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I am trying to create a cheap system that check if there is someone sitting on a chair.

I would like to know if there is someway I can configure a RFID reader so it identifies when there are interference in the radio signal from a RFID tag caused by an obstacle - in this particular case, a human body. So I can put a RFID on the table and a RFID tag on the chair. I read it every 10 seconds, and when there is a human body (or any phisical obstacle) it notice some interference in the radio signal and register that that chair is occupied.

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    \$\begingroup\$ Possibly, but RFID isn't really designed to be sensitive to that. Higher frequency systems could mean good directivity with small antennas, helping to make sure you are measuring a direct path and not an indirect one leaking around the obstacle. Light sensors would represent an extreme of this, but you start to get similar behavior at microwave frequencies. For chair occupancy, you might also want to look at a capacitive sensor, or a load cell (weight sensor). \$\endgroup\$ – Chris Stratton Aug 4 '14 at 16:44
  • \$\begingroup\$ These weight sensors (capacitive or load cell) are for sure the ideal technical solution, but are too expensive. I would need to put a sensor on each chair of a restaurant. Can't spend $5-$10 on each. That is why I am hoping to make it work with cheap passive RFID tags... \$\endgroup\$ – soneca Aug 4 '14 at 16:58
  • \$\begingroup\$ Tags might be cheap, but readers won't be. At least for a non-conductive chair a capacitive proximity (not weight) sensor shouldn't cost more than a dollar worth of parts as it consists of almost any microcontroller with a high impedance input and some sort of plate. Installing it and reporting the results may be more of a challenge. \$\endgroup\$ – Chris Stratton Aug 4 '14 at 17:08
  • \$\begingroup\$ Forget sensors. OpenCV, i.e. facial recognition, motion sensing, tracking, etc done with a camera and a computer would work nicely. \$\endgroup\$ – Passerby Aug 4 '14 at 22:25
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A main problem with RFID is the power transmitted by the reader module to the tag. With some on-PCB designs, such as Texas Instruments proposes through an evaluation kit for the 7960A, you can support a reading range up to 4 cm, where ID Innovations will propose a all-in-case reader up to 12 cm. Unfortunately, human body is larger than these metrics.

You can still get readers with a better range, but this is really not the same price I swear.

A cheap alternative could be the usage of a modulated IR signal as a fence. It just needs a MAX555 to modulate and power the IR diode signal on the emitter side, and a photo-transistor plus an analog comparator on the receiver. You add a peak-detector (diode + RC dipole) at the output in order to get major changes (i.e: your signal will go down only when somebody is seated, not due to the signal reconstitution) and it could sound good. You can get it for less than $8 I suppose.

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  • \$\begingroup\$ Agree wit the IR solution. \$\endgroup\$ – tcrosley Sep 4 '14 at 0:02

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