Pocket Pi!
This is a mini computer with 3.5 inch touchscreen. May be a little smartphone. You can build it under 30$!
Required Metarials
Collect these things-
1. Raspberry pi zero - 5$
2. 3.5 touchscreen with pen - 15$
3. 16 or 32 gb sd card - 6$
Process
Very simple.
Connect the touchscreen (HDMI) with HDMI connector with raspberry pi. Input the sd card in the pi slot. You should upload the Raspberry pi official OS. Now fit it in a box. You can make it with cardboard.
Power
You must need a power supply. I recommend you to use 2x3.7 volt battery. Lipo battery is better.
Thanks
Thanks to all. Vote me for pocket sized and microcontroller contest. My other instructables-