The expanses of WolfWings' land
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August 28th, 2004
August 28th, 2004
August 28th, 2004
August 28th, 2004
August 28th, 2004

[User Picture]06:00 am - Nosing about web-pages...
...due to a discussion I had with a friend about digital car readouts.

Am wondering why everyone seems to use such complicated methods of doing digital tachometers based on wierd, velocity-measuring circuits that seem overly complex... why not just run one cable into the engine compartment, and use an inductive probe to sample one spark-plug wire? Then just hook that 'pulse' up to a delay-circuit such that the first 'pulse' after each timed interval latches the display, the next resets the count. If you want to be really 'precise' and have a smoothly-changing display instead of a once-a-second update, have 10 counters, and 'time out' each one once a second, set out of phase with each other by 1/10th of a second. As long as you adjust the counter circuit based on the number of cylinders in the engine, the approach would work, and be very simple to construct as far as I can tell.

Each counter would require four 555 chips set up for base-10 counting to handle up to 9999 rpms, a static flip-flop bank to 'store' the currently-displayed number, and a four-digit base-10-to-7-segment-LED decoder to actually display the stored number. The 'time out' circuit would be another 555 chip. The only 'complexity' would be making the cyclic-phased reset for 10 of those, but even that's trivial. Just set up a 1/10th of a second 'trigger' that cycles another 555 chip as a base-10 counter, triggering each of the 'time out' sub-circuits in order.2 commentsLeave a comment
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