This was my first electronics project – a simple self-balancing robot.
The wheels and chassis are from a toy truck. Onto that I salotaped a breadboard with two Light-Dependant-Resistors, attached in series. On one end I feed 5V, on the other end, ground. This gives a voltage divider. And in the middle I connect that to a resistor and a capacitor in parallel. This gives me a Proportional Derivative signal (out of a PID controller).
This means that we can read off the current ratio of brightness that the two LDRs see, then add on to that signal the rate at which they are changing. This helps prevent driving the motors too fast when we already moving rapidly towards the balancing point.
We then read this analog voltage on the Arduino, and use it to control the direction of the motors.
There is a huge amount of gear slop which is why it can only barely balance.