GBC Speed Regulation

Custom power functions speed regulators in a Castle Enclosure
Custom power functions speed regulators in a Castle Enclosure

This page will cover how to control the speed of your great ball contraption (GBC) modules. First the traditional 9V train controller will be covered followed by instructions on creating a cheaper, custom controller like the one above for about $10 each.


Traditional/Pure Approach

When chaining /gbc modules, it is important to synchronize their speeds so that one slow module does not pool all the balls. This is typically done with an old 9V train speed regulator, Figure 1. Since these have not been in production for a while, they are about $50 used.

Figure 1: 9V Train Speed Regulator
Figure 1: 9V Train Speed Regulator

In order to connect a power functions (PF) motor, you will need an extension cable, either 8” 8886 or 20” 8871.

Custom Speed Regulator

Due to the increased price and scarcity of the train regulator, one may be built from after market parts. This page will instruct you on creating a custom speed regulator - no soldering required!. Built in pairs, the total cost is $20 or $10 per plug. A quick reference card with the schematic diagram is shown below, followed by complete a tutorial on constructing a pair of regulators.

Quick Reference with Schematic Diagram
Quick Reference with Schematic Diagram

Equipment for creating a pair of custom speed regulators
Equipment for creating a pair of custom speed regulators



Total time

15 minutes


The two biggest downsides of the custom regulators are that they are not ‘pure’ (produced by LEGO) and that they are not bidirectional. You can gain bidirectional control by adding either a power functions switch (8869) or a 9V 2x4 (or 2x8) electric plate with an additional PF extension cable.

Component (Details)

9V Power Adapter

You will first need to acquire a 9V power adapter. Used ones can be found for free if you keep your eyes open. Pay attention to the two numbers on the “OUTPUT” section of the label: “9V” and “x mA” for voltage and current respectively.

9V is most important. Anything lower will not drive the motor at full speed and anything greater exceeds the rating of the LEGO motors.

The current listed (mA or A) is the maximum amount. You want something that is at least 600mA to drive two M-Motors and 1A (1000mA) to drive two XL-Motors. A higher number is fine.

Also, be sure it is an AC to DC (referred to as AC/DC or DC output) adapter.

9V power adapter capable of 850mA (0.85 Amps)
9V power adapter capable of 850mA (0.85 Amps)

DC PWM Motor Speed Controller

This component takes in the 9V power source and outputs a stronger or weaker signal based on the turning the dial. The 1803BK 1803B controllers linked above use Pulse Width Modulation (PWM) rather than controlling the output voltage directly (like the train control does). There are some advantages to this, mainly that the motor has more torque at lower speeds.

The controllers linked above may be hooked up without soldering, making this a doable project even for those without experience with custom electronics.

At the time of writing this tutorial, you could buys these in packs of 1, 2 or 3 from several different sellers on Amazon.

Power Functions Extension Cable

The PF extension cables have four terminals and four corresponding wires. The outer two are always ground and power (0 and 9V) and are not connected inside the motors (but are used for the IR receivers). The inner wires are C1 and C2 (control) and are used to drive the motors. Figure 3 illustrates these connections.

The bottom of the light bley plug connects to the old 9V system using the C1 and C2 connectors only. This is why you cannot drive an IR receiver with the old train controllers.

The voltage difference between the C1 and C2 lines controls the speed of the connected motors. The train controller does this directly, by delivering 0-9 Volts as the dial is turned. A PWM signal may also be used; this is what is delivered by the off-brand speed controllers described above.

Figure 3: Power Functions Wiring Diagram
Figure 3: Power Functions Wiring Diagram


Before beginning, make sure the power adapter is unplugged. While 9V should not be enough to hurt, do not take any chances.

1. Strip the power adapter wires

Cut the power adapter cable near the plug. Then cut another 6” of cable off. Use the wire stripper to strip off the jacket/covering of the two wires on each of the three ends, about ¼”. Figure 4 shows all the stripped wires.

Figure 4: Stripped Wires
Figure 4: Stripped Wires

2. Strip the power function extension cable

Cut the power function extension cable in half. Use the wire strippers or scissors to split the four wires apart, about ¾ from the ends. Then cut the last half inch off the middle two wires. Now strip all the wires, leaving about ¼” of wire exposed.

Your wires should appear as those in Figure 4.

3. Connect the wires to the first controller

On the bottom of the motor controllers, there are four labels, one for each connector:

  • Power -
  • Power +
  • Motor +
  • Motor -

Unscrew each connector a bit (from the top) and insert the two center power function extension cable wires from one of the cables into the Motor + and Motor - terminals (from the side). The order you put them in determines which direction the motor will spin. For the sake of example, position the cable so that you can see the studs facing up and insert the two center wires accordingly. Screw the two motor screws down tight. Leave the two outer wires unconnected for now.

Now find and insert the positive and negative wires from the power adapter cable into the Power + and Power - ports. The positive wire should have a white stripe or line printed on it. Alternatively, you can plug the power adapter in and measure the voltage using the multimeter. Hook the black cable to the wire you believe is negative and the red cable to the other wire. If the voltage is positive, you have properly identified the wires. Note: the voltage may read high (e.g. 11.25V) when no motor is running. Unplug the adapter before continuing.

Take the 6” cable you cut and insert one end into the Power + and Power - ports as well.

Now take the outer wires of the PF extension cable and insert them into the Power + and Power -. The negative cable is the one on the right, if the plug is facing studs up; see Figure 3 above. These two wires are not required for powering the motors, but will allow you to power an IR receiver (independent of any connected motors).

Now screw down the two power port screws tight. Figure 5 shows a single motor controller complete - without the 6” wire. Click the image to zoom in and see the routing of each wire. You can narrowly see the white stripe on the positive power adapter cable.

Figure 5: A completed single motor controller
Figure 5: A completed single motor controller

4. Connect the wires to the second controller

Repeat step 3 with the second board, but skip the power adapter cable part. There should only be two wires going into each power port: the 6” wire and the PF extension cable’s outer wires.

You may want to flip the inner PF extension cable wires. If you do this, the motors will spin in opposite directions. I have done this in Figure 6.

Figure 6: Completed Project
Figure 6: Completed Project

5. Test

If you have a multimeter, you can use it to test for voltages and shorts. Note that the motor output is a PWM signal, which the multimeter may not measure well.

Plug in the power adapter and make sure nothing gets hot or smokes.

Turn one dial on, then off. An LED light should turn on when the dial is turned on. Repeat with the other. If an LED light does not turn on, remove power, make sure all cables are connected securely and your positive and negative wires are proper.

Now attach motors to the PF extension cable ends and turn the dials on. The motors should spin faster as you turn the dial up.

If you have an IR receiver, connect it to the PF extension cable as well. Connect a motor to the IR receiver and use a remote to turn it on and off. NOTE: this can be done as long as the power power adapter is plugged in and is independent of the dial position.

If anything does not turn on, verify all wires go to the proper place. If your motors turn in the wrong direction, flip the two center wires on the corresponding PF extension cable.

6. Enclosure

You can optionally build a LEGO enclosure. The boards may fit in a 4x4 stud area. Some of the ones I ordered fit fine while others were cut a bit too big.

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