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Sub Micro Servo Motor Hack

April 1998, Written by Jim Brown
August 2002, page format revised, links updated by NCC

The first time I saw these wonderful servos was when Ed Koffeman showed them to me on his robot the Echo Gecko which he was building for the IEEE Fire Fighter's contest. He was using these tiny servos for his camera's pan and tilt. I asked him about them, and the first thing he mentioned to me was that they were about $25 each. Ouch, I thought since normal servos run about $12 each. However, they were too cute to pass up. After several of us guys went to watch the IEEE fire fighting contest, we stopped by Roy's Hobbie shop in Hurst and Ed pointed out the price of these Sub Micro Servos. They were $21.99 - a real steal! Well, the first thing I asked Ed, was, "are they modifiable?" He said he wasn't sure, but he told me to follow him out to the parking lot and he would take apart one that he had off his IEEE robot. We went out there, and took a look at one's insides, and after careful inspection, we both agreed, that it looked like it would be possible. So, I ran back inside, and purchased two so for a really tiny robot that I thought I might be able to make.

Side View Top View

At first I was a little sceptical, and I started to have regrets about buying these expensive servos, since they were more cute than practical. Then, after the initial shock of remembering that I should tell my wife what I spent about $47 dollars after tax for, I started thinking of how cool it would be to have such a tiny robot. I remembered a web site that the Seattle Robotics Society had about hacking a normal servo, and I remember how they did it and how well it was written (so I didn't even need to go back and look at it). I also thought that since it helped me so much that I would document what I did on the Sub Micro Servo in hopes that it might also help someone.

Expanded View Side Body View

I started by removing the four screws on the bottom of the servo's case. You need a super small screw driver to do it as is expected. When you open up the case, what you find is the bottom shell, the body in the center which contains just about all of the guts, the top shell, and the top wheel for attaching things.

Bottom Body View

The body contains a super small motor about 1/2" by 1/2", a circuit board for reading the pot and controlling the motor speed and direction, a 5k Pot with a long shaft for the gears, and of course, the gears on top. The circuit board is in the case at an angle and there's just enough room to add a couple of resistors to perform the modification. I really like the cute size of that tiny motor. It's about the size of a large pea! The circuit board is about 1/2" by 1/2" too, and there's very little in the way of extra wire inside the case.

Gears Removed

The first thing I did was to remove the gears. The first gear is tightly fitted to the top of the Pot shaft, so you have to pull hard to get it off. On that top gear is a really, really, tiny bearing. If you do this hack, you just have to take a look at this cute little bearing with the tiny balls inside. Next, I removed all of the gears in order, and placed them carefully so that I would know how they go back in order. Actually, though, I found out the two shafts are different size, and they won't go back in any other way than the correct order, so it wasn't really that big a deal.

Motor Removed View of Pot

Well, next, I tried removing the motor, and it was no trouble to push the motor from the top down to remove it. It is still connected to the circuit board, and I didn't try to clip the wires. I tried inspecting the Pot to see if it would come out as easily. I couldn't remove the pot at this point since it has a plastic carriage underneath to prevent me from pushing from the top down, and it's connected to the circuit board to prevent me from pushing it from the bottom up.

Clipping Pot Leads Motor Detached

So, I got out some wire clippers and clipped the three leads to the Pot. There was a very tiny 2.2k resistor attached to the pot on one leg, and I left it with the circuit board, or in other words, I clipped the pot's leads between the resistor and the pot so I could keep the 2.2k resistor.

Removing the pot View of carriage

So that separated the pot from the motor and the circuit board. The Pot sits in a plastic carriage, much like a spare tire hangs under a pickup truck. If you just pull upward on the pot, at this point it can be removed very easily. In the picture, you should see the little plastic carriage T that I'm talking about.

Pot Tabs Pot Tabs Removed

With the pot now removed, I needed to make the Pot where it could turn 360 degrees for my servo hack. The Pot has two metal tabs that are slightly bent downward that hit two plastic stops inside it's black case. To modifiy the pot, I used my Dremmel Tool to buzz out the metal tabs, the metal contacts that touch the resistive material (to keep it from hanging up later), and I went ahead and removed the plastic stops. With the metal tabs and the plastic stops now removed, I could turn the pot shaft smoothly in 360 degrees in both directions. Of course, this modification ruins the original function of the pot, and make it just a type of axel.

Gear Tabs Gear Tabs Removed

Besides the Pot Tabs, there are also Plastic Tabs on the top gear which keeps the Wheel from rotating 360 degrees. For 360 degree rotation, these tabs must also be removed. Again, using the Dremmel Tool, I buzzed out these plastic tabs.

Gears Replaced

Next, I put the gears back on their axels.

Resistors Added

To replace the Pot, a resistor network needs to be added. The Pot was a 5k pot, so two 2.2k resistors are close enough to substitute for the 5k pot. So, hook one 2.2k resistor from what used to be the center pot lead to one of the outer leads, and the other 2.2k resistor from the center lead to the other outer lead. Note that one of the outer leads already had a built in 2.2k resistor. I tried it first with the built in 2.2k resistor only, but that didn't work, so I went ahead and added 2 resistors as if the built in resistor wasn't there. So yes, there are 3 resistors crammed in there. One side has a 2.2k resistor that I added, and the other side has the built in 2.2k resistor plus another 2.2k resistor that I added. Go figure.

So, the configuration is something like this:

                  built in
                     2.2k     2.2k
circuit board top -/\/\/\--*--/\/\/\--+
circuit board center-------*----------+
circuit board bottom-------*--/\/\/\--+

* The asterisks represent where my cutter cut the pot leads

Reassembled Works!

Here's a view of the reassembled modified sub micro servo! I've run it for a while, and so far it hasn't quit. I assume that means the modification for this servo is ok and will likely last for a long time to come, hopefully. The servo I modified was a Cirrus CS-21 BB High Performance Sub-Micro No. 444225-Futaba-J.

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