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In my truck a warning light comes on when I go to take off if a passenger is sitting there but the seatbelt is not secured. After some research and thinking it may be their weight, capacitance or maybe an infrared beam (although I cannot see a sign of that). Maybe other things I haven't thought of?

I am a furniture manufacturer and would like to incorporate a night light into furniture to have a light come on at night automatically when someone stands up. The truck gave me the idea and I was thinking because they sell so many they have probably put a lot of thought into how they work. Does anyone know the solution the auto industry uses or have other good ideas on ways to go about it?

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  • \$\begingroup\$ Since the actual problem is about how to detect whether a person is sitting down or not, the specific mechanism used in a vehicle may not be relevant. A suggestion: Sensing load is probably easiest at a DIY level, so why not just run with that? \$\endgroup\$ – Anindo Ghosh May 25 '13 at 10:10
  • \$\begingroup\$ For seats with sufficiently thick padding: magnet and reed switch separated by foam, brought into proximity by its compression. \$\endgroup\$ – Kaz May 25 '13 at 15:58
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you can use FSR(force sensitive resistor) to detect whether a person is sitting on a seat or stands up. The working of FSR is similar to LDR (Light dependent resistor the resistance of LDR would decrease if light falls much),in FSR if the applied pressure/weight increased the the resistance would get decreased,you can calibrate this resistance change in terms of weight/pressure.

fig(1) weight Vs Resistance and conductance enter image description here

fig(2) circuit diagram- connection of FSR enter image description here

fig(3) Output voltage Vs wight(response of circuit diagram fig(2)) enter image description here

 For more details http://www.tekscan.com/flexiforce/force-sensors-for-design       
  http://learn.adafruit.com/force-sensitive-resistor-fsr
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The seats are triggered by a pressure sensor and not a beam or capacitance. The problem w/ your design is wires to the seat, which you do not want. A simple pressure plate in the seat w/ a RF remote would work. A reed switch is glass and could break. Another solution would be to provide a beam in the desk so when the chair is pushed under the desk the beam is broken and the light comes on. Not sure if the design is really necessary. Do people really need a light to come on when they stand? I don't really understand what your trying to achieve.

Good luck w/ your project.

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The company I work for makes a Nurse Call system used in Retirement Communities and Elder Care facilities. As part of our system we have a pressure sensor pad (made by someone else) linked to our own Radio system. This is frequently used on a wheel chair to report that a senior (perhaps at risk of falling) has gotten up from the wheel chair (or bed). In your case you may want something that is more "built in" to the chair, but then you're also trading off ease of replacement / maintenance, etc. I'd still start by looking a commercially available pressure sensors.

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