Measuring presence of an object at a distance of 10cm


I need help with choosing the right sensor for a small project.

In my project, I have to sense a hand at about 5-10cm distance and I am not sure about the sensor I should use. The sensor will trigger a switch (relay) and the device will transmit data through bluetooth, etc.

The sensor will be either behind transparent plastic, or if the sensor is small enough I might drill a hole in the plastic and the sensor can be in the open.

I ruled out ultrasound sensors because of the transparent plastic. I am guessing the sensor will not work?

IR sensors are probably the best choice, but which one? I have a few TCRT5000 sensors lying around, but according to the datasheet their max range is 8mm.

Any help would be appreciated.

You will have to test the plastic to determine if it is transparent to IR.

One of these should work, but probably not through plastic.

IR sensors may or may not work through plastic, you have to try.

First of all the plastic must be transparent to the IR, or it won’t go through to begin with. Check the IR wavelength of your chosen sensor against IR absorption spectra for the chosen plastic material.

Secondly, your sensor may be upset by reflections against the plastic. This you have to try out. Reflections may mean part of the IR is reflected back, and you detect the plastic. It may mean it goes through, but on the return it’s reflected away and you simply don’t see the return. It could be a combination of the two even, or there may even be other effects. Reflections may occur on both the air/plastic and plastic/air interfaces, or on scratches in the plastic.

Best is to have nothing in the way at all. If properly aligned a VL53L0X sensor can measure through a hole just a few mm diameter, that’s the smallest I know of. I’ve had them measure the distance of a hand held above it through an opening of 3-4mm diameter in a metal screen. You may also look at the VL53L1X which is the short range version.

Those sensors make the TCRT5000 look like a giant!

Cant be much simpler than this capacitive sensor

5-10 cm is quite a distance for a capacitive sensor, but indeed should be doable. That link claims 300mm, that's a long distance. My personal record for capacitive sensing is some ~700 mm but it's terribly sensitive to just about anything (even air currents may affect the fields), making it useless in the real world. The overall electric environment also starts to play a role - meaning you probably have to make sure there no other electrics within that range, or changes in current (switching on and off an LED, for example) can seriously mess with your detection.

Capacitive sensing is also not very directional, which may or may not be a concern for OP.

Thank you all for your replies.

I think I'm gonna go with an IR sensor, but I dont know which one yet. I have to look at the properties and decide.

As for the casing, to simplify the project I will design a gap/hole in the casing for the IR sensor, so any reflection from the transparent plastic would not be an issue. The casing is probably the most difficult part for me.

As for your comments, directionality is not a concern for me. My main design criteria are:

  1. Detect a hand without a touch, 5cm to 10cm is a reasonable distance. The hand could be coming from any direction but the distance of detection needs to be more or less consistent.
  2. I don’t need to measure the distance, just the presence of an object is sufficient.
  3. Minimise false detections as much as possible.

Thanks again

I'm not sure how an IR proximity detector would respond to human skin depending on its albedo in the IR region.

Would a PIR sensor be a possibility?

Thanks John,

I am not sure about the skin colour argument, I am not arguing that it does not exist. As I already said, I need the sensor to be consistent, but it does not need to be 100% accurate. I can tolerate a few centimetres, but I cannot tolerate false or missed detections. At this point, I assume that a hand of any colour or size would be detected within a tolerance of a cm or two.

I did consider PIR sensors, I guess with the right PIR sensor and calibration they can work at about 10cm? I don't know enough about the PIR sensors.

Wouldn’t an IR sensor do the same job and just be simpler?

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