Have an old toy car? I hacked mine using Arduino and an H bridge circuit to control the motors, used my Sony Z1 Android phone to control it with the recent 1Sheeld I got from Kickstarter.
I thought to use the Gyroscope sensor in to drive it but after experimenting, I used the gravity sensor instead, will explain in details why later in step 5.
Step 1: Required stuff
1- Android smartphone / tablet ( x1)
2- Arduino uno ( x1) $26
3- 1Sheeld ( x1) $60
4-Cheap RC car ( x1) (you can get it from a toy store)
5- Connectors male female (x4) $5.45 per 40 set
6- H bridge ( x1) $9.51
7-- Mediabridge USB cable (x1) $7.99
8- Male Dc Plug (x1) $2
2- Wire cutter