Electronic T-Shirt: How to Make a Sensor-Filled Shirt

The E-tshirt project uses human computer interaction techniques to assist the people classified as autistic by helping their caregivers understand their emotional status via their autonomic data. This open source project consists of a t-shirt embedded with an electrophoretic (eink) display that can display the wearer’s hear rate, skin temperature, and galvanic skin response. Sensors and a custom designed miniature PCB are embedded into the fabric to allow this.

The E-tshirt was made to help autistic people and their caregivers by allowing them to understand what kind of objects, sounds, colors, and situations trigger meltdowns within the person. Here is a short video of the concept:

This tutorial provides only the relevant technical information for recreating this project in order to suit your own applications and to help people in distress. All the source code and design files for this open source project can be found at the end of this page. It is recommended to read the original concept and development story before starting with this tutorial:

E-tshirt: Concept and background
E-tshirt: Technical story of Development 

Prerequisite Knowledge

1. STM32L152 Discovery board Reference Manual  STM32L152 datasheet STM32L152 command manual
2. ChibiOS embedded development environment for RTOS, HAL, etc. Reference manual Troubleshooting forum
3. uGFX embedded library for integrating displays and developing GUIs. Drawing commands Gdisp
4. Eagle PCB design software

1. Driving an E-ink display using STM32L152
2. Introduction to ChibiOS
3. A quick view of ChibiStudio
4. Integrating DHT11 sensor through STM32 and ChibiOS
5. C pointers
6. Arrow member selector operator
7. Typecasting in C
8. Structures in C
9. Structures and pointers
10. Static data type Tutorial 
11. Typedef Tutorial

Required Materials

How Does the E-tshirt Work?

Sensors

The data obtained from these sensors are then processed by the microcontroller to generate accurate values of the autonomic data. This is then displayed through the e-ink display.

E-ink Display

Power Circuitry
In this project, the system uses a 3.7V 600mAh Lithium Polymer battery that powers the microcontroller and the E-ink display. As shown in the block diagram, the LD3985 provides the 3.3V power required for powering the STM32L152 controller. The E-ink display requires a range of voltages for driving it because of the FETs inside them. It requires +22V,-20V,+15V and -15V for powering it. These voltages are produced by the onboard circuitry. 

The LT3463 takes power from the battery and produces +22V (GVDD) and -20V (GVEE) based on the VPOS_CTRL and VNEG_CTRL signals coming from the controller. The +22V is then fed to the LM78L15 to generate +15V (VPOS) and the -20V is fed to the LM79L15 to generate the -15V (VNEG). LM358 is also used to produce a common voltage (VCOM) from VPOS and VNEG that is fed to the E-ink display as well.

Installing ChibiOS, GCC, openOCD and uGFX

After installing ChibiOS and setting up the GCC (for compiling the C projects), openOCD (debugger used for deploying the hex code onto the microcontroller) by following the tutorial above, we can proceed to install uGFX. 

Testing the Blink Program on STM32 Discovery Board

while (true) {
    palSetPad(GPIOB, GPIOB_LED4);
   chThdSleepMilliseconds(500);
   palClearPad(GPIOB, GPIOB_LED4);
   chThdSleepMilliseconds(500);
 }

GPIOB_LED4 is the LED on the bottom right labeled as LD4 on the discovery board. This is the LED that will be blinking. To upload this program, do the following:

1. Save the project (ctrl+s).
2. Right click on the project and build the project.
3. Click on run openocd button on the top toolbar.
4. Browse and select the new file created in the build folder inside the project folder with the extension as .elf. Example: ch.elf

Once you finish uploading you should see that the LD4 LED is blinking. If it is not, try going through the previous steps and checking your install. You can also try asking in the ChibiOS forum. 

Note: If you want to try setting/clearing other GPIO pins, you will have to edit the board.h file and change the pin configurations for that respective pin inside it.

Uploading the E-tshirt Code

If the project gets built successfully without errors, then we can move to the next step.

Connecting the Sensors

Heart Rate

Note: You would have to remove the LCD on the discovery board, as several of the GPIO pins are connected to it. 

GSR (Galvanic Skin Response)
The GSR of the body is basically the skin conductance. Our sweat glands open up when we are in situations of stress and hence increases the skin conductivity. We can measure this skin conductivity using the 12-bit ADC on our STM32L152. Connect the PC0 (Port C, pin 0) pin to a long connecting wire attached to an electrode. This electrode needs to be placed on the skin to be able to sense the conductivity. We need to provide a 3V line via an electrode to the area near the first electrode as well. 

Skin Temperature

To detect skin temperature, I am using the LM35. When placed in contact with our skin, it can detect the skin temperature using this formula: temp = (3.0 * analogRead(tempPin) * 100.0) / 1024; The signal line from LM35 is connected to PA5 (Port A, pin 5). 

Note: All the pin definitions can be seen in the board.h file (src->board.h). 

Upload the E-tshirt code and uncomment lines inside each TestThread() in the sensor_task.c file to view the data from the sensors on the serial monitor. For example, in TestThread2() for getting the ADC values from LM35 temperature sensor, you will see a commented line for:

//test_println(""); //test_printn(temp);

Uncomment those lines and save, build the project. Next connect your Serial to USB adapter to PA9 (TX) and PA10 (RX) pins on the discovery board. You can now view the data from the sensor on your serial monitor (I use the arduino’s serial monitor) after you upload the edited code onto your discovery board. Make sure you have a common ground between the Serial to USB adapter and the discovery board. You can do this procedure for each sensor to check if they are connected and working properly.

Designing the Electronic T-Shirt PCB

Beta Board

For this project, I had designed and made two different PCB boards. The first board or the Beta board was made to test the eink display. This two layer board only had the power circuitry required for driving the eink display. And connected the data lines for the eink display from the STM32L152 discovery board to the Beta board. You will anyway need a breakout board to connect to the 39 pins on the eink display, so you can download the schematic and board file for making this two layer PCB and fabricate it. I had my boards fabricated from PCB power, you can then use this board to test the eink display and verify that the code is working properly. Solder the components onto this board and test it out.

Alpha board

This is the final four layer PCB board that consists of the STM32L152 controller and all the power circuitry required for driving the E-tshirt. This board will be embedded onto the tshirt along with the sensors. The minimum trace width for these boards are 0.007 inch/7mils and via size diameter of 0.002 inch, drill of 0.015 inch. You will need a hot air blower to solder the parts onto this board.

Download the Eagle board design files for both the boards from here: E-tshirt PCB designs and the component list.

Tips for Soldering

Uploading the E-Tshirt Code to the Board

For testing the beta board, you have to connect all the data lines from the Discovery board to the connector on the beta board. You can refer to the schematic of the beta board and the board.h file from the E-tshirt source code to understand which pin is which.

Embedding the System Onto the T-Shirt

I also used the same material to create pockets for holding the eink display and the controller board. This lets you remove those components so that you can wash the-shirt later. Hot glue is also another good adhesive that can be used to keep the parts in place.

Resources for the Electronic T-Shirt Project