- Design of the robot

My primary inspiration for the design was [1]. The source does not provide many details of the mechanical construction and dimensions so I had to experiment but it wasn’t hard. The drawing of my final design is below. The dimensions depend on the size of the servos used. I used standard size servos which yields robot about 12 cm long and 9 cm wide. It would be interesting to use smaller servos and make a tiny “spider” robot.
I choose to develop my own controller board instead of using a commercial one, such as BasicX. This was mainly because of my desire to get some experience with Atmel microcontrollers and also to cut down the price. Typical controller costs about 100 USD while the Atmel Tiny 2313 MCU costs about 4 dollars and with all the other parts the price of the board is still below 15 USD.

Basic theory
Don’t worry, I don’t like theory any more than you do, so this will be just brief description of the things you could find helpful.
The servos
Servo is used primarily in radio controlled models (airplanes, cars, ships etc.) but they are also very popular in robotics. The advantage is wide choice of sizes and strengths, low price and easy control with microcontrollers. There are tiny servos weighting some 5 grams but also huge ones with metal gears able to lift several kilograms on its lever.
Cheapest servo can be obtained for about $10 and for our purpose it will suffice. The prototype robot uses Hitec HS300 servos which are about 10 years old, have served their duty in various radio controlled airplanes and survived several crashes, yet they still work very well. From today’s market Hitec HS311 can be a good choice, but you can also use any of the less-known brands without problem.

Important servo parameters are:
Suppy voltage – 5 V (max 6V for some types), required current is about 1A
Strength – given in N . cm, for standard size servo typically 30 N . cm.
Speed - given in degrees per second, typically about 0,15 s for 60 degree travel
Standard size servo is 40x20x36 mm and weights about 50 grams. There is cable with 3 strands, usually red (+), black (ground) and yellow or orange (signal for the servo).
Position of the servo lever is controlled by input signal which should have frequency of 50 Hz and pulses with length between 1 and 2 ms. change of the pulse length changes the position of the servo lever. See the picture:

The mechanical calculations for the servos in this kind of robot can be found in [1]. For me it was sufficient to find out that the robot with 10 years old servos walks. How long the servos will survive is a different question, but it doesn’t seem to me that they would be overloaded when the robot is walking.