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So it seems that RPM is done purely magnetical, not by hall effect.
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I agree, it seems like it might be easier that way.
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The 7812CT is a TO220 12V-1A max voltage regulator. The 2N3904 is a small signal NPN transistor, probably for buffering the opamp output signal and triggering the relay.
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That's what I thought, it's been a while since I did anything at the component level, but those parts looked real familiar
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The CA3140 is a mosfet opamp and probably works as a comparator between a fixed (user settable) DC level and the integrated pulses from the sensor.
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That was the part I didn't recognize, though that is about what I was guessing for a function - something had to be doing it and I didn't see any other suspects! Thanks for confirming my suspicion.
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Best thing to do is to try to reconstitute the schematic following all the traces and components on the circuitry and drawing it down on a piece of paper, and then try to understand it (with the help of the datasheets of the components). I suspect a usable RPM signal (AC pulses from the sensor) to be available just before the opamp, and before the integrator (which is probably just a (rectifier) diode, a serial resistor and a parallel cap). All you should have to do with the AC pulses is rectify them with a diode, put them through a schmitt trigger to obtain clean square wave pulses, and then feed them to an open collector transistor.
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Agreed, that is what I'm planning when I have a chance to sit down and play with it. I just hope the board isn't multi-layer. It doesn't look it, but when I hold it up to a light, I see shadows that I can't account for.
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Ofcourse a scope (even // port based or LCD based) would be usefull.
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Agreed. I have a cheapo single trace, no real calibration, low frequency scope that I got when I was in ET school, I just need to get it out of storage.
Gooserider