Thanks for the reply.It appears that what you have is a fairly standard magnetic impulse sensor, generating an AC signal. As already mentioned I wouldn't be terribly worried about using a 1KΩ sensor, you can simply add an 820Ω resistor in series if absolutely necessary. The important consideration is the number of pulses per mile, and the signal level. You will need to determine what is the lowest signal voltage that the speedometer will work at and the peak voltage at max road speed.
A magnetic impulse crankshaft sensor or abs sensor will do the job, or if you are fussy about the voltage level, a 5V Hall type sensor (often used for camshaft sensors) should work with a voltage level spot in the middle of your desired range. It will have the added complication of requiring a 5V supply , which can be easily obtained, and it will have a square wave output. I would be surprised if the speedometer is fussy whether it is a square wave or AC pulse.
Before you start, you will need to accurately determine the number of pulses per mile. By far the best option is to use a signal generator to drive the speedometer and graph frequency V's speedometer reading. If you don't have access to one you can easily make one with an arduino.
. Your Tyres are 205/65-15 which turn 792 times per mile. Your top speed is 140MPH = so your tyre turns at max speed are 30.8 times per second.
Going by the max figure you quoted in your original post of 400HZ, you will need to generate 13 pulses per wheel rotation.(30.8x 13 = 400.4) which is accurate to 0.1%
This is very easily achieved by making a new split collar in two halves,the same diameter and approx weight as the existing one on the OE driveshaft, with 13 evenly spaced teeth attached to it. It is then a simple matter of mounting the sensor on a bracket so that it is pulsed as each tooth passes.
I do have one question about this circuit. The input signal shown is a constant amplitude of +/- 300 V the output is a constant amplitude of +10V. If the input amplitude were to periodically change as mine does, would the output remain the same?The zener diode would (a) need to connect in parallel with the F-to-V circuit, not in series as shown above, and (b) be normally reverse biased. Here's a pessimistic simulated setup with and without a zener diode present:
View attachment 175530
This setup cuts off negative peaks, but I doubt that will worry the speedo circuit. One concern with this simple circuit is whether the action of the diode in effectively shorting the sensor at high revs could cause undue heating of the sensor coil.
The green trace shows the power dissipation in the zener diode. Average power is 165mW with peaks of 1.5W. Your mileage may vary. I'd be looking for a 2W diode (plenty listed in 1-off quantities at Digikey etc) and expect it to get moderately warm at high revs. If your sensor output under load is less than in the sim then a lower rated diode will fit the bill and sensor over-heating is less of a concern.
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