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?
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.
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.
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)
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)
