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From: "Tenho Hatonen"
Subject: Re: How do i measure signals without affecting them ?
X-Newsreader: Microsoft Outlook Express 6.00.2600.0000
Date: Wed, 20 Nov 2002 08:25:41 GMT
NNTP-Posting-Date: Wed, 20 Nov 2002 10:25:41 EET
Organization: Sonera corp Internet services
> > be at least 9.9Mohm. The reactive input/output impedances complicate the
> > calculations required a bit, but the principle remains the same: the
> > and load impedances must be known and the descision about the need for
> > additional buffering must be made according to the required accuracy.
> Maybe i should explain my problem a bit clearer. I want to build and
> connect a PIC based circuit to my cars main engine control harness and
> to log the signals that are available for each of the sensors. These
> sensors range from output from speed sensor, crank angle sensor, o2
> sensor, oil pressure sensor, etc.
> I am probably not able to obtain information as to each of the sensors
> impedance's but i still want to be able to connect my circuit to the
> harness without affecting the signal going to the cars engine control
> I have seen a few devices out there such as an Apexi RSM that can be
> used on a huge amount of different vehicles and im wondering how they
> manage to connect to the harness without affecting signals.
> Preferably i would like to do it in a simmilar universal manner so i
> can use my device on many vehicles safely as well. Is there a best way
> to do this ?
you have a very ambitious project! A couple of years ago I installed a
cruise control to my Peugeot 406, without any wiring diagram of the cars
electrics. To find a AC/pulse signal that is related to the speed of the
car or the engine rpm I used an oscilloscope. Finally I found a signal that
was usable, but the intersting point here was the following: the engine room
turned out to be electrically VERY noisy. The scope is a high input
impedance ( 10 Mohm, as usual) device, and picks all the disturbances
available and there are a lot of them.
In your project you probably need a high ( just a guess, 10 to 100 kohm?)
input impedance buffer to avoid loading the probe signals. To avoid the
disturbances, the screening of the PIC system must be leakproof. You might
also consider to have filtering in the input signals, whenever possible. If
you have a scope available, have a look on the signals you are interested
in, this will clearly reveal the nature of the signals and helps in
designing the input circuits.
Unfortunately I have no knowledge about the commercial things (like Apexi
RSM?). One more thing: there seems to be a connector (at least in my
Peugeot) in the engine room into which the engine analyser is connected,
may be it has buffered outputs that could be used?
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