12. Skin Electronics#
skin electronics 2 from Lara Campos on Vimeo.
It’s a silicone mask with SMD leds embeded in it.
SILICONE#
To begin with, I shaped a piece of silicone drawing it over my own face to get the exact dimensions I wanted. In this case I didn’t use any mold nor did any casting. It was a left over of silicone from last assignment, from which I cutted the piece I needed. Anyway, if you want to do it from the start, you need to use a flat mold and pour the quantity of silicone as thick as you want the mask to be. Also the shape can vary depending on your design.
To prepare the silicone I used Eco.flex 0-30, and mix same parts of A and B.
Then you need to introduce the mix in the vacuum to take the air bubbles from it, and so reach a smooth result. Make sure you prepare it quickly, around 3 minutes mixing and 3/5 in the vacuum, so the silicone doesn’t dry before casting. Afterwards I added some spirulina powder to make it green. Though the correct way would be to mix it before vacuuming it. Then leave it dry for at least 4 hours, or check the data sheet of the silicone you are using to have precise information about the drying time.
CIRCUIT#
Materials
• copper wire – sand it to take the coating, and try with a multimeter the conductivity of it in all the lenght I’m using
• 17 smd Green leds – sold them to the copper wires
• soldering machine
I designed the circuit thinking of the light intervals I wanted, taking into account that each circuit will need a resistor. So it was better to group leds than to have each one with a resistor. So I group them in 4: eyebrow (6 leds), nose(5 leds), upper lip (4 leds)and ‘moustache’ (2 leds). Each one is a parallel circuit, and their grounds are all connected to the same copper wire.
Soldering SMD leds
Skin electronics from Lara Campos on Vimeo.
To calculate the resistance that each circuit needed I did some research, which helped me to make it possible, by calculating the specific resistance needed in parallel circuits.
Useful links#
To do my calculations I needed the data sheet of the SMD leds I was using, and therefore know the voltage they need to turn on, and the current the circuit needs to work.
In this case, the power source is 5 volts. There are different quantities of leds in each circuit, but every led needs 0.020 amps (20mA) to light up at an optimum level. The current that we need to flow from the battery is the sum of the currents (i t): i 1, i 2 ,i 3, i4 , etc.
In a parallel circuit, the battery supplies the same voltage to each one of the leds - it is as if each led is connected across the battery separately. In this case, Green SMD leds need between 2.8v and 3.6v supply.
So you can now calculate the value of the resistance needed to drop the supply from 5 volts.
Resistance = (Vsupply - VLED) / (i 1 + i 2 + i 3) which is 5.0v - 3.6v / i t = X ohms (Ω)
So my calculations for ech circuit where:
Circuit 1 – 2 leds i = 0,020 + 0,020 = 0,040 amps
R = 5v -3.6v = 35 ohms (Ω) 0,04
Circuit 2 – 4 leds
i = 0,020 + 0,020 + 0,020 + 0,020 = 0,080 amps
R = 5v -3.6v = 17,5 ohms (Ω) 0,08
Circuit 3 – 5 leds
i = 0,020 + 0,020 + 0,020 + 0,020 + 0,020 = 0,1 amps
R = 5v -3.6v = 14 ohms (Ω) 0,1
Circuit 3 – 6 leds
i = 0,020 + 0,020 + 0,020 + 0,020 + 0,020 + 0,020 = 0,12 amps
R = 5v -3.6v = 11,7 ohms (Ω) 0,12
You can’t get resistances in every value, so look at the resistances available and pick the exact value if it’s available or the nearest higher value. In this case I used 36 Ω, 18 Ω, 15 Ω and 12 Ω.
Testing the circuit with resistors using a board.
CODING#
Once I had the circuit working, I started coding the secuence with arduino. Actually it’s a pretty easy code, same as the blinking example Arduino has, but just playing turning them on and off.
This is the code I used:
void setup() {
int led1 = 1;
int led2 = 2;
int led3 = 3;
int led4 = 4;
pinMode(1, OUTPUT);
pinMode(2, OUTPUT);
pinMode(3, OUTPUT);
pinMode(4, OUTPUT);
}
void loop() {
digitalWrite(4, HIGH); // turn the LED on (HIGH is the voltage level)
delay(1000); // wait for a second
digitalWrite(3, HIGH);
delay(1000);
digitalWrite(2, HIGH);
delay(1000);
digitalWrite(1, HIGH);
delay(1500);
digitalWrite(1, LOW);
delay(500);
digitalWrite(2, LOW);
delay(500);
digitalWrite(3, LOW);
delay(500);
digitalWrite(4, LOW);
delay(500);
digitalWrite(3, HIGH);
delay(500);
digitalWrite(3, LOW);
digitalWrite(1, HIGH);
delay(500);
digitalWrite(1, LOW);
digitalWrite(1, HIGH);
delay(1500);
digitalWrite(1, LOW);
delay(250);
digitalWrite(1, HIGH);
delay(250);
digitalWrite(2, HIGH);
delay(250);
digitalWrite(3, HIGH);
delay(250);
digitalWrite(4, HIGH);
delay(500);
digitalWrite(4, LOW);
delay(250);
digitalWrite(3, LOW);
delay(250);
digitalWrite(2, LOW);
delay(250);
digitalWrite(1, LOW);
digitalWrite(2, HIGH);
delay(250);
digitalWrite(1, HIGH);
delay(250);
digitalWrite(4, HIGH);
delay(250);
digitalWrite(4, LOW);
delay(250);
digitalWrite(4, HIGH);
delay(250);
digitalWrite(4, LOW);
delay(250);
digitalWrite(4, HIGH);
delay(250);
digitalWrite(4, LOW);
delay(250);
digitalWrite(2, LOW);
delay(250);
digitalWrite(1, LOW);
delay(250);
}
I used pins 1,2,3 & 4, as to have same pins in the trial with Arduino Uno than in Attiny85. As I told you before in Wearables II, Attiny has 8 pins, from which one is voltage and one is ground. Then you have 3 normal pins and 2 which you can use for PMW (Pulse Width Modulation). PMW is used to convert the digital signal into an analog by varying the width of the pulse and this pins are used for giving the desired analog output.
FINISHES#
Once everything was working, I sticked the circuit to the mask with some more silicone. I helped myself with some tape to keep leds and copper wires in place while sticking it. Let it dry, and connected it again with arduino to check if everything was still working good.
PROGRAMMING ATTINY85#
For programming AtTiny85 I plugged it to a sparkfun AVR programmer that will be connnected to the usb port to download the code in it. To be able to do it you need to change board type, port, internal clock & processor that meets the AtTiny85 characteristics.
In this case I’ve a basic code so the language and the pins where the same (1,2,3,4).
Once done, upload the code.
CREATING MY OWN PCB BOARD#
Breakout boards are customized boards designed to be plugged with other devices like Arduino or AtTiny with different types of connections. A PCB or Printed Circuit Board is a resin & copper board where you embed the circuit. The thickness of the traces depends on the current you need, so it’s important to check out how much you need before building it. For mine I needed space for the 4 wires with each resistance, a switch, the power source (which in the plans is represented by a led), a socket for the battery and a socket to place the AtTiny.
For designing it I used a program called Fritzing.
The program to mill the PCB in the Roland machine reads different extensions. In this case it is important to save the file in .PNG, but first we need to get the file ready.
I used just the file Top Copper file and edit it using Illustrator.
After some tests I realized the size of the traces must be 3pt otherwise it is too thin for the board. Also the circles must have at least 1,6mm thickness which is two times the size of the drill that makes the holes. For the holes I used the smaller one. Then saved it as .png.
Once ready, put the components together, welding resistances and wires in their place.
The last step was to embed the battery and weld the slide switch. These stay open in one position and closed in the other.