There is an old fashion toy that you put on a ramp and gravity will walk it down. I wanted to propel the toy forward without using a ramp so came up with an idea to use a swinging weight to move the center of gravity over the front leg and then the rear led which causes it "walk". And it works. Not very elegant or efficient but kind of funny.
Check out the video as I show how the principle works and also have some footage using the robot in an unpowered mode on a ramp; it still works with the power off. This version of the robot I had to add a couple of cambered feet pads so that it would rock back and forth a little bit.
Step 1: The Servo
This photo shows a standard sized servo that holds an arm with a counterweight at the end of the arm.
The servo is powered the 6 volt battery pack on the back led (left leg).
The legs are just two pieces of wood connected with a metal hinge and each having a foot.
Step 2: Arduino...
On the front leg(foot) is an Arduino Uno and lying on top of it is a 9 volt battery. I use the 9 volt to power the Arduino and the 6 volt battery pack to power the servo. All the ground wires are hooked into the Arduino.
Step 3: Counter Weight
I glued a magnet to the end of the servo arm so that I can easily add or subtract weight from the arm.