How to Make (Almost) Anything

MAS 863 2010
Week 8 assignment
  • design and make a circuit with a sensor
  • write a program to read and display its value

  • background: this week was very good for me. it gave me ideas about my research in the biomechatronics group. ideally, i would want to measure blood flow in a prosthetic socket and map that with measurements from a pressure sensor.

    the field of photoplethysmography includes the use of light sensors to measure blood volume/flow from the analyses of reflected light from LEDs on the human skin. my interest in this is to measure tissue perfusion in the residual limb/prosthetic socket interface. it would be important to map pressure in the socket with blood flow data to give an indication of pain/comfort for amputees.

    how: first step was to use eagle to generate my schematics and my board files which i exported to mill my circuit board using the modella.

    eagle schematics eagle board

    after milling, i soldered the components (attiny 44, phototransistor, red led, 50k ohms resistor for the phototransistor, 500 ohms for the LED, 10K resistor to stabilize current, a 1 uF capacitor, AVRISP). i did not use a switch in this circuit because i decided I would want the LED to be blinking at a constant rate to make it easy for synchronist detection (take a measurement when LED on, take one when off, subtract the two and spit that value out). These below changes were made on Neil’s python file

    "low = ord( high = ord( value1 = 256*high + low low = ord( high = ord( value2 = 256*high + low value = value2 - value1"

    the witches from halloween were all over my business this week. after soldering my new board complete, and before I started programming, i went to pick up my fabisp. on my way up, i tripped and fell. i lost my usb and my header along with the copper on which the components were soldered. thanks to Jie, i was able to breathe and solder jumper cables all over the place to make a sexy fabisp with strings all over the place. i also had to make a connection on my board because my phototransitor was not connected on one of ADC pins on the microcontroller.

    i had my LED working which was great since my LED did not work last week. look at the fabisp on the left. :)
    eagle schematics

    and then the sensor was turned on, python program modified to give me synchronist detection.

    sensor working sensor working

    next steps:
  • take the sensor and light off the board (so that we can apply pressure on specific tissues and see how that affects blood flow and map that with pressure measurements.
  • make a soft circuit and embed the LED and sensor in a fabric (sock) so that it can be put inside a prosthetic socket to measure flow at that interface. any ideas are welcome.

  • thanks D. Mellis and Jie Qi.