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Tachometer interface for old OMCs
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Circuit Description, First Half
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The first half of the circuit is primarily a filter and protection for the second half. The signal coming from the magneto is a negative rapidly decaying ring, starting at well over 100v and ringing at about 15KHz. D10 and D11 rectify the signal from the magneto while R1 and R25 prevent loading the magneto down and weakening spark. C1 filters out the high frequency component and lengthens the pulse for better compatibility with a wide range of tachometers. R5 pulls the line up to battery voltage (remember the signals from the magneto are negative). D9 protects against pin 6 going too high in the case of an arc-over or some such anomaly, or below 0v.
I have D11 and D10 as 1N4006, but really that's overkill. Really anything with about a 300V PIV would be just fine, and I think even 200 PIV would work. D9 is a zener marked 7-30v: anything between those two would be just fine, and 1/4w or so. Say 1N5242 (12v). All resistors should be 1/4w for durability. C1 can be polyester, ceramic or any other superior capacitor. It should not be a tantalum bypass cap. Working voltage should be at least 20v.
The value for the input resistors is a trade off. Higher resistances load the magneto less, but make the circuit more susceptible to noise. Experimentally, I determined that loading the magneto by 10K ohms had no discernible effect, and then I doubled that for the circuit.
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