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Splitting signal from PWM based sensor

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vvanders

New Member
I'm looking into a project that will read the wheel speed & RPM from a motorcycle and use this data elsewhere as a form of telemetry. To do this I don't want to disrupt the function of the tach/speedometer so I have a couple questions:

1. I assume a microcontroller(PIC18 series) would be fast enough to read the hall effect sensor and output a PWM that matches that passed on to the speed/tach?
2. Is there a more tolerant way to implement this without a microcontroller inline to pass the signal along relatively unchanged while allowing me to sample from my microcontroller?
 

Nigel Goodwin

Super Moderator
Most Helpful Member
It doesn't need to be in line, just tap off the sensor and leave the sensor feeding the original circuit as it was. You may need to build a high input impedance preamp to do this, but probably not to be honest - try it and see.
 

vvanders

New Member
Cheers!

I'm still in the process of just figuring out how the whole thing will work. My DC theory is still really rusty(only on class in my college that really covered it). I had somehow figured that two lines pulling voltage from one sensor would cause a drop in voltage.
 

tcmtech

Banned
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In theory yes. But in reality a signal booster or isolator circuit can easily be built that runs at several megaohms input. Its actual load is not even measurable in real terms.
 

vvanders

New Member
So another quick followup question, what would be the best way to measure the PWM frequency I get from my sensor? I've done a bunch of googling and there seems to be a bunch of literature for Analogue Voltage -> PWM but not in the reverse except for fixed PWM Duty cycles. I'm worried that if I just use interrupts on the microcontroller it won't be fast enough to capture an accurate view of the PWM's frequency.
 

tcmtech

Banned
Most Helpful Member
You need one of these. its a LM2907 frequency to voltage converter IC.
I have used this with very good results. It just goes by the frequency of the input signal and nothing else. It works well withevery type of signal I have tried it with.
As long as you meet the minimum voltage requirements for the input signal you should have no problem getting a good clean referance voltage out of it.
 

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Ubergeek63

Well-Known Member
why do you want it analog? speed and RPM are not PWM signals anyhow, only frequencies.

normally telemetry implies a digital format and all you need to do is to use the micro to measure the frequencies. this is normally done by averaging the a number of readings of the amount of time from one rise to the next and inverting (1/x).

capture registers can latch their counters on the rising edge and interrupt the processor. as long as the cpu can do all it needs to and still respond to the interrupt before the next one comes all is right with the world - or your measurement at least.
 
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