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.



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(ser.read()) high = ord(ser.read()) value1 = 256*high + low low = ord(ser.read()) high = ord(ser.read()) 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.

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


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



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.