Audioguru once mentioned this circut, which wd generate a variable square wave output to emulate the negative ignition wire of a car engine, hence making testing of RPM meters and shift lights much easier.

Can anybody point me in the direction of a good circuit that would generate the optimum frequencies that could be generated by a car?

I actually came accross one here.. is this allright?

http://www.electronic-circuits-diagr...rsimages/1.gif

Thanks!

 

Hi Indecided,
How's your shift-light circuit? Have you ignored it and over-rev'd your engine yet? :lol:

Your 555 circuit has way too much range. It will go from about 200Hz all the way to about 100KHz. Its pot will be so sensitive that it will be impossible to set a frequency.
Sorry, I can't remember what range you need. Do you have a 18,000 RPM formula-1 engine or a little putt-putt?
First of all increase the value of the 1K resistor. It mainly sets the highest frequency.
Secondly, add power supply bypassing right at the IC, like most ICs need.
Lastly, add a 100 ohm or 1K resistor in series with its output, since a 555 is powerful enough to destroy itself and a lower-voltage load.

I pasted together parts of its datasheet for you:

 

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Audioguru! my LM2907 is going well, extremely well in fact.. made a couple for my friends and they all love it.

about the 555, i'd like the range to be able to test from say, 1000RPM to 10000RPM... which would be in 4-cylinder terms, about...let me check.

 

I would probably need the frequency range to go from 33Hz - 333Hz

based on 4-stroke frequency calculation : (RPM * 4) / 120