For 2019’s iteration of The Long Con, I volunteered to help wrangle up some brain teasers for their “capture the flag” contest – a tech convention standard wherein attendees compete to solve assorted technical puzzles. As my experience with “cool stuff” mostly lives in the realm of circuit board design, I opted to contribute a circuit-board based puzzle. I think it looks pretty sharp with the silkscreen logo!
As this is a small local convention, there wasn’t much budget available to go off-the-wall with technical capability – so, my three design goals were: 1. it has to look cool, 2. it has to be cheap, and 3. it has to do something neat. With these design goals in mind, I began my digikey search for an inexpensive but capable microcontroller. I settled on the attiny84a based on it’s capacitive touch capability, as well as an internal temperature sensor. These on-board sensors meant I wouldn’t need to add external sensors, which would increase the unit cost. You can get more info about the attiny84a here.
Designing the circuit board was relatively easy, based on Atmel’s design guides for capacitive sense (this is via their QTouch library). The hardest part was designing the capacitive touch sensor – I opted for the simpler rotary sensor design due to its ease of entry into KiCAD. I tried a few different ways of importing their recommended design, but couldn’t quite get it to work.
I had intended to use svg2mod to import the auto-generated footprint, but the polygon approximation didn’t seem to work out to a workable resolution. I didn’t feel like going any further down this rabbit hole, so I opted for the simpler design that Atmel’s documentation suggested.
The rest of the design was reasonably boilerplate MCU work – I stuck to standard components that were available from Digikey, and preferred components that exist in their KiCAD libraries. I have uploaded the KiCAD project to github, check it out!
The software was very dependent on Atmel’s QTouch library – most of the challenge was actually in configuring their IDE to work correctly. I wrote up my experiences on their forums here. Based on this experience, I have resolved not to use Atmel components for a while… I got the feeling that Atmel Studio is a second class citizen since the Microchip buyout.
The actual software itself is not terribly complicated (or optimized) – Atmel’s QTouch library, for all of its installation issues, was easy to code with. I had wanted to configure the firmware to spend most of it’s time in a low-power sleep state, but after doing some quick calculations, the battery would last the duration of the conference on a coin cell without the low-power modes. And so, I spent no time optimizing this device for power consumption.
You can see the source code for my solution on github here. It is far from the “cleanest” code I’ve written, and was written in a time crunch. But, it did work!
For this unit, I opted to use reflow solder techniques. I purchased a Whizoo upgrade kit to modify a toaster oven, but I did not get the parts in time. I ultimately used a thermocouple and manually rode the thermostat on my toaster oven to follow something like a soldering profile. It seemed to work reasonably well!
For flashing the firmware onto the attiny84a, I placed a contact pad that mated with a spring loaded 6-DIP connector on the back of the device. To make for an easy flashing process, I exported a 3d model from kicad and used it to make a 3d-printable jig. I had to manually line up the spring loaded connector and hot glue it in place, which was a little tedious, but worth it in the end.
The project was an overall success, and people enjoyed the challenge. Unfortunately, out of the 30 that I had tried to manufacture, only 6 worked to their fullest extent! I attribute this to my ambitious use of tiny resistor arrays – it appeared most of the failed units featured poor connections on these components, and my reflow repair skills were simply not up to the task. Luckily(?) only a couple teams got far enough in the CTF to get to the “PCB challenge”, so we were never short on units despite the poor yield.
Another conclusion – try a different MCU vendor next time, Atmel Studio/QTouch installation was a torturous experience!
The general intention was for participants to look up the datasheet of the single IC to see what peripherals it had – QTouch and temperature sensing – and work from there.
Puzzle 1 – this conference takes place during the daylight savings time change. Users had to “turn back the clock” by using the capacitive rotory sensor on the clock face. 4 complete rotations would prompt the LEDs to display a byte pattern.
Puzzle 2 – the weather had just begun getting cold in Manitoba (where this conference took place). Users had to get the unit 15 degrees Celsius colder than when they plugged in the battery. This could easily be done by taking the unit outside. This was a fun puzzle to figure out from a software point of view – the calibration on the sensors was not great out of the factory, and I didn’t intend on taking multiple calibration points for each board. So, it simply looks at the temperature delta since the unit turned on instead of comparing against an absolute temperature.