grnbrg's $10 (Or less!) rotor tach







Well, I'm new to RC helis, and have been wondering what headspeed my rotors are spinning at. Rotor tachs are available, and are in fact cheaper than I had originally thought at $25-$50, plus shipping. But I built this one for around $10 -- if you work at it, you can likely do it for less.

Updated February 2010


I recently saw this Youtube video:

For those interested in measuring the headspeed of their RC helicopters, I think this method is actually much safer than what I came up with. If nothing else, using the Spectrogram function of Audacity with the IR sensor below will be quicker and easier to read...

YMMV. And check the related video links for another by this author that shows you how to calculate the speed of a pass based on a doppler calculation.


The tach outlined here is an audio tachometer made from an IR emitter/phototransistor pair ($5 at Radio Shack) wired to a AA battery, with the phototransistor wired to a microphone jack (the other $5) and plugged into the microphone port of a PC or a laptop. As the blade (or any other object) moves past the sensor pair, the IR light from the emitter is reflected into the phototransistor, which allows current from the battery to flow into the microphone input. By (digitally) recording these pulses, a very accurate reading can be taken.

The following is a detailed set of instructions to build your own.





Build it!

These are the parts I purchased:
  • An IR emitter/phototransistor pair. These might be salvaged from an old TV or VCR remote. The emitter will be in the remote and the phototransistor in the TV. Some disassembly required. :)
  • An 1/8 inch mono phono plug. This might also be salvaged from an old pair of headphones.
The emitter and phototransistor need to be mounted on a handle that will allow you to safely get it close to the blades. I used a damaged rotor. Mark out where the holes for the leads will go, and drill them out.
The components look very similar to LEDs. (And the emitter actually is one.) However, because they are so close together, they'll interfere during operation, so some shielding is needed.
Cover the sides of the emitter and phototransistor with something to block the light produced during operation. I used two small pieces of heat-shrink tubing, but electrical tape would work as well. Be careful not to obscure the round tops of the components.
The emitter and phototransistor can now be placed in the handle.
Time to wire everything up. You'll need a way to attach a AA battery to the circuit. Note that the positve battery terminal is wired to the long leg of both components. Make sure the wire to the microphone jack is long enough to reach from your work space to your PC or laptop.

Tape the wires and battery to the handle and you're done!



Use it!





That's it! Enjoy your new tach!

If you found this project interesting, I've also got a instructions for making a paper pitch gauge.

These instructions are distributed under the Creative Commons Attribution-No Derivative Works 3.0  You may freely distribute and link to it, but may not change it, claim it as your own, or incorporate it in other tools.

If you find this tool useful, please consider sending $5 to the author via PayPal.  


Thanks for visiting! -- grnbrg@grnbrg.org