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R C Servos

Servos or RC Servos: Servos are DC motors mechanically linked to a potentiometer (variable resistor). Signals (or pulses) are sent to the servo and are translated into position commands by electronics inside the servo. When the servo is told to rotate, the DC Motor is powered until the potentiometer reaches the value corresponding to the position transmitted. Typically servos have a range of motion of 90 to 180 degrees. They can be modified to turn 360 degrees but this disables the ability of a Servo to stop at a certain position.

RC Servos are are used in radio control vehicles (planes, cars, etc.) and are also used in robotics. While specialized controllers for Servos can be constructed it is also possible to use PICS such as the Basic Stamp to control Servos. Some of the lesson plans for the Basic Stamp II show the relatively simple wiring required to control the position of a servo using a potentiometer. The control demonstrated in the following video was constructed with a Basic Stamp I using schematics and a program from Basic Stamp educational materials.

Picture of typical servo

Video: Servo Controller Demonstration
Close-up of Basic Stamp I servo controller circuit

Servo's are closed loop devices in that they must be "reminded" regularly the position they are to hold. If a PIC is used to control a servo then the programmer is responsible for making sure the servo is "reminded" of its position at the proper interval. There are specialized controllers for Servos that will hold the position of the servo arm without regular intervention from the PIC.

Servos come in a wide variety of sizes and toques (the amount of power the motor can exert). New Servos start in the $10 to $12 range and can run into the hundreds of dollars. My careful shopping at surplus stores, ebay or by watching sales at retail sites you can often find servos for less than $5 each.