Build a sensor network with sensors and microcontroller connected wirelessly

Discussion in 'Wireless & RF Design' started by Vimal Venkat, May 3, 2019.

  1. Vimal Venkat

    Thread Starter New Member

    May 3, 2019
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    Hello, I am working on a device where the microcontroller is stationary but the capacitance stretch sensors are moving. Right now, I have the sensors hooked up to a CAV444 (capacitance to analog voltage) chip, via a multiplexer, that inputs into Analog In pin of Arduino Uno. But I realized that it is not feasible if the sensor network is moving, as wires are an issue. Is there any better way to design this circuit in a way that can get rid of wires between the sensors and microcontroller possibly using a bluetooth or RF module?
     
  2. LesJones

    Well-Known Member

    Jan 8, 2017
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    604
    Can't you keep the microcontroller close to the sensors and multiplexor and send the information in digital form using a Bluetooth serial link or a serial link using HC-12 transceiver modules.

    Les.
     
  3. Vimal Venkat

    Thread Starter New Member

    May 3, 2019
    3
    0
    That's definitely one way of doing it. I am wondering whether I could get rid of the CAV444 chip and directly convert the capacitance to digital and then transmit that information using a low power bluetooth or RF module. The capacitance of the sensors have a range of 50 pF to 300 pF. Ideally I want to create a really small bendable PCB for wearables. I am also really new to electronics as I am a mechanical engineer. So I do not know that many things exist. Thanks a lot!
     
  4. LesJones

    Well-Known Member

    Jan 8, 2017
    2,050
    604
    I have never seen the CAV444 chip before but I don't think you will be able to make anything as small to convert capacitance to voltage. You will need to consider the bandwidth of the RF link. Will it be capable of sending the data from all of your sensors at the number of samples per second that you need.

    Les.
     
  5. Vimal Venkat

    Thread Starter New Member

    May 3, 2019
    3
    0
    Thankyou Les. I will definitely try that.
     
  6. danadak

    Distinguished Member

    Mar 10, 2018
    2,768
    595
    I am not sure this will work at 50 pF, would have to do some testing.

    16 channels shown, could be possibly > 50, although takes bigger package.

    Basically a current source is switched onto external sensor, and a timed wait, 100 uS,
    for cap to charge followed by a SAR converting at ~ 1 MSPS gets the V, then C is
    calculated from Q = C x V, I = C x dV/dT......

    Its possible to add another channel to mux with no pin connection to get error due
    to stray C. One could. by testing, create a least squares error fit or power fit equation
    if low C range gets non linear to linearize the channels.

    This is all onchip, as you can see in rightmost window lots of resources still left to use.

    upload_2019-5-3_20-15-33.png


    Cypress PSOC 4, could have gone to PSOC 5LP for a lot more resources if needed.

    Cypress has some parts, multichip package, that have wireless and PSOC in the
    package. Although I would be inclined to use PSOC + ESP8266.


    Regards, Dana.
     
    Last edited: May 4, 2019
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