Before I dwell into the technical details - a brief overview: NXT uses 6 wire ports. All passive/active RCX style sensors use pins 1 and 2. Digital sensors typically use pins 4-6, unless they require also a 9V power supply from pin 1.

The Lego Sonar sensor fits the last category. It requires 5 pins to function - +4.3V, ground, 2 digital (I2C) bus wires and the 9V power supply.

Soon after I started working on this project I realized that the sonar sensor is not an option. The reason is simple - all commerical cord untangler I could find only have 4 wires. These untanglers are used to connect a phone handset with the phone itself, and thus need only 2 wires for the loudspeaker and 2 for the mic. I could not find any manufacturer producing 6 wire untanglers (Maybe LEGO will make one some day?). The untangler I did find was this:

This untangler fits perfectly into the base of the old Technic turntable (Peeron #2856), and can be easily secured in place with thin 1×4 lego plates.

What of the sensor? well, I found out that mindsensors.com Sharp IR sensor does NOT require pins 1 and 2 to function, so this sensor can be incorporated into a radar-like sensor.  But before I could build the radar itself I needed to make some soldering. First, I used a Dremel to expose the contacts on the untangler:

Then I cut one of my Lego wires, and an old phone cord and did some soldering. Make sure you put all heat shrinks on before you solder! The resulting units look like this: (click to enlarge)

It required some practice till all wires were soldered correctly, but at the end it worked. Mounting this into the turntable, I soon discovered that rotating it became more difficult (probably since the untangler is not concentric), which required a very stable construction - connecting the turntable to the platform with studless connectors, mounting gear very tightly and having the NXT motor connected directly to the turntable base. After all this was done (summerizing many hours of failures in two sentences…) the radar was ready!

Next post - programming using LabView…