A year ago (Halloween 2014), I attended my first MakeCation at Parachute Factory. I got into a conversation with several others about capacitive touch devices. These devices are based on capacitive sensing. They are touch (input) devices – similar to the screen on modern cell phones – that are used to transmit a signal to a processor – cell phone, computer, electronics, etc. We discussed how easy and inexpensive these touch devices are to make … that’s when I fell into the rabbit hole!
Since my personal interests are in (inexpensive) accessible devices, I wanted to know how to make these. Many of the inexpensive capacitive touch or capacitive sensing processors I found are based on Freescale Semiconductor’s Proximity Capacitive Touch Sensor Controller (MPR121). This sensor uses I2C communication. Each MPR121 is capable of handling 12 distinct input. But these are pretty tiny and not to use as is.
Fortunately, several manufacturers make breakout boards with these MPR121 processors. I purchased my first MPR121 breakout board from Adafruit as their website had a nice description and tutorial. These breakout boards are then connected to a microprocessor which interprets the touches/signals and does something with it. As many as four of these breakout boards may be connected to one I2C bus. At that time, I was playing with the Arduino Uno. There is just one I2C bus on the Uno.
Plugging one of these MPR121 breakout boards into my solderless breadboard was OK but once I added more, it looked like a mess of tangled wires! But this mess allowed me to learn to use them in different setups. But soon, I grew tired of debugging the messes I made! My first prototype board with four MPR121 breakout boards worked well (stable) but was not very pretty.
Although not pretty, my prototype (MPR121-PB) allowed me to work and understand the rest of the circuit. The Tutorial at the Adafruit website showed me everything I needed to get started. This Tutorial used aluminum foil as the sensors. I had some aluminum tape from my local hardware store which worked just as well. The nice part is that the tape will stick to any place I put it — sometimes intentionally, sometimes not.
Here’s a picture of several strips of tape put onto a clear piece of acrylic I had lying around. For each of the pads, a strip of aluminum foil is taped around the bottom of the front to the back. This back part is where I taped a pieced of wire. Note that I used a separate piece of foil tape to keep the wire in place but the top of this second piece of foil tape is not electrically connected to the one below it! The only purpose for this second piece of tape is to keep the wire secured onto the (conductive side) of the first piece of foil tape.
Using alligator clips, I connected the bare wire on each of these touch pads to one of the 12 sensors on the MPR121-PB . Soon, this became a wire-mess I wanted to avoid.
In my next version, I used flat ribbon wires, separated each branch, stripped an inch of plastic from the ends of the branches and taped the (bare) stranded wires directly onto the touch pads. OK, now we’re getting somewhere!
I can plug this set of 12-touch sensors directly onto any one of the Adafruit MPR121 breakout boards on my MPR121-PB. With these working, stable pieces – MPR121-PB and touch pads – I can start to play!