ten cent tach - RC Groups
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Nov 06, 2012, 10:12 AM
RC beginner

ten cent tach

after minor issues finding good motors for my diy quad thread (https://www.rcgroups.com/forums/show....php?t=1710948) i decided to resurrect an old method for measuring rpm. it depends on the fact that leds are actually photovoltaic devices. basically small solar cells that can function as optical tachometer. normally i use an ohm meter to get an estimate of kv but it dont always tell exactly how the prop/pager combo will perform. the thrust stand in my indoor micro thread is obviously most accurate but not as convenient to set up as this simple led tach. and that scale dont cost 10 cents either.

i was talking about the led tach this weekend and somebody wanted to see pics. if its of any interest i can go into more detail but lets start out with the photo. ive added a couple scope traces from my tektronix 210 but you can use one of those free pc soundcard scope programs. the 1st trace shows a cheapo aliexpress motor that worked out pretty good but had .8mm shaft which required tricks to fit 1mm prop hole. the other one is of replacement motors he sent that had 1mm shaft as advertised but much weaker. looks like 1.2ms per blade for 23k rpm vs 1.9ms at 16k rpm. i was surprised a small difference in rpms made such a big difference in flight ability.
Last edited by dave1993; Nov 06, 2012 at 01:24 PM. Reason: scope traces
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Nov 06, 2012, 01:30 PM
Registered User
Zen09's Avatar
Nov 06, 2012, 02:03 PM
RC beginner
oh MANNNNNN! (bart voice) and i thought this was my "bright" idea only. lol! thanks for that link. those programs seem perfect for this application:


i havent tried either one yet but they look to be much better than a simple soundcard scope.

btw the problems you had there may have been related to the ir sensor used. ir led was the first type i tried because they have far more output than visible ones (longer the wavelength, higher the voltage). and it worked great on the black props i started with. then a problem cropped up when switching to orange udi prop. NO SIGNAL AT ALL! turns out ir passes right through those like they arent there. switching to green solved that problem but signal amplitude was very low for reasons just mentioned. finally settled on plain ol' red which happen to be cheapest too. also note that only clear lens, not diffuse, will work.
Nov 07, 2012, 01:33 AM
Registered User
Zen09's Avatar
The DYNO program is much much better, it can also calculate the torque and power curve for a motor by deriving the rpms (taking into account the moment of inertia of a flywheel).
It doesn't show live rpm though

The revcounter does show live data, but it was initially developed as a frequency counter (and later modified by my suggestion) and is quite limited.
Nov 07, 2012, 10:27 AM
Registered User
23K rpm is over a 40% increase from 16K RPM - that's not a "small difference" -
Nov 30, 2012, 04:52 PM
RC beginner
yes, but the difference in lift and controllability seems much greater. maybe 2 or 3 times more so idk.
Dec 04, 2012, 01:47 PM
Registered User
midelic's Avatar
Of course because the relation between lift and speed is not linear ..it is quadratic.
It sounds better "exponential".
In fact the lift increases with the square of speed(rpm).For example if the speed doubles the lift increases 4 times.
Last edited by midelic; Dec 05, 2012 at 02:42 PM.
Dec 05, 2012, 01:34 PM
Registered User
Zen09's Avatar
It's not exponential either, but the second power of rpm, though I know what you meant.

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