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We need to get some kind of a simple sensor, that can sense someone is standing close to a product . But, it has to be behind plastic, and not seen at all . We only need to detect someone 1 meter away from the product .

  1. Using PIR sensors is a problem because its covered.(is it possible somehow?)
  2. using any lights resistors/sensors also a problem when it covered .
  3. ultrasonic is a problem because its too big and battery eating .
  4. infra is no brainer because its covered .

Is there a simple yet powerful sensing method that i am missing here, that takes no energy and detects a small distance human body ?

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    \$\begingroup\$ Can you put a pressure mat in front of the product? \$\endgroup\$ – Roger Rowland Oct 25 '15 at 9:14
  • \$\begingroup\$ Not all plastics are fully opaque to mid-wave or short-wave IR. For example, the daylight filter on an infrared photodiode is made of plastic that appears black in the visible, but transmits around 800nm. So depending on the plastic at least two of these options are feasible in principle. \$\endgroup\$ – Oleksandr R. Oct 25 '15 at 18:22
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You might consider a capacitive proximity sensor. Commercial models are mainly aimed at detecting smaller objects much closer than a meter, but the same principle can be used over larger distances too.

Place a large metal plate just inside the plastic cover of the product, or metallise the inside of the cover. Measure the capacitance between this plate and ground. The capacitance will change when a large conductive object like a person is in front of the plate, even if they aren't directly connected to ground. Capacitance measurements can be made with very low power.

Whether or not this will work depends on a variety of things you don't mention in your question. The size of the plate you can fit in your product and the dielectric properties of the product's cover will matter. A forklift 3 meters away might look a lot like a person 1m away. And the capacitance level to trigger the sensor will vary depending on the product's installation site, so you'll need to either provide an adjustment method or a circuit/microcontroller to do self-adjustment.

It's not the easiest way to sense a person, but if your requirements have ruled out the easy options, you could probably make it work.

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I think first you should determine the Radiated energy response of the plastic metarial.

Test the plastics back-radiation pattern and reflecting power for various frequency. I guess the useable frequency range may be between 49 - 21 GHz. Because this spectrum contains less harmonics and intermediate distortion radiations.

This is basically a short range radar for special purpose. Radiate and listen for the response. Compare the RSSI. The antenna must be directional and output power of the antenna and receiving sensitivity must be properly arranged through the test. This requires a hardcore R&D.

But the simplest solution would be to use RFID. Every person passing through the plastic sheet must hold a transponder and reader should be behind the plastic metarial.

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put an infrared thermometer behind an infrared transmitting, visually opaque acrylic window.

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  • \$\begingroup\$ yea..we have a problem with the acrylic thing. The design of the product is very unique .. it has to be fully covered .. \$\endgroup\$ – Curnelious Oct 25 '15 at 9:15
  • \$\begingroup\$ if we could have a very very thin hole, like 1/2 millimetre , can it help finding a right sensor ? \$\endgroup\$ – Curnelious Oct 25 '15 at 9:21

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