of RPM very accurately by a device called a
tachometer. For short, it’s often called a “tach,” which is pronounced “tack.”
Most cars have tachometer displays built into the dashboard. The gauge usually goes from 0 to 8, with
numbers over 6 in the red portion of the dial. Multiply the number by 1000 to get the RPM. Therefore, the
gauge is actually displaying 0 RPM to 8000 RPM.
“Contact” tachometers are physically connected to the motor shaft, which can reduce speed, thus
providing an inaccurate reading. “Non-contact” tachometers usually employ brightness sensors to detect
rotations. Some non-contact tachometers, like those placed on car hoods during emissions testing, take
advantage of the expected period of vibrations.
Figure 17-17 shows a DC brush motor running at 4038 RPM being measured by a homemade
tachometer. The motor spins a half-black and half-white disc attached to the motor shaft. A motionless
image of the disc is pictured at the far right of Figure 17-16.
The tachometer illuminates the rotating disc with an LED. The tachometer sees the disc change from
light to dark by the amount of light reflected into a brightness sensor. A chip on the tachometer counts
the amount of time it takes the disc to complete a light-to-dark-to-light transition (one rotation). This
measurement permits the tachometer to mathematically determine the speed.
Professional handheld laser tachometers can be purchased for as low a price as $100.
Mounting Putty Aids Motor Experimentation
Poster tack or mounting putty is a rubbery clay-like substance. This reusable putty is reasonably sticky yet it
rarely leaves residue and it doesn’t dry out. It’s primarily used for cleanly attaching posters to walls.
I find the putty to be ideal for holding motors upright while experimenting. The putty also quickly
affixes the testing disc to any motor shaft. You may be able to spot that I used a bit of putty for both purposes
on the top and the bottom of the motor in Figure 17-18. Of course, the putty is too soft for permanent
mounting in a robot.
Figure 17-17. Homemade tachometer measuring a DC brush motor