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WEEK 11: INPUT DEVICES

Assignment:

Read a microcontroller data sheet.

Measure something: add a sensor to a microcontroller board that you have designed and read it.

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// GOALS

For this assignment I decided to experiment with conductive fabrics to create sensors, this is research is part of what will be part of my final project.

Create a bend sensor

Make board to measure sensor

Read data from sensors

// USEFUL INFORMATION AND REFERENCES

- Velostat: a piezorestive material made of polymeric foil impregnated with carbon to make it electrically conductive. Apparently it was designed for the protection of items or devices that are susceptible to get damage from electrostatic discharges. It was developed by 3M. It has the property of decreasing it's electrical resistance when being bent or pressured (when receiving mechanical stress). When sandwiched between two conductive layers,it has a range of making pressure and bend sensors (depending on the support fabric that you use).

-Conductive fabric: There's a lot of different types of conductive fabrics, with a variety of densities, yarns, etc. For this test what I actually used was High Performance Silver Mesh Fabric made by Less EMF Inc. For this sensor what you only need is a nice conductive surface so it does not have to be exactly the same that I used.

-Conductive thread: Conductive threads can carry current as wires sou you can make flexible circuits. I used Sparkfun's stainless steel thread, but there's also silver made ones. Some information about threads and sewing recommendations learn.sparkfun.com.

// MAKING A BEND SENSOR

I followed Plusea's (Adafruit) bend sensor tutorial that you can read ---> here . She uses stretch conductive fabric and neoprene, these materials weren't available for me at the time so I replaced them with conductive fabric and 1mm thick lycra.

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>> BILL OF MATERIALS :

(2) 10x100Mm. Velostatstripes

(2) 11x100x1 Mm. Lycra fabric.

Conductive thread.

(2) 10x5 Mm. Conductive fabric.

Process:

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I sewed 4 stitches of the conductive thread to the lycra, making 2 equal peaces.

Then, I sewed at the ends a little peace of conductive fabric.

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Then I put the velostat stripes between the 2 lycra pieces and sewed everything with a normal green thread.

IMPORTANT: Make sure you sew the conductive thread measuring equal distances.

To test the functionality of the sensor I used a multimeter and measure resistance changes in Ohms.

Bend sensor test + multimeter from Citlali Limonada on Vimeo.

// MAKING PCB FOR THE SENSOR

I followed Niel's photoresistor / light sensor PCB design.

BOM

Part	Value	Device	Package	Description
C5	1uF	CAP-US1206K	C1206K
JP1	/	PINHD-2X3-SMD	2X03SMD	PIN HEADER
R3	10k	RES-US1206FAB	R1206FAB	Resistor (US Symbol)
SL1	/	M02S	02P	AMP QUICK CONNECTOR. I used this to test different value resistoris.
U$1	FTDI-SMD-HEADER	FTDI-SMD-HEADER	1X06SMD
U$2	SEWTAP1	SEWTAP1	PETAL-LONG-1	Edited it from originals Lilypad eagle file.
U$3	SEWTAP1	SEWTAP1	PETAL-LONG-1	Edited it from originals Lilypad eagle file.
U$4	ATTINY45SI	ATTINY45SI	SOIC8

>> FIRST VERSION

For connecting the sensor I added two sewing tabs that I imported from the LilyPad-Wearables library.

I also tooked a look on the actual Lilypad Eagle file to understand general distribution, dimensions and design. You can get more information here: Arduino Lilypad website.

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Board view of my design.

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If you haven't milled holes take the following information into consideration:

Holes must be in a separate .png file

Holes must be coloured in black and the rest in white. Remember that the milling bit will follow black traces. If you do it the other way, you'll get undesired bigger holes.

Use 1/32 milling bit.

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Milled board, ready to be soldered.

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As you can see, I added a female header in order to test different value resistors. Plusea recommends 10K - 20 K.

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_PROBLEM!

After trying to read the sensor several times, I found out that I had forgotten make the connection between SCK of the ISP to PB2 on the ATtiny45.

Solution 1 : Adding a horrible wire.

Solution 2: Making a new board.

>> SECOND VERSION

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//PROGRAMMING BOARD, READING SENSOR & CODING

For programming the ATtiny45 I did the usual Burn Bootloader from Arduino, connecting my FabISP programmer.

Later on, I used cocodrile clamps to conect the sensor.

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Bend sensor + PCB + Arduino Test from Citlali Limonada on Vimeo.

For reading data I based my code using "Analogue In" from Arduino examples.

You will need to use SoftwareSerial library, this is because ATtiny 45 does not have RX, TX pins, so you have to declare and set them. SoftwareSerial library allows this to happen.

Arduino code:

I also did a test without using my board but Arduino UNO to compare results. For this I based my code on the "Anaogue In" example, and added a smoothing process avoiding noise and instability from the sensor.

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Note: I also did the .py test, but you will find more information about it on the "Interface and application" week assignment.

//Very useful links & references

Plug and Wear online shop. They sell conductive thread, and all kind of conductive fabrics.

How to get want you want KOBAKANT DIY Wearable Technology Documentation. Created since 2007 as acollaboration between Mika Satomi and Hanna Perner Wilson.

>>FILES

Eagle files, RML, and PNG's: \ (•◡•) /

Simple Arduino file: (ʘ‿ʘ)

Citlali Hernández - Fab Academy - 2016