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I have a small machine that uses a 1 cm wide conveyor belt to transport small rocks. I am looking for a sensor that will identify if there is a rock at the end of the conveyor. This sensor will need to have a minimum range of 1 mm and does not need a max range of more than about 1 cm.

Am I looking for a IR sensor?

Do there exist IR sensors with that small minimum range?

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  • \$\begingroup\$ You could use an ultrasonic sound sensor, but that goes up to 50cm. You might be able to change the range though. \$\endgroup\$ – user17592 Apr 23 '13 at 8:43
  • \$\begingroup\$ why put the sensor up front, why not use a sensor along the edge of the conveyor belt? (A photoemitter-photodetector pair) This would eliminate the range problem, you align it right at the end of conveyor belt and get a trigger whenever the circuit breaks? \$\endgroup\$ – Anshul Apr 23 '13 at 8:48
  • \$\begingroup\$ its on the side of the end of the conveyor. It will tell the belt to stop when a rock is there \$\endgroup\$ – bogen Apr 23 '13 at 8:49
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    \$\begingroup\$ Pressed Enter button too soon as a matter of habit :P , I've edited the comment. \$\endgroup\$ – Anshul Apr 23 '13 at 8:51
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If actually measuring the distance isn't important, rather just detecting presence, then I think a basic reflective IR sensor might work okay. At these short distances typical ultrasound detection isn't practical.
If you can mount at either side of the belt, using a "beam-break" type detector (as opposed to reflective) might be worth considering also.

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  • \$\begingroup\$ Do you know where you can buy that kind of sensors? \$\endgroup\$ – bogen Apr 23 '13 at 8:51
  • \$\begingroup\$ Yes, any decent electronics vendor, such as Mouser, RS, Farnell, Digikey will stock various types of IR sensor. For the most basic setup all you need is an IR LED and IR photodiode (or IR phototransistor) plus the logic necessary to cut the power to the belt when the sensor is activated. \$\endgroup\$ – Oli Glaser Apr 23 '13 at 8:53
  • \$\begingroup\$ Beam break as used in supermarket checkout conveyors, seems popular & reliable for them. \$\endgroup\$ – John U Nov 18 '13 at 12:14
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This kind of small distance detection is simple with an IR led and IR Sensitive Receptor/Photodiode. In fact, it's so common, that almost everyone has used one at some point. They are found in computer mice with trackballs or scroll wheels.

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The light emitters and photo detectors form a basic optical encoder. The detector changes its output when the light gets blocked and unblocked. And by counting how often/fast it changes, it could even tell you the speed of something. Or if you know the speed (of say the conveyor belt), you can use the amount of time it takes for the sensor to change to tell how long a rock on the belt is (Distance = Speed x Time)

enter image description here

As you can see in the picture, the distance between the two parts is very very small, maybe 0.1"/0.254 cm. These can work at 1cm no problem. You could mount them across the belt (Sensor changes when the line of sight is blocked), or if needed, angled on the same side. (Sensor changes when something reflects the light)

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