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Measuring VSS (SOLVED)

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truhlik_fredy:
Hi guys.
I want to measure VSS (Vehicle Speed Sensor) signal from my car. It's analog device, just magnet and coil and it is going to another analog device (speedometer). And I'm in situation where it gets more complicated just because I want to have it digital.

I'm working now with Mega16 (but if it's necessary I intend to upgrade to Mega644) and I have 1 external IRQ free, 1 or 2 Timers (and their inputs) and some ADC should be free as well. The sensor is designed to make 5khz signal when going 3600kmh :) so with 90khm speed signal should be 125Hz. But problem is with low speeds the signal amplitude is very weak like 50mV - 100mV and I would like not to miss them. I don't want go with car 30kmh just to force it that I'm moving at all.

What you think guys would be the best approach? Amplify it, filtering spikes with zener and then sample it all the time with ADC? Somehow when I expected that the signal will be strong enough and only I had to do is just filter over voltages and I will be grand with 1 or 4 passive components.

For any advice thanks in advance. Anton.

PS: I recorded the waveform, it's here (in description on youtube is explanation for the spikes)

PS2: I didn't post it into AVR micros section, because I think it's irrelevant. If I will be able to make digital signal from that analog it won't matter to what I will feed the signal.

Zero999:
It's pretty easy to make an amplifier and Schmitt trigger from CMOS logic gates.

truhlik_fredy:

--- Quote from: Hero999 on February 17, 2011, 09:14:43 pm ---CMOS logic gates.

--- End quote ---

But the 50mV - 100mV signal will triger the cmos logic levels?

Zero999:
Not normally but I've biased the first stage as a linear amplifier with plenty of gain which is fed into the Schmitt trigger with Vdd*0.1V of hysteresis.

truhlik_fredy:

--- Quote from: Hero999 on February 17, 2011, 10:21:47 pm ---Not normally but I've biased the first stage as a linear amplifier with plenty of gain which is fed into the Schmitt trigger with Vdd*0.1V of hysteresis.

--- End quote ---

It looks like I'm too stupid for this :) I thought when you put output of NOT to input it will start oscilating.

So I can just get cmos NOT gates and I should be OK?

Do I need to put there some protection? Like zener diode. And some current limiter resistor? (it needs to go into instrument cluster without any change like minimal curent and voltage diference, so my device needs to be transparent to the car, if you know what I mean)

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