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Frequency to voltage conversion

I'm trying to remember my limited 20-year old electronics knowledge and failing miserably...

Some background: the school I work at is involved in a science competition run by Rolls Royce. Our project is to make a wind tunnel and test turbine designs. I am in charge of the instrumentation.

The problem: I need to be able to record the rpm of the test pieces.

What I've done so far: I have availed myself of the National Semiconductors' LM2917 (14-pin), as detailed here:

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It's a dedicated frequency to voltage IC, and I'm running it off a 9v battery. I've set it up so that it works - I can input an ac voltage and get a proportional dc output.

What I can't do: I can't make my rpm detector talk to the chip. The detector set up is a bike computer sensor and a 3v battery. Output is pulsed dc which does not fall below ground - and the chip requires that pin 1 goes at least -30mV on each cycle.

What I can't do (part2): I know I have to use something like capacitor coupling to strip out the dc and keep the ac. I know I have to keep RC

Reply to
Simon Morden
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Read the 2nd paragraph on page 7. You need to set up the chip to use dc.

It has a comparator that can be setup to be ground(or actually a bit above ground) and it says something about hysteresis in their too.

Optionally you could shift the voltage of the pulsed dc down by using a negative supply or possibily shifting the ic chip up. e.g., put a diode on the ground pin or use an active resistor(if the ic doesn't pull enough current for the diode to turn "on"). The output signals will all be shifted up of course but you can unshift them. At least this should work in theory ;)

Reply to
Jon Slaughter

You star. My only excuse (and I'd read that document *and* the AN-162 applications pdf) is that I'm a geologist by trade, and only picked up the physics I needed...

I now have the sensor and the chip talking to each other, with only one more problem to solve - it only works when I have a voltmeter between pin 1 and ground (I was measuring V(in) so I could balance the V(bias)). I have a feeling a M-ohm resistor will do the trick, but not sure why.

But I've stopped the night - I can't see the numbers on my capacitors anymore.

Simon

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Reply to
Simon Morden

Well, try it and see ;) If it works then it works ;) I'm not sure what is going on though. Maybe adding just enough capacitance to the circuit or something else? It could be acting as a pulldown to ground which is needed if the input ends up floating for some reason.

Reply to
Jon Slaughter

A pull up resistor is needed most likely.

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Reply to
Jamie

I will experiment. My sensor (and thus V(in) is working at +3v, my f-to-v IC at +9v. Would a pull-down on V(in) be better? Or would I be better off trying to run the sensor at +9v too?

Simon

--
Visit the *all new* Book of Morden (www.bookofmorden.co.uk)
"I haven\'t had that much fun with a novel for a while." - Bookbag
The Lost Art - from David Fickling Books
Reply to
Simon Morden

Just to reply to myself. Everything is now working perfectly (I used a

2k pulldown - it's probably too large, but I need to go to the shops again anyway).

The sensor is still on the +3v, with a bias (controlled by a variable pot) of half that. I actually feel like I almost understand what's going on in the circuit, and can make adjustments accordingly.

Thanks again for everyone's help.

Simon

--
Visit the *all new* Book of Morden (www.bookofmorden.co.uk)
"I haven\'t had that much fun with a novel for a while." - Bookbag
The Lost Art - from David Fickling Books
Reply to
Simon Morden

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