Page 238 - The Unofficial Guide to Lego Mindstorms Robots

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Outside the tidy world of LEGO MINDSTORMS, however, robots have all sorts of different actuators. Three good
possibilities for do-it-yourself actuators are servo motors, solenoids, and Shape Memory Alloy (SMA) wire:

servo motors
Servo motors are special motors that are used in radio controlled cars and airplanes. They are actually an assembly of a motor,
some gearing, and some electronics. In response to an electronic signal, they rotate an output shaft to a certain angle.
Interfacing a servo motor to the RCX is a matter of making the RCX produce the right signal. You'll probably also need a
power supply for the servo.
The bottom line, however, is that you can get just as much done with a LEGO motor and a LEGO rotation sensor. The whole
point of a servo is that it rotates to a precise angular position; but that's just as easy to do with a motor coupled to a rotation
sensor. As a matter of fact, the LEGO-only solution is more flexible. While a servo has only a limited range of angular motion,
the motor and rotation sensor have no such constraints.

solenoids
A solenoid converts electrical power into a small linear motion. You can harvest solenoids from discarded fax machines and
cassette players, or buy them new from a supplier like Jameco.

SMA wire
Shape Memory Alloy is a special kind of metal that changes shape dramatically when it's heated. You can buy wires that
contract when they are heated. Conveniently, you can heat SMA wires by passing current through them. SMA wire should be
simple to interface to the RCX's outputs. Note, however, that heated SMA wire can melt LEGO bricks—mounting the SMA
wire may be a challenge.

Online Resource

General Information

Mindstorms Sensor Input

http://www.plazaearth.com/usr/gasperi/lego.htm
This is the definitive online resource on the inputs of the RCX. It contains a wealth of information about the inputs themselves,
the official LEGO sensors, and sensors you can build yourself. It's full of schematic diagrams, graphs, explanations, and links
to other people's RCX sensor web pages.