After converting my old phone to run without a battery, I needed to find a use for it. Smart displays seem to be quite popular these days, so I thought I’d try and make one!
Tidying it up
Before I could do anything, I needed turn my setup into something more permanent. I housed the buck converter and USB PD trigger board in a cheap plastic project box and fed the wires through a hole drilled through the back of the phone.
Being a budget phone, the back cover is plastic and there are no magnets or wireless charging coils in the way.
At this point I also added a Schottky diode to the output of the buck converter; that way I can still use the Pixel's USB port for data access without the worry of it attempting to charge the buck converter. A 1N5822 would be ideal, but as I only had a 1N5819 in stock I used that instead.
Optimising the software
I could've put this to use straight away, but I wanted to turn my attention to the software. Because this is going to be running 24/7, I want it to draw as little power as possible. Phones are already very power-efficient, but they have a lot of background apps and services that I don't need.
The easiest way to debloat my phone was to install a custom ROM. I went with LineageOS, which is designed to be clean and customisable. I also rooted the phone with Magisk, which allowed me to customise things further.
Once installed I went through and disabled all of the apps and services I didn't need. I also installed a firewall to block access to the internet by default, but allow something like a weather widget through if needed.
I also disabled cosmetic things like the navigation bar and battery indicator too.
Wait, why is the battery level dropping?
Curiously - despite the voltage staying constant - Android gets a little confused and still reports the battery level dropping. Why is this, if this setup can power it indefinitely?
It seems the way Android calculates battery life is quite simple - it measures the rate of discharge and subtracts it from its pre-programmed battery capacity. It takes battery voltage into account when it boots up to get an initial reading, then either doesn't realise or doesn't care that it stays constant and doesn't tally up with its calculations.
In my experience it seems to drop to 1% and doesn’t cause the phone to shut down, but this may vary depending on the manufacturer.
Energy usage
I was curious to see how much power it was drawing after my modifications, so I used a charger doctor to measure the current flowing. It jumped around a lot, but at 50% brightness, it drew well under 1W at idle - not bad!
At current UK prices of electricity (26.11 pence per kWh at time of writing), this would cost a little over £2 a year to run if kept on 24/7.
Designing the setup
Next I set about designing the setup. I took inspiration from the new-ish standby mode on iOS devices - I could've displayed a lot of information, but I wanted something simple and readable at a glance.
I waded through a lot of junk on the Play Store, but in the end I used Nova Launcher, and this selection of widgets:
- Analog Scribble Clock Widget
- Event Flow Calendar Widget
- KWGT Kustom Widget Maker
- Today Weather: Radar & Alerts
Result
After some tweaking, I was pretty happy with the result, and I have a couple of designs to choose from.
It's still fully functional, so I can pick it up and use it as a timelapse camera, or for anything else that I may need it for in the future.