Circuits for magnetic pickups

Donald Whisnant dewhisna at ix.netcom.com
Wed Jul 31 15:05:36 GMT 1996


>> From: "Tony Bryant" <bryantt at psc.fp.co.nz>
>> Date: Wed, 31 Jul 1996 08:43:51 +1200
>> Subject: Circuits for magnetic pickups
>>
>> I'm using the following circuit for my inductive pickups:
>>
>>  <snip>
>>
>> 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?
>
> From: Gary W Harris <Gary_W_Harris at ccm.ch.intel.com>
> Date: Tue, 30 Jul 96 17:26:00 PDT
> Subject: Re: Circuits for magnetic pickups
>
> 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


Tony...

The LM1815 that Gary mentioned is good as well as Harris Semiconductor's
HIP9020AP... The Harris part was specifically designed as a Vehicle
Speed Sensor Buffer chip -- used to pickup the inductive pickup on the
output gear of the transmission ...  Plus it has an on-chip pulse
frequency doubler and some selectable dividers as well... Works with
100mv to 130V ...  And is about a $3 (US) part (I think -- as I got all
of mine as samples) ...  I've tried both the National LM1815 and Harris
HIP9020AP and both are very good for this application --- They take a
lot of heartache out of the design process...  Also, you may want to try
a filter cap on the signal.  The HIP9020 with the VSS sensor on the
transmission requires this! (I can tell you from experience!) -- I had
so much noise before putting on the cap that while starting the car (in
park of course), the ecm (as it was told by my circuit) thaught the car
was traveling at 60-65 MPH!.. This totally confused the computer -- as
it must have been there "saying" something like: we're starting the car,
the car isn't running, we are in park, we're traveling 60 MPH... hmmm
.... It didn't want to start right ...  Here is the front end I used
with the HIP9020 --- it works like a champ!! :


                     40K res
        Pickup  >----^^^----*-----> To HIP9020
                            |
                           ---  0.1uF rated at 250V
                           ---
                            |
                           Gnd
        Pickup  >--- Gnd

The resistor is needed as a current limitor and it together with
the capacitor form an integrator (i.e. filter) that will get the
noise out....   I mention this because the Harris datasheets vaguely
mention something about a capacitor, but none of their drawings show
it in there...

Good luck...
Donald Whisnant
dewhisna at ix.netcom.com



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