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Pressure sensors

by jermaindefoe
Tags: pressure, sensors
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Sep30-13, 02:01 PM
P: 6
I am not sure if this is the right place or not for this post so i apologise in advance if not.

I am trying to find some small pressure sensors that can go inside a seat cushion or something and that can be connected somehow to a computer system to indicate when they might have been triggered.

I was also wondering if there were any that could transmit a wireless signal :D

I hope someone could help me out with some suggestions

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M Quack
Sep30-13, 04:12 PM
P: 669

you will have to connect it to an arduino or similar small computer if you want to process and transmit the readings.
Sep30-13, 06:11 PM
PF Gold
P: 866
The automotive industry uses tens of millions of these sensors every year as part of the air bag sensor set.
However, it seems you need only a few units, so M Quack has the better answer.

Oct1-13, 01:58 PM
P: 6
Pressure sensors

do you know if it could be hooked up to a rasberry pi?
Oct1-13, 02:32 PM
PF Gold
P: 866
Don't see any reason why not.
That said, how do you plan to do this? You can monitor the pressure as a variable with a suitable sensor/A/D converter combo, but will still need to program your arduino/raspberry pi processor to integrate the data and launch the appropriate response. Of course, the job is easier if all you want is an on/off sensor to indicate weight has been on the seat, but then a simple switch is ok, no computer needed.
I guess I don't really understand what you are trying to do.
Oct1-13, 02:41 PM
P: 6
im just trying to make a system as a personal project to monitor a number of seats and display which have people on or not :)

trying to make it a proof of concepts using as cheap materials as possible. I want to order 10 of those sensors as soon as i can assertain if 1)its possible and 2) if a rasberry pi / arduino can handle it :)
Oct1-13, 04:18 PM
P: 6
if anyone can answer how i might go about hooking a few of these up to raspberry pi then i will proceed to buy a few today - thanks guys
Oct1-13, 04:46 PM
PF Gold
P: 866
As described, this seems a very undemanding application.
Each seat is either full or empty, so just a binary 1 or 0, no actual pressure measurement needed.
The simplest way to set this up is to have each seat wired to a light, so seat full, the light is switched on, seat empty, it goes off. The hang-up is that this needs a wire pair for each seat, so it scales poorly, but no computer is needed either.
A computer comes in when you need to be able to address each seat individually without hard wiring each seat separately, basically multiplex the signals across a common data line. Then each seat has to have an identifier, so it transmits not just the full/empty code, but also an associated seat ID. Also, there needs to be a protocol that sets out the sequence in which the seats are addressed and answer, else the signals get mixed together and confusion results.
That means each seat needs to have a minimal computer, to manage the signals back and forth from the readout console. So think one raspberry pi per seat, plus one for the console. Your data volume is small, so their minimal computer capacity should be quite enough. You will need to program the system, which should be a snap, as there is lots of ARM software support.
Unfortunately, I've never worked with ARM systems, my software experience was with Fortran long ago.
M Quack
Oct2-13, 02:27 AM
P: 669
The RPi has 17 GPIO pins that could be used to detect an occupied/unoccupied signal each. The problem is that
you need a wire to each sensor/seat. Of course you can use less, e.g. one RPi per row of seats. Programming
the RPi to read these ports is a piece of cake, transmitting the data over wireless ethernet is also easy, you just
need a wireless USB dongle as the RPi does not have wireless built in. You can probably run the RPi off a battery.

You will need some minimal analog electronics to convert the resistance of the sensor to a digital signal appropriate for GPIO (3.3V or 0V). A simple opamp comparator with a couple of resistors and maybe a potentiometer for tuning should be enough. Just make sure you use an opamp that can operate with just 3.3V and 0V as supply voltages. Many need +/-4.5V or more.

If the opamp scares you, then you can try a beaglebone or beaglebone black. That has 4 analog inputs. You hook up the sensor with just one resistor per sensor, and the tuning of the setpoint can be done in software.
Again you need a USB wifi dongle.

The new arduino Yun has built-in wifi and 6 analog inputs, so that may be the most economic solution.

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