Circuits for magnetic pickups

[Gary W Harris]

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You might also look at using the National LM1815 Adaptive Variable Reluctance 
Sensor Amplifier chip.  It is inexpensive (well, $5.70 from DigiKey) and 
operates over an input voltage range of 100mV to 120V!  Check it out!

Gary

I'm using the following circuit for my inductive pickups:

LM3900 norton op amp,
       10K
+5v---/\/\/\---<pickup>---(inverting)
+5v---/\/\/\------+-------(noninverting)  (output)
      10K         |                        |
                  \---/\/\/\---------------/
                       470K

seems to work relatively well, but it suffers from the occasional
extra pulse (noise I assume).

The requirements I had when designing this circuit were:
1) circuit is stable, when pickup is not picking up (i.e. it needs
hysteresis)
2) I can pickup both approaching & receeding teeth (i.e the output
should be a squared up version of the pickup current)
3) 5V only supply

My question is what are other people using?
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