[cs2818]

Any good solutions for getting GPIO, I2C, SPI on a phone?

[NullPrefix]

You can use ESP chip and make it talk GPIO/I2C/SPI while communicating with your phone over WiFi.

[singularity2001]

Or use ESP chip and make it talk via USB serial (android host mode adapter < $5). Much superior to RPi+shitty screen.

But rooting can be a PITA if java@android isn't enough for you.

[rutierut]

On some the volume buttons can be used as GPIO

[Jaruzel]

You can get USB based GPIO boards for ~$20. Some need drivers, some emulate serial. I'm guessing that with a USB-to-go cable, you could connect these to the phone.

[imtringued]

Then you're back to $30 and can just buy an ESP32 with a screen module but suffer from the indeterministic USB stack. However you could also buy a $6 ESP32 and let it communicate with the smartphone over blue-tooth at a price point of $10 (phone) + $6 (ESP32).

[Jaruzel]

Agreed. There are many ways to skin this particular cat. It all depends on your use-case.