Door sensors used in elevators/lifts are very expensive and my budget is not that much. But I need a similar mechanism or solution to detect the presence of someone if he/she is standing in the given line where the elevator door closes.

I can't use a PIR sensor as it checks proximity and the range is very small and I need it to be around 5-6 meters. This means that the transmitter and the receiver will be 5-6 meters away from each other in a straight line so that whenever there is some individual standing at the given line, the receiver will not receive the light from the light source(transmitter) because of the obstacle between the transmitter and the receiver.

For eg, I can use infrared transmitter or laser source at one end and light-dependent resistor or light or any photovoltaic sensor to detect the given source light at the other end. So, when there is no obstacle the LDR reading in the range of 0-255 will be around 200 due to illumination from the laser source, but when there is some obstacle (the presence of some human/objects between the transmitter and the receiver), the light source will not be able to reach to the LDR side and the reading will be around 80-100. Hence, I can apply some logic or some LDR range to distinguish between the two situations - one without any obstacle and other with an obstacle in between.

I hope you are getting what I am trying to say. I really need help here as I am not able to get any long distance infrared source/detector. I want it to be invisible at nights as well ((means no humans could feel that there are sensors at either side of the walls/doors), so I can't use laser source. Any solution would be appreciated and any other approach to accomplish the same thing of detecting objects between the transmitter and the receiver will be a big help as well. Please let me know if you need any other information about the project.

  • \$\begingroup\$ Why not use invisible IR LEDs? \$\endgroup\$ Commented Dec 19, 2018 at 4:58
  • 3
    \$\begingroup\$ you can get an infrared beam sensor for about $10 online, what's your budget? \$\endgroup\$ Commented Dec 19, 2018 at 6:31
  • \$\begingroup\$ What do you mean by long distance? \$\endgroup\$
    – Jeroen3
    Commented Dec 19, 2018 at 7:16
  • \$\begingroup\$ What about a pressure sensor in a pad under where the person would stand? \$\endgroup\$
    – Pranab
    Commented Mar 26, 2019 at 2:52

1 Answer 1


One method can be to use infra-red laser and a photodiode on the other side with the setup you described.

Be sure to use a low power laser and perhaps a beam that is not too narrow so that it can't cause eye damage.

If it has to operate during day-time, you would need a pass-band filter in front of the photodiode so that it is sensitive only on the spectral range of the laser, those can easily be found online.

To be even better, you can modulate the laser at a known frequency, like 1khz on-off, and the detector can then look for this modulation, this will help remove parasitic ambiant light that would offset the signal of the photodiode.

You could do the same with an IR LED, but the signals would be significantly lower and would be more difficult, but totally doable.

  • \$\begingroup\$ Just a note taken from other experience: It's not sufficient to modulate. The photodiode and associated circuit, if linear response and not logarithmic, will rapidly saturate from ambient light and there will be no modulation present to see. The thin film filter (and baffling) are almost always needed in the presence of strong ambient light. So with varying and strong ambient I'd start with the thin film filter and directed tube to reduce acceptance angle as a necessary 0th order requirement and add modulation as a 1st order improvement. \$\endgroup\$
    – jonk
    Commented Dec 19, 2018 at 14:44
  • \$\begingroup\$ Thank you, Damien, for answering. Could you please share me an Amazon link to the infrared laser you are talking about because I am unable to find laser source with wavelength over 700nm (infrared region) online? \$\endgroup\$ Commented Dec 23, 2018 at 6:54
  • \$\begingroup\$ You better search on specialized reseller, like this one digikey.com/product-detail/en/quarton-inc/VLM-780-03-LPA/… \$\endgroup\$
    – Damien
    Commented Dec 24, 2018 at 4:23

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