by Forrest M. Mims III


The Mystery Component is a Small DC Motor.

While watching his son do wheelies with his radio-controlled car, Bob Wheels realized that small DC motors are very tough gadgets. They also generate a voltage having a polarity dependent on the direction of rotation. So he built a test circuit to determine if an inexpensive motor could be used as a rotation detector for the video game he was designing.


How It Works

Figure 2 shows the circuit with the missing component displayed. Both LEDs glow when the motor's armature is still or when it is rotated in a clockwise direction. When the armature is rotated in a counter-clockwise direction, the green LED switches off and only the red LED glows.



Figure 2. The mystery component in Figure 1 is a DC Motor.


Background

My first project that used a small DC motor to detect rotation was an airspeed indicator for a homemade wind tunnel. Details are here: An $11 Wind Tunnel Design, Model Rocketry, July 1970, pp.13-15.

Going Further

Any general purpose, single-supply op amp should work in this circuit. Different effects can be had by connecting a small capacitor between pin 3 of the op amp and the motor.

About the Author

Forrest M. Mims III has been an electronics and science writer and photographer since 1969. He received an IR 100 Award from Industrial Research Magazine for developing an eyeglass-mounted infrared travel aid for the blind. He received a Rolex Award in 1993 for developing a handheld instrument that measured the ozone layer accurately enough to find an error in a NASA ozone satellite. He was named by Discover Magazine (December 2008) as one of the "50 Best Brains in Science." See his home page and publications here and follow him at Twitter.