I am trying to measure a speed from a barrel with two sets of diodes. I was planning on using IR diodes to measure the speed. Does this sound like a good design? The barrel speed will be relatively slow like a maybe 10-30 ft/s.

 

Sounds simple enough. I hope you plan on using photodetectors too along with diodes

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Planning on it. I'm not really sure what photodiodes to get. I'd prefer not surface mount. That's all I saw. What wavelength IR diode would be good to? How do photodetectors usually work. What is the output. Can they have either on or off? Inputing it into DAQ so wondering.

Thanks for the help.

 

THe best wavelength LED is the one that can't pass through the bullet and the one that the detector can pick up. Simple enough.

You could have the detectors be either dark on or dark off...doesn't matter. You could even use one of each! All you're measuring is the time between when one changes state and the other changes state, so the state doesn't matter as long as it changes state.

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I just didn't know if it was automated in the chip or not. Would you recommend a photodiode or phototransistor? Looked up both. Seems like photodiodes are for set frequencies where transistors can be used in a range. Is this correct?

 

I think one is faster and less sensitive, while the other is more sensitive but not as fast. I think the photo transistor was more sensitive, but slower. I don't think it matters- you comparing electrical speeds to the movement of mechanical things. THe difference would not be noticable.

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Thanks. Grabbed two diodes and two phototransistors today.