Peter L. Peres email (remove spam text)
On Wed, 28 Oct 1998, Andy Kunz wrote:
> >circles of patterns, one for each desired speed. If you figured out how
> to make
> >the patterns, you could print up anything you like on your laser or inkjet.
> Some more info on the problem might help...
> I'm looking at an 1/8" dia shaft with a 1.25" propeller on the end,
> rotating between 1K and 60K RPM.
Why didn't you say so ;) Is it going to be wet ? If so, a twin plastic
optical fiber arrangement with an IR laser and a common IR photodiode will
likely help you a lot. Prop blades are reflective enough for this in my
experience and the flexible twin fibers can be taped to a convenient place
for temporary use ;). You can probably use an IR led if pulsed fast. In
this case some trickery is required to get rid of the beat frequency. PICs
are as good as made for this. The input amplifier for the photodiode needs
to be a better one, probably a faster FET op-amp with the detector wired
as current generator (connect polarized photodiode to - input). 60,000 rpm
is only 1000 rot/sec or 4000 Hz for a 4-blade prop (not likely, more like
2 or 3 blades). If the LED is pulsed with 1 Amp pulses at 30 kHz as for IR
remote applications it should work ok.
The audio tach can be of two kinds: The one where you hear the speed as a
tone and the one where a tone is to be displayed as speed. The 1st method
is easy, connect an audio amp to the IR circuit described above. With 3
blades you get 50 Hz for 1k and 3000 Hz for 60,000. The 2nd method
involves a microphone and a Schmitt Trigger amplifier that supplies a
frequency to a counter. If the amplifier has some AGC, it will lock on the
loudest frequency around (such as, a nitro engine running full throttle
near it). This last method works very well as I can confirm ;)
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