rpm signal from cigarette lighter?
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rpm signal from cigarette lighter?
I saw this SHIFT LAMP CONTROL UNIT BLACK W/RED LED HYUNDAI SCION:eBay Motors (item 120429796394 end time Jul-03-09 12:46:19 PDT) and wonder exactly how it works?
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Not very well.
Seriously, though. It senses the electrical noise produced by the alternator. Since the alternator generates AC internally and then rectifies it to DC through a simple diode stack, the current waveform coming out of it has a fair bit of ripple. If viewed on an oscilloscope, you'd see that it's not a nice, smooth 14 volts, but rather that it has a high-frequency noise component that varies in frequency as to the speed at which the alternator is turning.
By sensing this ripple, you can derive an imprecise approximation of engine RPM. There is no absolute relationship of the noise frequency to engine RPM, as it varies with the pulley ratio, which is why the device is adjusted by a couple of analog potentiometers rather than something scaled in exact RPM. If methodically adjusted, it will probably be accurate to +/- a couple hundred RPM.
Seriously, though. It senses the electrical noise produced by the alternator. Since the alternator generates AC internally and then rectifies it to DC through a simple diode stack, the current waveform coming out of it has a fair bit of ripple. If viewed on an oscilloscope, you'd see that it's not a nice, smooth 14 volts, but rather that it has a high-frequency noise component that varies in frequency as to the speed at which the alternator is turning.
By sensing this ripple, you can derive an imprecise approximation of engine RPM. There is no absolute relationship of the noise frequency to engine RPM, as it varies with the pulley ratio, which is why the device is adjusted by a couple of analog potentiometers rather than something scaled in exact RPM. If methodically adjusted, it will probably be accurate to +/- a couple hundred RPM.
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