Error free (or very close) optical tach output

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SETEC_Astronomy
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Error free (or very close) optical tach output

Post by SETEC_Astronomy » Mon Jun 05, 2006 4:01 pm

I asked about this on another post but I don't know if I didn't get an answer because I piggybacked (the original poster had the same project but I asked for more specifics on a circuit) someone else's post or if no one had an answer. I want to build an optical tachometer and I need a circuit that will output a high or low pulse for each beam break (or beam make). I have tried several designs off the internet but I can't get a clean pulse. Every time I try this project I get jitter and noise and every other unwanted signal on the output. Anyone have a good circuit for this? I would like to use a phototransistor since they respond quicker than than Cds cells (right?). Is it possible to get a signal clean enough to feed into the clk input of a CMOS or TTL chip? I will probably end up using a uC but I want to try both versions. Also if I measure the number of revs in a given time and then multiply to get 1 min that's RPMs right? For instance, 10 revs in 100 msecs = 100 RPM (right?)

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Chris Smith
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Post by Chris Smith » Mon Jun 05, 2006 4:14 pm

Schmidt triggers

Gorgon
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Post by Gorgon » Tue Jun 06, 2006 3:41 am

10 revs in 100ms = 100rev/s = 6000RPM

TOK ;)
Gorgon the Caretaker - Character in a childrens TV-show from 1968. ;)

SETEC_Astronomy
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Post by SETEC_Astronomy » Tue Jun 06, 2006 2:54 pm

Thanks for pointing that out I forgot my answer was in seconds not minutes. Any ideas on a schematic?

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philba
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Post by philba » Tue Jun 06, 2006 3:07 pm

what have you tried? I'm kind of suprised to hear that you've had so much trouble. I'd bet you are doing something wrong. noise can be mitigated through several techniques:
- bypass all ICs
- make sure you have clean power
- keep the tach signal away from other signals
- low pass filter the tach signal for something like a 1khz 3db point
- schmitt triggers

Gorgon
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Post by Gorgon » Tue Jun 06, 2006 3:25 pm

I would also take a look on your detector and filter or screen it from ambient light with 50/60Hz frequency. This optical 'noise' may well ruin your pulse detection, by adding pulses or overloading the detector.

TOK ;)
Gorgon the Caretaker - Character in a childrens TV-show from 1968. ;)

ecerfoglio
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Post by ecerfoglio » Tue Jun 06, 2006 3:28 pm

Philba's advices address the "electronic" part of the issue:
- bypass all ICs
- make sure you have clean power
- keep the tach signal away from other signals
- low pass filter the tach signal for something like a 1khz 3db point
- schmitt triggers
Once you have taken care of that "electronic" part, look at the "optical" one:

>> Make shure that when the beam ** is not ** broken the fototransistor has a good saturation level (ie if common emmiter then Vout is << 1V)

>> Make shure that when the beam ** is ** broken the fototransistor is in it's open state. If necesary, block out other sources of light (ambient, sun, other leds).

>> you can block out other light sources placing the sensor in a piece of tube or by adding small baffles - Paint everithing flat black.

>> If the other sources of light have diferent colors you may try filters (optical filters, perhaps a simple piece of colored plastic)

>> You can modulate your light source and AC couple your detector

>> In some "extreme" cases (long distances, etc) you may have to use lenses, but it´s not ussualy necessary in a simple tachometer application


Edit: I posted before seeing Gorgon's advice
E. Cerfoglio
Buenos Aires
Argentina

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