It has been an interesting week here at Smart Central, most of which I spent wrestling with the helpful language used on websites selling components and in their respective datasheets. Even though I have now been trying to get serious about prototyping for more than three years, some instructions given with regards to how to activate certain functions offered by components still baffle and confuse me deeply. So much prior knowledge is just assumed to exist on the part of the creative technologist by the authors of these sites and datasheets, and unless you happen to know someone you can ask what something means exactly, and more importantly how to execute a certain instruction, you run the risk of ruining your components. So I thought I’d write this post about my recent experience with the Adafruit Standalone Toggle Capacitive Touch Sensor breakout.
In theory, this is a beautiful component to use to 
incorporate a touch sensitive switch into your project – you can even replace the integrated touch pad with a conductive surface to make touch sensitive keys that blend in more discreetly with your project by soldering a connector into the pin below the touch pad. There is a momentary version of this sensor available, which is only active when contact is made, but for my current purpose the on/off toggle function works nicely. Now, because space is at a premium when working on a jewellery scale, even the tiny dimensions of this sensor (about 1.5cm x 2.5cm) were too big for my project and I decided to be daring and simply lop off the redundant integrated touch pad and status indicator LED with a jeweller’s saw. I don’t advocate this as the best way of quite literally ‘hacking’ a component, but in this case I felt the value of the learning experience outweighed the risk of ruining the sensor (I bought a spare just in case). However, much to my surprise the maimed component still worked perfectly afterwards – I made an educated guess about the connections I was savaging, and it seems to have paid off. Great – so far so good!
The next step is where things really started to unravel for me – I wanted to make use of the automatic timer function the sensor had to offer. I had read the following instructions in the Adafruit guide for this part (which covers all their touch sensors but none in greater depth):
It also supports a configurable time-out to turn off the output automatically after a delay. To select this mode, cut the ‘TIMER’ jumper and connect a resistor & capacitor to the TIME pin. For a circuit diagram and resistor/capacitor calculations, see page 13 of the datasheet.
You can also just connect TIME to Vdd and the chip will turn off approx 15 minutes after being turned on. Connect TIME to OUT and the chip will time-out approx one hour after being turned on.
Wow. There are a lot of assumptions of prior knowledge in that paragraph. What is the ‘Timer’ jumper (or indeed a jumper)? How do you cut it? Does the second part of the paragraph about the pre-programmed time-out function also require the jumper to be cut? Do you have to add a resistor/capacitor in that case? I decided to look at the datasheet to gain clarity. Unfortunately, the datasheet is not for the actual breakout board, but for the processor used on the breakout. It is highly technical. It did not address any of my questions, as it is clearly written for a highly specialised audience of electromechanical engineers, who know exactly what they’re doing. I was just about able to decipher some of the instructions relating to the timer function, but what I really needed was the map of the different connections and resistors used on the breakout board, also known as an eagle schematic. These I found tacked on at the very end of the guide thankfully, and soon things started to become clearer. Let’s take a look at the back of the board to start with.
A ‘jumper’ I found out after much googling, is a short length of conductor used to close a break in or open, or bypass part of, an electrical circuit. This can be either a separate component, a simple wire or a printed trace on a PCB. In this case it turned out to be the latter – the toggle breakout indeed has two jumpers, one for the timer function and another for the LED indicator of the integrated touch pad (hacked off in my case):
To ‘cut’ the timer jumper, I discovered, means simply to use a sharp scalpel and scrape away the small bridge between the two larger pads:
This, according to the eagle schematic, changes the state of the timer pinout to ‘high’ (or active) by removing its connection to ground (which rendered it ‘low’ or inactive). It is apparently possible to undo this change by connecting the two pads with a blob of solder, but I haven’t tried this as of yet. It is then merely a matter of soldering a wire between the TIME pin and either the VDD pin (15 min auto turn-off) or GND pin (60 min auto turn-off). You can also set the auto turn-off to any interval you like, by adding resistors and capacitors of the appropriate value as specified on the datasheet, but for me 60 minutes will be just fine.
This may seem like a lot of work merely to figure out a single component, but in the process I have also demystified the language used in PCB instructions and gained more knowledge, which is always a good thing…until the next time!