Circuits for magnetic pickups

[Donald Whisnant]

>From: Sandy <sganz at westworld.com>
>Date: Fri, 02 Aug 1996 21:16:04 -0700
>Subject: Re: Circuits for magnetic pickups
>
>Tim and others...
>
>I was thinking much of the same idea, but was afraid that it was way too
>simple. I guess that I have to go out and get a sensor, and a wheel on a car
>and see how bad the noise really is. I'm guessing that it is really bad in
>some cases, but if I can rid my design of the lm1815, that would be very
>good. The circuit that you used on the Direct Ignition looked good, but
>again looked like it would be susceptible to noise. 
>
>  <snip>

Actually the noise really isn't bad at all....  I've got a 120v power 
inverter and have actually put my oscilloscope in the car (had someone else
drive :) and looked at the signal under real-time operating conditions.
The key factors I found were: be sure to twist the wires from the sensor
together, and a simple integrator type filter is more than adequate to remove
all noise -- even running it single-ended.  ...  The LM1815 and HIP9020
both have zero cross detection which eliminates a lot of noise problems
that you'll find in op-amp setups (this probably accounts for my "clean"
signal -- as I didn't try it with just op-amps) ...  So, you may want to
look in some data books for zero-cross detectors -- I seem to recall a
circuit that employed several diodes in the feedback loop of an op-amp to do
zero cross detection.  ... ...  As for "voltage" limiting, the key factor
actually is reducing current flow (for example, the HIP9020 requires a
40K resistor in series with the input lead) --- A well designed voltage
divider ladder with resistors but choosing the correct size --- (i.e.
a 1k and 2k in series gives a 0.66666 ratio and so does 100k and 200k --
the current flow in the 100k/200k is a lot less than the 1k/2k ladder)...
But, all in all, I believe you'll find it simpler just to use either the
LM1815 or HIP9020 -- it will definitely be cheaper in the long run (from
both time and parts), unless you are pressed for board space...  

Donald Whisnant
dewhisna at ix.netcom.com