I was not using the front shaft of the motor, so I decided to read the RPM from that shaft. I painted the shaft in light and dark bands: Two light and two dark. The light bands will reflect IR light and the dark bands will not.

I made the mounting bracket from aluminum left over from the previous work on the truck. The box is a project enclosure from Radio Shack. This photo shows the Radio Shack Infrared Emitter and Detector mounted in a box with an aluminum separator.

About 100 mAmps drive the emitter LED. The phototransistor's collector connects to +12. A 10k resistor connects the emitter to ground. The voltage at the emitter changes from 0v to 12v.

The voltage comparator processes this signal. The comparator's output activates the base of an NPN transistor. As the comparator changes from open to closed, the voltage at the NPN's collector will change from 0 to 12v. This square voltage wave is the output signal. It is important to note that there are two pulses per revolution.

Here is a photo of the system in place

Update: July 2010

I reworked most of the wiring this past few weeks. I had some problems with the display and decided to make some modifications to the system. The output from the RPM sensor incorporates two transistors in a push-pull formation. It is capable of much higher current.