Hi Bastian, answers below:
Does it need to be connected to a WiFi/second device to log/transmit the presses outside the button?
No, the 1-Button isn't a network device. Part of the experiment in open instrumentation is to have simultaneously "maximum (personal) access" and "maximum privacy." So the instrument records the observation on an internal data store, which is accessible via mini USB cable. When you plug into your computer with the standard cable, it shows up as a drive, and the data is a .csv file with a time stamp.
Are there schematics for how to build your own?
You can definitely make your own! This is a prototype and a bit fiddly in some ways but the nice thing about the prototype is that the board is just an Adafruit board like this and you are welcome to the firmware. The casing is 3-D printed. It uses a GPS chip to get the time. There is some soldering involved.
I'm asking as less than 2 weeks ago I started tracking my own subjective sleep quality through two re-purposed Amazon Dash buttons that are on my night stand. After waking up I press one of them, respectively logging "slept well" or "slept poorly" into a Google Sheet. But I'd love to hear what alternative ways there'd be to do this!
My intuition is that for recording observations like the exact ones you're describing, where you can keep the instrument in the same place, on a table, within easy reach and connected to your own wifi network, your solution is going to be easier. But if you want to record observations from other places then a solution like the 1 Button starts to make sense.
We have one here you can try out for a project if you want.