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R/C Heli Optical Tach Project

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Although commercial R/C helicopter tachs are expensive, I was not trying to find a low cost way to obtain one. Rather, I was just interested in seeing if I could build one.

There are different ways to tach the heli's main blades. But, the most useful (to me) are the optical designs that allow you to measure RPMs from a distance of several dozen meters.

These gadgets use a motor driven shutter that the operator manipulates until the two blades appear to stand still. By looking in a view finder that is interrupted by a spinning hole, the heli blades can be "frozen" in time. It is essentially a strobe that uses natural light.

I used a surplus LCD display and tiny 40mm PC CPU fan. Both parts came from All Electronics. The LCD was $4.50 and the fan was $2.50. The LCD is used to show the measurement and the fan is used as a shutter wheel.

A PIC 16F872 microcontroller is used to control fan motor speed and manages the LCD readout. The two upper left buttons are for Up/Down RPM control. The software was written in C.

Photo 1, the completed board (note that it is a fairly small board and could be made smaller):


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The shutter view is through a large hole in the board that can be seen at the lower right edge of the LCD (see photo #1 above).

There is a stiff paper disk glued to the fan hub. A small wedge cutout acts as the flying shutter. By looking through the top of the board, through the PCB hole, then through the spinning disk shutter, the strobe magic occurs.

The disk cutout also is used as the index for a photo sensor that reads the motor RPM. I originally tried to use the special motor speed output of the fan (a popular feature on PC fans), but it had too much signal jitter for my tastes. So, after a lot of effort, I gave up and added a optical sensor (photo transistor and detector).

When I first tried to use the tach outdoors the sunlight interfered with the IR sensor. The fix was to use some black tape and light-proof the fan housing.

From the backside (and with fan removed), the shutter view port is seen near the three colorful wires. This photo was taken before I decided to add the photo detector. See photo below.


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The RPM range is set up for 1000 to 3500 RPM. The resolution is 10 RPM. Accuracy was checked against a frequency counter, so it should be close enough for my tastes. I expect it is within a dozen RPM's or so.

The design has low battery alert. If it is time to change the 9V battery, the LCD will flash a warning message, and the piezo speaker occassionaly beeps. Pressing both buttons will reveal the battery voltage too.

To conserve power it enters a low power standby state after a couple minutes of inactivity. Pressing the Up/Down buttons wakes it back up. A 9V Alakline battery should last a long time with this scheme.

It works great on model Heli's. From over 100' I tach'd one without any trouble at all. All the pilot has to do is hover in one position, for several seconds, while a tach operator takes the measurement.

I was also able to tach my SlowStick airplane (which swings a big geared prop). I just tied the model down on the bench, walked several feet away, and conveniently measured the prop speed.

Overall, the project cost me about $35 and a few million hours of my time. Now I know why they cost $130 in the hobby shop. ;)

Below is a photo of the Tach in operation.


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