This circuit is crude but it does provide an inexpensive way to drive or test servo motor systems without the need for a complete system.
This circuit has also been used to drive the - Victor 884 Speed Controller. The purpose of this was to test a controller that was thought to be damaged. The circuit could also be used to operate the controllers without the need to have a full robot controller unit available.
http://home.cogeco.ca/~rpaisley4/xServoTest555.html