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IR sensor 1-bit SPI slave

Discussion in 'Sensors and Actuators' started by Daniel Zetterman, Dec 9, 2014.

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  1. Daniel Zetterman

    Daniel Zetterman

    Dec 9, 2014
    I'm working on a led table kind of project. The table will be of 16x16 pixel size, controlled by a beaglebone. On the output side i am using RGB led circuit of type ws2801 which are acting as SPI slaves. On the input side my plan is to use a IR photo transistor coupled with an IR led acting as a touch sensor.

    For the IR-circuit I have a few doubts/questions and I'm hoping that someone with experience with using IR as multiple touch can give me some hints:

    1) Does it sound ok as a general solution to use IR for the touch sensor? It will be behind a tinted glass, sunk down a few centimeters to make place for the circuitry for each pixel
    2) Will the bright led which is used for each pixel give me problem with the IR-sensor?
    3) Will the glass give problem? Seems that I'm detecting even the glass. If the glass is dirty etc?

    4) My plan is to make 8x8 modules which I can connect together to make bigger tables. I need some way to serialize the input and I think that the SPI protocol can be used for this. So I guess that each touch sensor can be a 1-bit SPI slave, and that each sensor is connected in series, finally reaching my microcontroller which will interpret the bitstream and control the clock and slave select signals. The question is how does a 1-bit SPI-slave circuit look like, my guess it is made up of a few NAND gates and one flip-flop?

    Thanks in advance for any help I can get, cheers!
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Welcome to our forum.

    You may have a look at the octolively module. This uses IR sensors and the various considerations when using a cover on top of the module are discussed there.

    As for the 1-bit SPI slave: An SPI is essengtially nothing but a shift register. As you noticed, a 1-bit shift register is a flip-flop. I suggest you use additional control line to steer the flipflops in the sensor input chain between "acquire" (read sensor signal into FF) and "shift" (read serially data from FFs) mode.
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