Quote:
Originally Posted by fdanielson
it seems that you may be making this more complicated than needed. How about using a 555 timer in a monostable circuit as a Schmidt trigger?
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A several reasons....
1) The magnetic pickup produces a TINY signal. About 500mV p-p at idle, dropping down to something under 250mV at higher revs. I don't have enough space to build the amplifier I need to boost the signal to trigger something like a 555.
2) I hate RC timing components. I never have the right values around, and they drift all over the place with age and temperature. I'd much rather just change some software code to adjust timing.
3) I also get the flexibility to do some interesting things. I believe I'll have enough code space to build in a self-test. When you switch the key to the glow position, the amp will execute a self test routine that sweeps the tach needle to 5k RPM and holds it there for 1 second. It then drops to 2.5K RPM and holds for 1 second, finally sweeping to 0, at which point normal operation commences. I like the visual indication that everything is working. Makes me sleep better.
Plus, my microcontroller is cheaper than an industrial temperature range 555.....