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Old 04-18-2013, 02:38 AM
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Need help in electronic speedometers and compators

I have a project on " Car Speed Alarm System " to present next week at the end. I am using an LM319 as comparator, a DC voltage representing my speed limit or reference, an old electronic speedometer together with an odometer, but I need just the speedometer. I want to simulate the car speed( detected by sensors ) that I have to compare with the reference in term of voltage,in the Lab using a generator to generate some kind of pulses which to send to the speedometer and also to be used signal to be compared with the reference( using its amplitude). But I am not very sure and am confused because of the nature of the pulses signal. So I want to ask some questions:

1. Am I in righ way to do that comparison?
2. What is exactely the kind (nature) of signal sent to the electronic speedometer and coming from sensors?
3. Can I compare that signal with the DC reference at the comparator or I have to use a signal conditioning?
4. Can I have a universal waring diagram of an electronc speedometer?
5. How exactely an electronic speedometer works? Many reading so far do not give me a better way to understand

Thank you in advance for your help.

Kind regards

Kabuya Kanyinda

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Old 04-18-2013, 02:59 AM
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Need help in electronic speedometers and compators

I forgot one thing:

The electronic speedometer I am using is from a car mark Ford Mondeo S771 JRV, year 1998. Thanks.

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Old 04-18-2013, 05:31 PM
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speedometers use 4000 pulses per mile, or 8000 pulses. Amplitude plays a small role as it needs to be sufficient to provide a signal that will drive the circuit beyond its static state . The amplitude of the signal the speedo sees is not linear with speed in any form.
Most circuitry has a clipper to "tame " the amplitude so the circuit can handle the signal with out overdriving it (overmodualtion) or applying overvoltage.
I dont get to mess with a lot of speedos and info can be hard to find .
Many times the signal is "handled" by another module like the engine control module, then handed off to the speedometer . Some times it is to convert a signal to a harmonic frequency (usually double) or to boost amplitude to drive the speedo , but only to satisfy circuit bias and not for analog measurment of speed.
Some of the GM systems have a "buffer" which I suspect is an amplifier to raise voltage/ current enough that the signal can be shared by 2 or three speedo, cruise control...PCM strategy etc. But it is the frequency that all the devices use for speed input
Not sure if this helps.Good luck
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