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contact sensor based on soft dance pad sensor

Discussion in 'Sensors and Actuators' started by max_torch, Jul 19, 2015.

  1. max_torch

    max_torch

    98
    1
    Feb 9, 2014
    I wish to make a contact sensor that will detect if someone sits on a chair. It will be a sheet that will be draped on the chair and indicate once a person has sat on the chair. I take the soft dance pad used for games such as DDR, StepMania, and Pump It!, for inspiration.
    Basically I opened my dance mat, and here is what I saw:
    P1050458.JPG
    Can anyone help me identify what kind of principle is applied here?

    My wild guess is that it looks like a capacitive sensor where the black conductor on top and light grey conductor on the bottom are the conducting plates (I'm talking about those grey and black lines) and the white foam in between is only a spacer and the dielectric is really just using the permissivity of free space. Stepping on it alters the distance between the plates which corresponds in a change in the voltage and that is what is detected by the microcontroller.

    Does that sound about right?
    How could I make a contact sensor with a larger sensing area based on the dance pad sensor from scratch?
    I'm thinking that really tiny wires can be my conductors..
    Also, I'm wondering how I can condition the sensor output to be suitable for input to a microcontroller, perhaps an op-amp may or may not be needed but the simpler the better.

    Any advice would be appreciated.
     

    Attached Files:

  2. Alec_t

    Alec_t

    2,629
    693
    Jul 7, 2015
    Have you considered using thin sheet rubber/plastic foam sandwiched between aluminized-plastic sheets? That would make a capacitor which could be used as a frequency-determining element in a simple CMOS-Schmitt-inverter-gate oscillator. The micro would then get a logic-level square wave and would detect a frequency shift.
     
  3. max_torch

    max_torch

    98
    1
    Feb 9, 2014
    Hmm that kinda sounds like what is going on with the dance pad in the picture
    The black and grey lines could be aluminum and there is that plastic foam in the middle.
    Can you show me what further reading I would have to do to know about this 'frequency determining' and how I can ultimately make an oscillator input to my micro and how my micro can tell when the frequency shift is present?
    Kinda sounds like frequency-shift-keying, wouldn't you say?
    Also, I have to use a mid-range PIC microcontroller because that is the one and only microcontroller that is readily available where i live.

    TLDR: How exactly do I get the micro to have a logic one when there is a frequency shift?
    Also, what kind of testing and trial would you guys recommend to get this thing into a presentable, working, functional condition?
     
    Last edited: Jul 19, 2015
  4. Robert Hill

    Robert Hill

    111
    12
    Mar 5, 2015
    A simple solution would be to buy one of these: http://www.amazon.co.uk/gp/aw/d/B00...+mat+alarm&dpPl=1&dpID=11JpQgYWIrL&ref=plSrch

    And then connect the wires into either a led with resistor and a battery or for something more fancy into a 555 timer chip set up in one shot mode.

    I've run this sort of thing from 9v batteries quite happily.

    The mat I linked to is basically a switch where two bits of foil are separated by foam. When you step on the mat the foam compresses and the foils come into contact creating the connection.

    Not sure if that is what you are looking for?
     
    Last edited: Jul 19, 2015
  5. max_torch

    max_torch

    98
    1
    Feb 9, 2014
    My dance pad already does precisely that function. What i am looking for is how to make one from scratch. The most important thing is to be making it from scratch.
    You seem to know something about it's theory could you elaborate? so it has nothing at all to do with capacitance and it is more like the closing of a switch when the foam is pressed? how can that be if the two foils dont actually come into contact? or do they actually make contact because of the holes in the foam?
     
  6. Robert Hill

    Robert Hill

    111
    12
    Mar 5, 2015
    Sorry, obviously missed the bit about it being from scratch.

    I've come into contact with these mats where they are used in care homes to set off an alarm when a resident steps on them (for example an unsteady resident getting out of their chair unaided).

    When one stopped working I opened it up and found foil and foam.

    The call bell system they attach to is normally triggered by a tactile switch so I assumed the mats perform the same function by as you say making contact through the holes in the foam.

    This chap makes a cardboard one on the same principle.
    http://m.instructables.com/id/Use-a-DIY-Pressure-Plate-Switch-to-Automate-Your-H/?ALLSTEPS
    I think even if the dance mat works in a different way, using the foil as a switch should work for your purposes. The downside of the foil approach is that it isn't super durable.

    Another option is using conductive threads which I don't know much about but seems cool.
    http://m.instructables.com/id/Fabric-bend-sensor/
     
    Last edited: Jul 19, 2015
  7. Alec_t

    Alec_t

    2,629
    693
    Jul 7, 2015
    A simple CMOS inverter oscillator (power pins not shown) is like this :-
    SensorOsc2.gif
    R1 would be chosen according to the no-load capacitance of the home-made sensor, so as to give a convenient oscillator frequency (a few kHz).
    There are various ways a micro could measure the frequency. One would be something like this :
    Use an input pin with an interrupt-on-change ability,
    apply the oscillator output (a square wave) to the pin,
    whenever an interrupt occurs note the time and clear the interrupt,
    calculate the time intervals between consecutive interrupts,
    average a given number (say 8) of the intervals.​
    You'd then check for a major change in the average.
     
  8. AnalogKid

    AnalogKid

    2,289
    646
    Jun 10, 2015
    The sensor in post #1 is not capacitive. It is a contact switch. The upper and lower pieces of plastic have conductive ink patterns screened on them in orthogonal directions. The center white foam piece is an insulator with holes in it. When pressure is applied, the center foam is compressed and the top and bottom plastic sheets deform into the holes. Conductive lines on the top and bottom sheets touch through the holes. This is a variation of the conductive rubber keypads used in TV remote controls.

    ak
     
  9. max_torch

    max_torch

    98
    1
    Feb 9, 2014
    Thanks for all the feedback so far.. I will perform tests on the device in order to observe that it really is merely a contact switch and not capacitive and I will return with results.
     
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