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An LED Bargraph Optical Tachometer

An LED Bargraph Optical Tachometer

A tachometer is still just as useful. By performing RPM measurements from time to time, problems such as a bad cell in the battery, deteriorating connectors, or damaged motor windings can be detected early before there’s a major failure.
For the electric R/C enthusiast, a tachometer can be a very useful piece of equipment. When I first built this tach back in 1995, it was essential, as there were very few off-the-shelf electric R/C power systems that just worked. At that time, you had to experiment with batteries, speed controls or switches, connectors, and wiring, and a tachometer was a tool to help you measure the results.

Specifications

Scales: 0-4000, 0-8000, 0-16000, and 0-32000 rpm.
Resolution: 250, 500, 1000, and 2000 rpm.
Power: 9V alkaline battery or 4xAA 4.8V NiCd/NiMH battery.
Sensor: NPN photo-transistor.

This is the “raw” resolution. Each LED in the bargraph indicates an interval of this size. In practice (and with practice), the resolution is higher. When an RPM reading is taken that falls between two of the LEDs, both LEDs will flicker, and you can interpolate between the two values by the relative amount of time that each LED is on.

To use the tachometer, merely point it at the propeller from behind, looking through the propeller arc. If doing this indoors (electric models only of course), it helps to shine an incandescent light through the propeller from the front. Be sure to turn off all fluorescent lighting when using the tachometer, or it will give erratic readings. The best light source to use is a battery operated flashlight.

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