Tachometer electronics question (general)

Paul Moir

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Nov 5, 2002
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I've got an old magneto type outboard that I want to hook up a tach to. I would like to use a modern gauge but since they don't work with magneto ignition, clearly I'll have to build some sort of electronic interface.<br /><br />I don't know much about tach's signal requirements and I'm hoping someone out there does. Specifically, my questions are:<br />How important is the signal's amplitude?<br />Does the duty cycle (length of the pulse) matter? If so, what % do I need?<br />Does the wave shape matter? Square would simplify things.<br /><br />I'm thinking that it would be much easier to start with an I/O type tach rather than an outboard one, since (I understand) it is expecting 0 - +10v pulses and can be set for 2 pulses per revolution. Am I correct in thinking that?<br /><br />Thanks, I appreciate any answerers, comments or opinions!
 

gaugeguy

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Jun 4, 2003
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Re: Tachometer electronics question (general)

As far as signal requirements go, a square wave of about 6V in amplitude will drive any tach on the market. % of duty cycle doesn't really matter, but 50% is ideal.<br /><br />Wave shape doesn't really matter as ignition pulses are sawtooth shaped with extremely high amplitude and the tach conditions the wave shape to a square wave.
 

Paul Moir

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Re: Tachometer electronics question (general)

Thank you very, very much!<br /><br />I should have mentioned, my mag pulses are exceedingly short (maybe 5% or less at idle). I'll strech them out to bring it closer to ideal.<br /><br />Thanks again - that's exactly the stuff I needed to know!
 
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