Frequency to voltage conversion

[Simon Morden]

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:
http://www.national.com/mpf/LM/LM2917.html
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<T
because I'm trying to just get the voltage spikes +/-. What I really
can't do is make what on the face of it should be a really simple
circuit to convert the pulsed dc train (at 10-100 Hz) into a pseudo ac
voltage with a large enough variation to fool the chip.

Any help gratefully received!

Simon
--
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[Simon Morden]

Jon Slaughter wrote:

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.

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
--
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

 

[Jamie]

Simon Morden wrote:

Jon Slaughter wrote:


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.



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
A pull up resistor is needed most likely.

http://webpages.charter.net/jamie_5"

 

[Simon Morden]

Jamie wrote:

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

A pull up resistor is needed most likely.


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

 

[Simon Morden]

Simon Morden wrote:

Jamie wrote:



A pull up resistor is needed most likely.


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

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