Hi, I am planning to build a device able to detect and tell a server via wifi if a chair is occupied. How would you do it? And how cheap do you think the device could be?
Hi,
I've not done this myself, but I have heard that a really cheap but effective pressure sensor can be made as follows:
Use that black conductive foam that is used to protect ICs from static during transit. Make small-ish squares of it and sandwich them between two pieces of copper-clad board (the type used for etching your own PCBs). Solder wires to the copper at top and bottom. Insert this into the cushion of the seat. The resistance between the two sides of the sandwich goes down as it is squashed by the weight of the chair's occupant. You can probably get a rough estimate of the occupant's weight! Use an appropriate resistor to create a voltage divider and feed the resulting voltage into an Arduino analog input.
Paul
PaulRB:
Hi,I've not done this myself, but I have heard that a really cheap but effective pressure sensor can be made as follows:
Use that black conductive foam that is used to protect ICs from static during transit. Make small-ish squares of it and sandwich them between two pieces of copper-clad board (the type used for etching your own PCBs). Solder wires to the copper at top and bottom. Insert this into the cushion of the seat. The resistance between the two sides of the sandwich goes down as it is squashed by the weight of the chair's occupant. You can probably get a rough estimate of the occupant's weight! Use an appropriate resistor to create a voltage divider and feed the resulting voltage into an Arduino analog input.
Paul
That doesn't seem feasible over repeated use. I doubt that foam recovers very well (or is very comfortable with copper plates). However, pressure sensor is a good way to go, since it is probably the most direct. Another option would be some form of thermal sensor (Probably IR), since bodies create heat, you could detect that. Or you could build a laser gate into the armrest near the legs. Possibly, also an Ultrasonic sensor or other proximity sensor, strategically placed, assuming you can install it so it doesn't get blocked (I don't know if the ultrasonic would pass through your chair's material, so it would need a hole to emit out of).
I think modern car seats have occupancy sensors, so you might see how they work.
albus_c:
Hi, I am planning to build a device able to detect and tell a server via wifi if a chair is occupied. How would you do it? And how cheap do you think the device could be?
Maybe a piezoelectric pressure sensor.
Unless the chairs are in a very hot area, a simple temperature sensor in the seat would work, although there would be some delay between the chair being occupied and the detection and vice versa. Temperature sensors are small enough not to cause discomfort for the chair's occupier and are sturdy enough to not be damaged by being sat-upon by even the largest posterior.
I'm desperately trying to avoid the 'A' word here! 
I think with the right set of sensor(s) you might even 'recognize' the person by weight or # moves or...