In my UME Mark II's (UME = "Useless Machine Ever"), originally I would program an Atmel AVR microcontroller to turn an RC servo forward and backward using timed pulses like you're supposed to. But, when you want to build many machines, microcontrollers are not the way to go. Simple polarity-changing circuits are the way to go. But, you still want the RPMs of a servo without the hassle of the pulsing control. So, you hack the servo and make it a geared motor. Easy! Here's how...
Grab yourself one o' them thar RC servos for a good price at your local hobby shop or on-line. They look so innocent. If you listen, they make a whimpering sound because they know that phillips head screwdriver in your hand is there for kicks.
I hope that my lack of knowledge in all that I do is entertaining and not a hinderance in the usefulness of these posts. :) So, that said, look at the above picture. I took off the star-shaped armature thingie.
Unscrew the screws in the bottom of the case of the servo-soon-to-be-geared-motor.
Remove the cover on the bottom of the servo. You'll see nifty circuitry and the spots where the three wires are soldered to the board.
Carefully remove the top of the servo that contains the gears and pins and lubricating goo. I bold the word, "carefully" because you need to put this all back in the way it came out. If you can't get it all back together, you will have a nice pile of gears, pins, circuitry, a plastic case, some wire, and a neato DC motor. Take pictures if these don't cut mustard. They can help back track the destruction and make it reversible.
Shimmy the neato circuitry and motor guts out of the plastic case. The potentiometer (black thing standing on thick leads opposite the motor) is used by the circuitry to indicate where the shaft is positioned at any given moment. Once the hack is complete, it will be unused. I'll give you some options for it later on.
I used my ACMG robot (aligator clip magnifying glass) to hold the guts while I worked. Desolder the three wires from the board. Remember not to heat the stuff you work on with the soldering iron for too long. That heat can travel to components that don't like warm weather and can cause severe rash or sunburn or death.
Solder the red and black wires to the motor leads. Once you do this, the servo is simply an inexpensive geared motor. The robot makes this very easy to do. You can use a wife, girlfriend, son, daughter, or even an uncle to hold the work, but they're nowhere near as steady as a the alligator clips.
Wires soldered. Route them between the pokey little component leads sticking out of the bottom of the neato circuit board because there isn't much room between the PCB and the case cover and the wires have to travel across the board to the hole in the case. I like the path I chose. It looks like a... Well, it doesn't look like anything, but you can pretend.
View of the servo... Er, almost-a-geared-motor (now) looking at the underside of the top of the servo (with the gears) and the top of the main body (at right). The bearings on the black gear are tiny and cool. That black gear has a little nub on it that acts as a stop at one of two positions 180 degrees from each other in the rotation of that shaft. We need that snipped off and trimmed.
See the nub? It's what I'm holding onto with the wire cutters. You need to trim that down flush with the shaft and with the face of the gear (the part of the gear parallel to the back side of the wire cutters in this photo). That will prevent the gear from stopping against the stops that are built into the case of the servo.
Look on the right side of the gear, just below the bearings on the rear of the face of the gear. Make sure there are not bits left behind or you will hear clicking as those remnants click past the stops in the gear box. I suppose you could clip the stops, as well. But, you can see in this photo that I have a little bit of trimming to do, yet.
Use your new geared motor as you wish. I learned to circumvent the circuitry on these from various posts on the IntarWebs. It's not hard at all. If you screw up, the servos I found locally at the hobby shop were only about $12, so it isn't the end of the world if you fubar one. In the photo above, you can see the unfinished tops and "robotic arms" from one of the Useless Machines (see post on my site about this device).