Error free (or very close) optical tach output
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Error free (or very close) optical tach output
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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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
- 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
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- Location: Buenos Aires Argentina
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Philba's advices address the "electronic" part of the issue:
>> 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
Once you have taken care of that "electronic" part, look at the "optical" one:- 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
>> 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
Buenos Aires
Argentina
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