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Tachometer

Olflyr

Mar 24, 2022
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I have a schematic for a tachometer using a 555 timer. I am going to use it on an old engine that runs the tach off a 4 lobe cam and points that are not part of an ignition system. I need to adjust it to full scale on a 1 ma meter at 3000 rpm. Can anybody tell me what values I need for the resistors and capacitors? I have attached the schematic
 

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crutschow

May 7, 2021
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How fast does the 4 lobe cam run?
Is it the same as the crankshaft speed or 1/2 speed?
 

crutschow

May 7, 2021
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Here's an LTspice simulation of the circuit, tweaked for your requirements.
As shown, the cam (lobe) output is 200pps (5ms period) for an engine speed of 3,000 RPM.
The average meter current is 1.02mA (small window on left) for a U2 pot setting of 27.5kΩ.

upload_2022-3-24_15-12-28.png
 

Olflyr

Mar 24, 2022
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Thank you very much. I sort of understand how this stuff works but my knowledge isn't very deep.
 

Olflyr

Mar 24, 2022
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I got the wiring done today and it sort of works. the meter only has a small variation from idle to full throttle. It only changes about 400 rpm. What do I need to do to increase the sensitivity?
 

Alec_t

Jul 7, 2015
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The schematic in post #1 has an error.
The wire from pin 2 should connect to both R2 and R3. (I assume the wire gap between R2 and R3 represents a bridge over the pin 2 wire).
 

Olflyr

Mar 24, 2022
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Oops! looks like I left that part of the circuit out. Thanks for waking me up. Duh!
I hope I'm just inattentive and not getting senile.
 

crutschow

May 7, 2021
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the meter only has a small variation from idle to full throttle
The circuit I posted should be quite linear with RPM.
The simulation shows an average of 1mA @ 3000 RPM and 0.1mA @ 300 RPM.
 

Olflyr

Mar 24, 2022
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I must have done something wrong. I put in the part I had omitted and the scale is slightly larger but as I increase rpm it will go up and then fall back. Sometimes it pegs at zero. I think I take it apart and do the whole thing over.
 

Olflyr

Mar 24, 2022
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Do you have an oscilloscope to view the waveforms?
I have one but I haven't had it hooked up in years. I'll dig it out and see what I can learn. Probably take a few days. Also, should pin 5 on the timer be hooked to anything? My schematic shows it unconnected,
 
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crutschow

May 7, 2021
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should pin 5 on the timer be hooked to anything? My schematic shows it unconnected,
You can connect a 100nF capacitor from pin 5 to ground to filter the trigger resistor network, but it's likely not needed in this application.
 

Olflyr

Mar 24, 2022
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Thanks. You could take all I don't know about this and write a pretty good training text.
 

Bluejets

Oct 5, 2014
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Don't know why you don't simply do some study....the necessary link was provided earlier.
 
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