piclist 2002\10\02\114315a >
Thread: slow sine wave freq counter
face BY : Wagner Lipnharski email (remove spam text)

From 2 to 10mVpp is not difficult to use any comparator, even the old
LM339, but this sensor output being magnetic pickup can be tricky, any
slower rotation of the magnet can drop down the pickup coil output so much
that the comparator can start to miss pulses. Anyhow, at this voltage level
you should take all possible extra care for noise, using shielded cable or
at least twisted pair.

To avoid most noise, the best deal would be to use a differential
amplification away from the Comparator power rails (Ground or VCC).

The R100 and 100nF cap offer a better filtered power to the bridge.

The Trimpot R2k2 should be adjusted to offer few +microVolts to the (-)
comparator input, so it will keep its output low when no signal from
pickup.  It would also compensate the lack of precision of the R100k
resistors and some possible temperature variations. Adjust the R2k2 for the
worst case, so it should work fine in all other situations.

This should not be a problem, but you "may" try to find the best polarity
for the pickup installation on the bridge.  In one way, the comparator will
detect the incoming magnet, the other way it will detect the magnet going
away.  As the comparator will be powered by pure 5V (no double supply), you
will have a single positive output when the magnet would be passing by.

Some pickup coils have a magnetized nucleus, sometimes for those, you get a
stronger signal when certain magnetic polarity alignment is done with the
moving magnet or piece of metal (metal gear or dent in a wheel).

 .-------o----------.         +5V
 |       |          |          o
 |     R100k      R100k        |
 |       |          |      |-_ |
_|_      o---COIL---o----(+)  '-_
---      |          |      |     '.___ out
 |100nF  o---------------(-)   _-'
 |       |          |      |_-'|
 |     R100k      R100k    '   |
 |       |          |          |  Comparator
 |     R2k2<--.     |         _|_
 |       |    |     |         GND

You can try also this combination, but I will not bet much on it:

 .-------o             +5V
 |       |              o
 |     R100k            |
 |       |          |-_ |
_|_      o---------(-)  '-_
---    COIL         |     '.___ out
 |100nF  o---------(+)   _-'
 |       |          |_-'|
 |     R100k        '   |
 |       |              |  Comparator
 '-------o             _|_
         |             GND

In this one, you can change the value of the pair R100k to change
sensitivity, but no very low values, since in this circuit you are forcing
a dc current through the coil.

Wagner Lipnharski - email:  KILLspamwagnerspamustr.net
UST Research Inc. - Development Director
http://www.ustr.net - Orlando Florida 32837
Licensed Consultant Atmel AVR _/_/_/_/_/_/

Mike P. wrote:
{Quote hidden}

Wagner Lipnharski - UST Research Inc
Orlando FLorida - USA - http://www.ustr.net
/_/_/_/ Atmel AVR Consultant /_/_/_/

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