Amplifier Update: Touch Part II, Being Tidy and Not Breaking Things

I swapped the touch IC in my amplifier project from the QT1106 I got from Rachel’s Electronics to a MPR121 from Sparkfun. There were two reasons for this: You can’t buy a QT1106 after Atmel bought Quantum (the original manufacturer) so it’s hard to find a supply or even one more of them. More importantly, I fried the QT1106 when I shorted out the LED board a few months ago so I had to choose something.

I’m starting with the Wire-based code from Bildr’s tutorial, which uses the standard I2C pins on an Arduino. I looked into using the softI2CMaster library from fat16lib so I didn’t have to use the hardware I2C pins but decided it isn’t yet worth the time to port the code.

The MPR121 is a 3.3V chip so you need to use a level shifter to connect it to a 5V Arduino. I found this great circuit to do just that, and I used one of the transistors on each of the SDA/SCL/IRQ lines. SparkFun uses level shifters in their Arduino shield version on only the SDA and SCL lines, which makes sense because the IRQ will still register high on the Arduino and isn’t bidirectional. Bildr’s tutorial doesn’t use any level shifters which will  work but isn’t a best practice for the longevity of the MPR121. Doing it on all three lines might be overkill, YMMV.

Something I’ve learned working on the project, especially after breaking almost everything, is to be tidy when you’re prototyping. Looking back at the first stages of the breadboard, it’s almost comically messy. I didn’t trim component leads and they stuck up like aerials, I left wires to the board longer than they needed to be, it was hard to operate on and laborious to trace connections. Now that components are arranged neater and they sit flush with the board, it’s easier to trace connections and rearrange components. It’s also easier to visualize the space that circuits are occupying and will likely occupy when on a PCB.

The other lesson is to never touch anything that’s powered (duh). You might not hurt yourself, but you could still very easily kill hundreds of dollars worth of equipment. And after you kill hundreds of dollars of equipment you need to build it all again. The easiest way to get the circuit up and running quickly is to have more of the components that you need ready to go. Mouser and Digikey both charge \$20 flat rate shipping, and it’s fast too, but it adds up quickly when you need to make five orders instead of two. I’ve now broken two Arduinos, and I’m using #3 while #4 sits by in case I need it. Yes they’re \$30 a piece, but that’s not much more than the cost of shipping alone.

I’m looking forward to having more time this summer to finish up this project, especially to see the electronics produced and put into the enclosure.