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.

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!

You'll need a piece of software on your PC that is able to record sound, and let you zoom in on the resulting wave form, and accurately measure the time and/or number of samples between arbitrary points.
Although it does far more than we'll need of it, Audacity is free audio editor available for Windows, Mac and Linux. Download and install it, unless you have another piece of software that will do what is needed.
Get Audacity ready to record, preferably with as high a sample rate as possible. The higher your sample rate, the more accurate your measurement will be.
Spool up your heli to the speed you want to measure, hit "Record" on the PC, and hold the tach over the spinning blades so that the sensor end is just over the blades. Note that overhead lights will cause minor interference in the reading. Measuring in a dimmed environment, and with the sensor facing down will minimise this.
Once you've taken a few seconds read, shut down the heli, and stop the recording.
Zoom in on the wave you've just captured, and you'll see distinct peaks where the rotor tip has passed under the sensor. Measure the number of samples between the start of one peak, past the second, and just before the start of a third. This measurement represents the number of samples taken over one full rotor rotation. See: for an example.
Once you know how many samples a full rotation takes, you can calculate the speed of the rotor in RPM. For example, the measure above indicates that a full rotation took 2,223 samples, and the sample rate was 96,000 Hz. Therefore, each rotation of the rotor takes (2,223 / 96,000) or 0.02316 seconds. To convert this to RPM, divide 60 by this result. ie: (60 / 0.02316) is 2591.09 RPM.

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