Servo modification for PWM Control
By Ringo Davis
This document describes how to modify a Futaba S148 servo so that it’s speed can be directly controlled by using 2 pwm signals, 1 for forward, and another for reverse. For details on modifying the servo for continous rotation, click Here. The servos electronics consists of an Hbridge (BAL6686), another IC that monitors the incoming signal and the onboard potentiometer and creates the appropriate signals for the Hbridge, and a few support components. What I did was to remove the support IC and the potentiometer and attach wires directly to the Hbridge.
To do this modification you will need the following tools:
Small Phillips screwdriver,
Soldering wick (or some other way to removing through-hole parts),
Start off by removing the 4 screws from the bottom of the servo case. Then remove the top of the case and make note of the arrangement of all the plastic gears. If you are modifying the servo for continuous rotation, you can clip the tab now. You need to remove the two tiny screws from the under the gear at the opposite end of the servo from the wires. These screws hold the top of the motor to the case.
Figure 1. Top of servo case removed, showing small screws that need to be removed. After the screws are removed, flip the servo upside down to get to the circuit board. The circuit board is held in tightly by friction. Use a very small screwdriver or similar tool to pry it up and out of the case.
Figure 2. Bottom of servo case removed. This is what it looks like when it comes out of the case. The IC closest to the motor is what we need to remove. Also remove the potentiometer. I used solder wick to desolder all the pins then just wiggle the IC out. Note that there are 2 rows of staggered pins on the IC.
Figure 3. Board before modification. After the parts are removed, the board looks like this.
Figure 4. Board after Parts removed. Now to attach wires to the proper signals, I used the white signal wire that was already on the servo as one of the control wires, and added a second yellow wire. As you can see from the schematics below the pins we want to attach to are pins 4 and 6 on the BAL6686, which happen to also be connected to pins 2 and 3 on the removed IC. Since the white signal wire goes to pin 12 on the removed IC, just solder a jumper wire from hole 12 to hole 3 of the removed IC. Also solder the new yellow wire to hole 2 of the removed IC. You will need to cut a small notch on the bottom cover of the servo to leave room for the new wire to come out.
Figure 5. Bal6686 pinout
Figure 6. Connections between the two IC’s.
Figure 7. Board after wires are connected. Now just reassemble everything and you are done. Just put 5V and Gnd on the red and black wires, then either pull the white wire high to go one direction, or pull the other wire high to go the other direction. Do not pull them both high at the same time.
Figure 8. Completed servo. Here is a simple circuit you can use to drive the wires using 1 PWM signal and 1 Direction signal.