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Re: Convert 6 cyl tacho for use with 4 cyl
In fact, my donor tacho is a veeery basic Ford unit, circa 1985 I
suspect. But is has a nice 6,000 RPM scale. The "electronics" are just
resistors, capacitors and diodes, with what looks like a fairly sensitive
270 degree moving coil movement (ie. *not* sine/cosine moving magnet).
Inside there is a 220k pot that may have enough adjustment anyway, but I'm
going to make up a CMOS circuit from scratch because I can't imagine the
original circuit could have been too accurate!
I recall that one of my first "TTL" projects was a tacho circuit using a
7400 that drove a 1 mA meter (90 degree movement) from a school's science
lab - I used this on a Hillman IMP - some years ago now!
BTW, http://www.national.com/pf/LM/LM1819.html indicates the LM1819 is
now an obsolete part.
Andy Whittaker - Chester UK wrote:
> Obviously you need to divide by one and a half.
You're looking at it from the wrong angle - I'd divide by one and a half if I
had a 4 cylinder tacho being driven from a 6 cylinder vehicle. Here's what I
said originally -
logically, the tacho requires 3 pulses for every
two pulses the coil generates.
So, I have to multiply by one and a half because there are 2/3 as
many spark plugs (and hence pulses) on a 4 cyl compared with a 6 cyl.
Peter
PS. to multiply by 2/3, You need to clean up the input waveform and then
differentiate it, then...
> Use an 'edge-triggered' binary up/down counter set to count-up. This will
> effectively multiply the input square-wave by two. Then, connect an AND gate
> between bits 0 and 1 of the counter, this will divide by 3. This should then
> give you the output you want.
I think you have to feed back the AND gate's output to the counter's CLEAR
so when the count gets to binary 11 the counter goes back to the 00 state.
The counter counts 00, 01, 10 -> 00, etc. - the CLEAR signal is the 2/3 signal
you want.
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