LEGO PF to NXT Adapter Cable    Send article as PDF   
I really despise the NXT Motor design. The shape of the motor was basically just designed to allow construction of the LEGO Mindstorms NXT Kit’s, AlphaOne‘ Robot easy. As a result, it makes it hard to use the NXT Motors in a lot of situations without extra bulk being added to your design. Often I just ditch the NXT Motor in preference for a LEGO Power Functions Motor. The downside to this approach is you have no feedback from the Power Functions Motors, unlike the NXT Motors with their Quadrature Rotary Encoders. You can use a LEGO Mindstorms RCX Rotation Sensor for a lot of applications.

To use the Power Function Motors with the NXT you need an adapter cable as you need to build your own cable as there is no commercial equivalent. You could use a 8528 Converter Cables for Mindstorms NXT and a 8886 Power Functions Extension Wire, but this can be a little on the long side for many situations. The alternative is to build your own by following the guide to creating your own Lego Mindstorms NXT to LEGO Power Functions cable, to control any PF motor or light.

What you’ll need:

You will also need some very basic electronic/soldering knowledge. An alternative way is to take you cable sets to you local Television Repairman who would join the cables for you, for a few dollars.

White Motor 1

The WHITE and BLACK wires (Motor 1 and Motor 2) deliver power to the motor. If the white wire is positive and black is negative, the motor will turn one way and If you reverse the polarity, the motor will turn the other way.

The RED wire is connected to the ground (GND). (Note: with sensors, the RED and BLACK wires are connected to one another.)

The RED wire is connected to the ground (GND). (Note: with sensors, the RED and BLACK wires are connected to one another.)

The GREEN wire is connected to the I2C, +4.3v NXT power supply.

The YELLOW and BLUE wires are connected to the quadrature encoder, also called an incremental rotary encoder.

Black Motor 2
Red GND (Ground)
Green 4.3 Volts
Yellow Tacho 1
Blue Tacho 2

Basic Rotary EncoderSquare waves from quadrature encoder

As shown in the diagram above from Wikipedia, ( the YELLOW and BLUE return square wave pulses that are 90 degrees out of phase. If the rising pulse on TACH00 leads the rising pulse of TACH01 by 90 degrees, then the motor is going forward. If it instead lags by 90 degrees, the motor is rotating backwards.

One complete square wave cycle corresponds to 2 degrees of rotation. In the diagram above, the motor is going backwards as Tacho 0 is lagging Tacho 1.



When it comes to the LEGO PF-to-NXT Adapter Cable, the only wires that concern us are the White and Black wires on the Mindstorms NXT Cable, which are to be joined to the centre two wires on the Power Functions Cable.

Cut the Power Functions in half. Remove part of the Two Outer Wires. Strip and Tin the Centre Two Wires.

Cut the Mindstorms NXT Cable in half. Strip back the insulation by 10mm. Trim back all wires accepting the Black & White to the Insulation. Strip and Tin the Black & White Wires. Slide on a Piece of 6-8mm HeatShrink Tubing well away from the Soldering points.

Solder the Mindstorms NXT and Power Function Cable Halves Together.

Apply some Hot Glue between the Joins of the Mindstorms NXT and Power Function Cable Halves Together. Be careful to have the two solder joints parted.

Slide Down the Piece of 6-8mm HeatShrink Tubing to cover the Cable Join. Using a Match or Lighter, Gently Heat the Heatshrink material to it fits tightly.

The Shrunk HeatShrink Tubing should appear like this.

Your Finished LEGO Power Function to Mindstorm NXT Cable should look something like this.

Next repeat, and make the second cable from the left over two cable halves.

The ‘Light Grey’ LEGO Power Function Plug will also fit the older Technics motors, including the Race Buggy Motor.

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