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Old 20th September 2008, 10:50 PM   #1
diyAudio Member
Join Date: Nov 2002
Default updating Tachometer circuit?

I was wondering if anyone has any ideas as to how I could update this 40+ year old tachometer circuit and use silicone transistors instead of the hard to find germanium ones. Any other ideas would be welcome as well.

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Old 21st September 2008, 12:21 AM   #2
Join Date: Jan 2008
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Does the circuit work? If it does, leave it alone.

If it doesn't try building one with AC128 germanium transistors, you can get these, it looks about right to me, although somebody else might know better.

These old auto circuits are sometimes a bit nadgery, I can't be sure without a bit more work, maybe you'd get away with substituting a 1A 30+V PNP silicon transistor but maybe the other values would need tweaking, the path of least resistance is just to try the germanium ones.


You could lash up a couple circuits to try this out on veroboard in a few minutes if you have on hand or get in the components, and it won't cost much in the scheme of things if you're restoring an old car. You want a Zener that will pass an Amp. Oh, the thermistor. I'll think about that...
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Old 21st September 2008, 01:24 PM   #3
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Join Date: Nov 2002
Thanks for the reply. I thought maybe there might be a monostable multivibrator, or one shot that was available in a IC.
I would still like to update the circuit if possible with a new board and parts.
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Old 22nd September 2008, 06:43 AM   #4
dangus is online now dangus  Canada
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Join Date: May 2004
Location: Vancouver Island
National Semiconductor has a chip that's ideal for tachometers and dwell meters: LM2907 or LM2917
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