My Wooden Airplane
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Most tachometers (especially those in cars from the 60's, like my Corvair) are connected to the negative side of the ignition coil. My engine has two ignition coils, which is convenient, because the Dynon has two tachometer inputs (right and left magnetos, for an airplane engine). Unfortunately, William Wynne strongly discourages connecting anything directly to any of the ignition components. He contends that a failure of a sensor that is connected directly to the ignition can cause the ignition itself to fail. While it is probably not very likely that the Dynon would fail in such a way as to take down the entire ignition system, I decided to play it safe. William recommends a tachometer sensor like the ones most often used for diesel engines. These sensors are NOT connected to the ignition (since there is no ignition in a diesel engine), but instead, are rigged to count pulses of some sort. Some sensors count fuel injection pulses, some count gear teeth, and some count the passes of a rotating magnet. William recommends a sensor that is set up to count gear teeth, like the ones on the starter ring gear. Unfortunately, there are 135 teeth on the ring gear, and the Dynon can only count up to 8 pulses per revolution, so I couldn't do it that way.

I ended up buying a Cherry GS100502 hall-effect sensor from Digi-Key. I made a mount for it that attaches to root of the alternator bracket. I attached four 7/16 x 3/4 bolts to holes that already existed in the ring gear. I adjusted the sensor so that it just touched the bolts, then backed it off a half-turn. There are three wires for the sensor. The brown wire goes to +12V. The blue wire goes to ground. The black wire is the signal output, and goes to the left tachometer input on the Dynon. There is also a pull-up resistor between the brown and the black wires. (It was supposed to be 2.4K, but all I had was 2.7K, so that's what I used.)

Total Time: 4.0 hrs.