Optimize a TCRT5000L

rhor... · 2021-05-29T12:57:59+00:00

I am using a TCRT5000L photo-interrupter to drive the input of a bank of 74LS90 decade counters to make a rotation counter. I will be biasing the diode transmitter at 20mA. Obviously, I will want to use something like an analog comparator or an op amp to make the detector more sensitive and a schmitt trigger to prevent instability. The question is, how can I optimize the sensitivity? Nothing about the rotating material will be known, including its IR reflectivity, except that the surface will have a well defined mark that does allow the reflected light to vary by a significant amount. If the collector resistor is too large, then the phototransistor may easily saturate, and the sensor voltage will never go high enough to set the comparator on each revolution. If the resistor is too large, the sensor voltage may never drop low enough to reset the comparator. I am trying to think of a simple way I could actively normalize the collector current so the sensitivity is right in the nominal range whenever the material and sensor gap change. Of course, this circuit would have to be relatively slow acting, or else it will change the collector voltage too quickly, either causing aliasing or lost data. The incoming pulse frequency may vary between 0.2 Hz and 150 Hz. I am thinking something like the following might work, but I am far from confidant:

May 29, 2021 Last reply: 5 years ago 21 Replies

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rhor... 5 years ago

'Good point.

'Also a good point. It's a bit harder to implement, however, and less flexible in terms of the target.

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whit3rd 5 years ago

If you could ensure the target (presumably a rotating cylinder) was , say, 1" diameter, you could blacken it and apply a 1/4" strip of aluminum tape for the mark. It should be possible to get quadrature with two sensors at 1/8" shift in the circumferential direction. The coil winders I've used had a Jacobs chuck for a holder, and such chucks have a cylindrical girth available.

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