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Understanding Signal Conditioning

Thread starter #1
I am embarking on building an electronics project for a old classic vehicle of mines. I want to build a RPM meter.

I have however found myself stuck at the very start.

The signal to be used is provided by the vehicles coil -ve "low tension" terminal. This terminal is ideally suppose to go 12v, 0 ,12V ,0 ... etc as the points open and close for each cylinder fire.

However due to the high voltages generated by the coil the responce on the negative low tension circuit can experiance short large voltages impulses and resonance.

I want to design a circuit that would condition this signal to something manageble without spikes and resonance. just smooth 12V 0V or 5V 0V etc.

Can somebody please point me in the correct direction? My objective is to obtain a stable input signal isolated from the harsh voltage changes in the coil.

Thank you
 
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MikeMl

Well-Known Member
Most Helpful Member
#2
Thread starter #3
Interesting,

Do you feel the zener coupled with the circuitory following, in your design, should provide adaquate protection?
and would you maybe suggest any further protection that could be implemented?

Could you please help me understand your design especially after the zener, is that cap resistor, resistor combination some sort of filter?

Thank you
 

MikeMl

Well-Known Member
Most Helpful Member
#4
R3, C1 and the cable capacitance is the RC filter. There is no Zener. R3 and D1 clamp negative excursions to about -0.65V. R3, R2 and the Vbe of Q1 clamp the positive excursions to ~0.65V. What is left is a 12V logic signal that reflects the dwell of the points. If you want a 5V logic signal, just return R1 to 5V instead of 12V.

Circuit repeated here for convenience; explanation in other thread.

 
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