This project can be a good introduction to soft circuits and how to build a basic circuit. It uses conductive thread, CNC cut conductive fabric and resistive plastic, or Velostat. An RGB LED is sewn in to the bird design as it's eye and changes color when it is on and the three homemade pressure sensors are pressed.
After building this project you will hopefully have a better understanding of what a RGB LED and a basic circuit is. As well as picking up some essential techniques for building soft circuits.
The bracelet circuit uses a basic circuit which consists of a LED, power source and a resistor. Let's go through and talk about how each of these components work and how they translate into a soft circuit bracelet.
This project swaps hard metal parts for soft metallized fabric and thread. Typically copper wires or screen printed metal traces on a circuit board connect all the components together. Metal is conductive, so it allows electricity to flow from the battery to the LED, causing it to light up. Instead of wire, this project uses conductive fabric. Connections on a circuit board between the wire and the components are made by soldering with metal alloys. Instead of using solder, we will be making these connections by sewing with conductive thread.
Before we talk about the resistor in the circuit, let's go over what it does and why it's needed. A battery provides a specified amount of voltage and current. The LED takes a certain amount of current and voltage to operate. If it gets too little, it won't light, if it gets too much, it will blow up (a minor explosion, finding an LED's limit can be quite a fun experiment). The resistor limits (resists) the electrical current to a safe level that won't blow the LED out.
The resistor in the above circuit diagram has a value of 220 Ohms, a standard value that works when powering a LED. LEDs usually operate with around 2.2 volts, depending on their color and manufacturer. This bracelet has a resistor too, it actually has three, one on each color channel of the LED. Instead of being a piece of hardware, we will use thin, sewable plastic called Velostat. It is impregnated with carbon black which makes the plastic electrically conductive. It has a pretty high resistance when left alone, much higher than wire. When the electrical current runs through the plastic and pressure is applied, the amount of electricity let through changes, which is why the brightness of the LED changes.
Want to know more about how current, voltage and resistance relate to one another? I recommend reading more about it over at Sparkfun, they have a great intro to current, voltage, resistance and Ohm's Law.