The expanses of WolfWings' land
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August 31st, 2004
August 31st, 2004
August 31st, 2004
August 31st, 2004
August 31st, 2004

[User Picture]10:52 am - Further ruminations about that digital tachometer...
...and some corrections as well. Namely, I kept saying '555' chips, when I was mixing up my components. What I'd actually need numerous of would be 74LS390 (or possibly 74HCT390) chips, with only one 555 chip as a clock source of 0.125sec.

The only other chips needed would be 4 74LS247 chips to drive the 4-digit display, along with 8 74LS74 chips to serve as 'buffers' for the 16 bits of 'data' I'm displaying in the 4-digit displays, and 16 74LS151 chips to select between the 8 'counters' to read from, one chip per bit of data transferred.

So, in total, I'd end up needing 16 74HCT390 chips, 4 74LS247 chips, 8 74LS74 chips, 16 74LS151 chips, one '555' chip to build the clock-generator from, and a small custom-crafted inductive-monitoring circuit to monitor all of the individual spark-plug lines. And the tachometer would update 8 times a second.

Total price for the chips themselves ends up being around $10-$20, and that's ordering them at the '1 at a time' price. And then I realize... this logic could easilly be replicated in a single $5 FPGA chip from XiLinx or similair.

So... over 40 chips, taking up a huge amount of space on a circuit board, and $20 just for the chips themselves... or a single, ultra-compact circuit board with nothing but 1 lead coming off of it to tap onto a single spark-plug wire with a little plastic clip-loop, for around $20 for the complete circuit board, mounting box, display, and wiring harness along with sensor lead...

Yeah, the 74xxxx series is officially just for mental prototyping now, FPGA really _is_ the god of prototyping. :-)3 commentsLeave a comment

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