The trouble with timer based tachs is they are only correct at one point in time. It
takes time to count pulses. There are two ways of doing it. Feed in a fixed freq into
a counter and read it every rev. The number of counts are inversely proportional to
RPM. The second way is count the ring gear teeth for a given time period. Same
problem. The count is inversly proportional to RPM. High RPM few counts. Low RPM more
counts. Consequently the resolution varies.

An analog tach on the other hand is always up to date only with a slight delay.
So I decided to put one on the new EFI board and read it with the analog to digital
converter. This is a revolutionary idea and you will see it more and more as time
goes on used on microcomputers. Another invention of mine like the hardware PW
generators. It is a major simplification of the software. No need convert inverse
numbers. The resolution is always 10 bits depending on the A to D. There is very
little processor over head as there is no need to mess with timers. Timers are a set
up nightmare. This simple circuit works so well it blew me away. It puts out zero to
five volts as a linear function of RPM up to 7500 RPM. I was stunned.

The CMOS 555 is an amazing circuit. There is no end to what can be done with a CMOS 555.


Paul Lamar
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