Complete step-by-step directions and general application notes for building a speed control like the one on the MB1 blimp from the November 1999 issue of RCMicroflight.

These throttles are great for use on blimps, small cars, boats or other creations. Their efficiency is not as good as that of "real" ESCs like the JMP 5bis ($56), but this is not as critical as it is on airplanes and being able to make one yourself for $10-15 seems to more than make up for it (:-) They are great for use with the new MB-1.5 kit, as well as with many other types of devices (shown at the end of the article).

The circuits shown were "harvested" from new Futaba 3003 and Hitec HS-300 servos and are good for current draws up to 1A.

I've also had good luck using beat-up servos from fellow club members and others. R/c car guys regularly abuse and wreck servos. If you make friends with one of them you can get all the free "funky" servos you can use. These servos will be sloppy and may have bad cases, pots, gears, or motors, but the amp board is almost always ok and this is the only part we need.

Servo circuits are designed to be very sensitive. They have very little "deadband" and a motor may buzz or turn over every now and then when the control is at idle. This is due to both the designed-in sensitivity and to minor inaccuracies of the tx stick assembly. It's normal and not a cause to worry. It doesn't consume very much current is not a problem in any other way.

Don't pair an amp board with the same size motor it was attached to in the servo. It will draw too much current with a prop installed and will burn out the circuit. In general amp boards will work well with motors from the next servo size down: standard servos will handle the motors like the N20s which are used in microservos and 1/4 scale amp boards will handle motors from standard servos.

The motors shown on the pan & tilt unit are pretty hefty, but they turn very slowly and don't overtax the small boards hooked up to them. It's difficult to generalize about this, except to say that heat is a good indicator: if something is baking hot it won't last long.

Converting a servo to use as an ESC overloads it tremendously compared to what the design engineers intended. A very few servos will not tolerate the abuse and would have a very short ESC lifetime. When you first take the servo apart, run it for about 10-20 seconds and feel the components on both sides of the board. If any of them are uncomfortable to touch either put the board back and use the servo normally, or else, if working with a trashed servo, expect it to have a fairly reduced lifetime.

If you've always been curious about how a servo operates (but were afraid to ask), here's a simple description to help you along: Your transmitter makes square wave pulses that vary in length from 1.0 to 2.0 miliseconds depending on the position of each control stick. Once these pulses are generated, another part of the transmitter converts them into a radio frequency (RF) signal which is then radiated out through the antenna. The receiver picks up the RF signals from the transmitter and it demodulates (extracts) the original pulses. It then sends the extracted pulses to the servos. The servo circuit board is only happy when the potentiometer (pot) is set to a certain value for each position of the tx stick. If these values do not match the circuit has a built-in feedback control loop that lets it move the motor (which moves the gears, which move the pot) to bring things into adjustment. The fact that it motion is imparted to the ouput arm is a happy by-product. When you remove the gears you disengage the motor from the pot and the circuit becomes unable to self-regulate. This will cause it to keep turning in whichever direction and at whichever speed the tx stick directs it to go.

The pictures show step by step directions for making an electronic speed control (ESC) from a Futaba S-3003 or S-148 servo. Both of them are very common, easy to work, have good quality and are quite cheap ($10-15).

Step 1. Remove the 4 screws that hold the servo's bottom cover.

Step 2. Remove the servo's bottom cover and top section.

Step 3. Lift up all the gears. To remove the last one need to first pull off the shaft. Grasp it firmly with your fingers and pull up.

Step 4. Push outwards in spots A and B, then press down on the brass shaft.

Step 5. Press out the motor by pushing down on the top of the gear.
Step 6. Wiggle the entire assembly free of the servo case.
Step 7. Plug the board in and check that all works ok.

Step 8. Unsolder the motor from the board using de-soldering braid as shown.

Step 9. Attach a set of wires to the contacts.

CONGRATULATIONS! You now have a miniature bi-directional micro ESC.

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If you're comfortable working with a soldering iron and want to make the ESC smaller, lighter and prettier there are 2 more things you can do:

Option 1. Shorten the servo lead. In addition the being lighter, a shorter lead will have less resistance and thus be more efficient.

Option 2. Replace the stock potentiometer with a miniature "trimmer" pot. You will need a 5K-ohm (5 kilo-ohm), linear taper model. Depending on where you shop, you might have a choice of single- or multi-turn models and also top, side, or front adjustments. The picture shows a few of the different shapes available. Some will fit better than others, but all will work and this choice is not too critical.

Option 3. Cover the entire circuit with heat-shrink and make a small opening so you can adjust the neutral (idle) position.

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Another good servo for this conversion is the Hitec HS-300. The procedure is roughly the same, but a few particulars differ:

To remove the pot, turn the nut with a pair of pliers, then press the shaft downwards.

The motor is glued in and difficult to remove, so it's easier to simply unsolder the leads.

Compared to the Futaba servos, the circuit board is smaller and easier to pull out but the contacts are located in the middle of the board and are slightly more difficult to work with. The picture shows (from the top) the "raw" freshly-harvested circuit board, the middle one has been modified with a trim-pot, shorter leads, motor connector etc, and the last one shows a slightly different type of board from another Hitec servo. (The single-turn front-adjust pot used here also works well with the Futaba board shown before.)

NOTE: I've used the 2 servos you see because they are among the most common and they are very representative modern servos.
All servos can be converted to ESC use and the steps required will be very similar to those shown.

To attach it to the motor I like to use connectors like the 2-pin Deans, but soldering the wires directly to the motor is also fine.

YOU'RE FINISHED!!

A selection of amp boards from various size servos.

Hitec HS-300 board - front and back.

A few motors that can be used with these controllers:
Kenway KR-1 with stock "Diamond" prop, Mabuchi N-20 with U-80 prop a Knight&Pridham KP-00 with included, adjustable-pitch 4-blade prop and, in front, a couple of even smaller Mabuchi motors.

Here are a home-made R/C duckie and a Kawada 1/24 scale car, both of which use ESC's like those described above.

Two more things that use servo ESC's. the first is a home-made, fully-functional submarine, and the other one is a commercial pan/tilt camcorder mount adapted for radio control.