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MindStorm Brainstorm

Updated: Dec 23, 2020

I have a 20+ yr-old first-generation Lego MindStorms set that's been hanging around unused for a long time. I decided to pull it out, go over everything and gift it to my Godchildren. They're both very much into Lego sets and I thought this might be a good way to teach them the basics of computer programming.

Although the Lego pieces themselves are "forever", and the MindStorms RCX 1.0 "brick" came to life with just a new set of batteries, much of the wiring used to connect the motors, sensors, etc. was pretty much shot. The rubber/plastic insulation had dried out to the point that it cracked and flaked off when I bent the wiring.

Here are a few pictures showing the typical condition of these wires:

While this doesn't present a shock hazard (the highest voltage used is 9V), if the bare wires touch together, the motors and sensors connected using these wires won't work properly.

It looked like the base of the square black connectors was just snapped together (it IS Lego, after all!), so I carefully inserted a very small screwdriver in the base of the connector and pried gently, like this:

I was pleased to see that it came apart without damage. I could see that the original wire was simply pressed into a couple of very small brass barbs.

It came off easily.

I used a small pair of needle-nose pliers to straighten the barbs, circled in red below:

Now the challenge was to find some replacement wire that was small enough to fit this tiny connector. I remembered that I had a stash of old computer ribbon cables lying around, so I pulled one out and stripped off a length containing two strands. It was just the right size.

I carefully bent the end of the wires into an "L" shape and used a small screwdriver to press the individual wires onto the barbs and over the strain relief "hump", like this:

Then I reinserted the bottom half of the connector and gently tapped it into place using a drift punch and a small hammer.

It snapped back in with a satisfying "click" and everything lined up just like new:

I used my ohmmeter to confirm continuity on both leads:

Here's the finished connector, all ready to go:

The housing for the light sensors was a different story. This one had six small tabs on the bottom which were melted into place during manufacturing. I had to carefully cut them off with an Xacto knife.

But then I discovered that the four corners were glued together. So, I used a small wheel on my Dremel tool to cut into each corner about 2mm. I then used a small screwdriver to pry the bottom half of the case off.

I could then remove the small printed circuit board from the blue half of the case and snip off the wires connected to it.

The next hurdle was that there's a deep hole in the blue half of the case where a loop of the wire is inserted as a strain relief. There's a corresponding post on the gray bottom half which holds the wire loop in the hole. During manufacture, they placed some kind of a gooey, black potting compound in the hole which effectively glues the post into the hole, along with the wire loop. When I pried the gray and blue halves apart, the post broke off. I then had to use a pair of pliers to pull the loop of wire out. I used a small screwdriver to methodically dig all the potting compound out and clean up the hole. Unfortunately, I neglected to take pictures of this process.

I unsoldered the wires from the PC board and used a solder sucker to clear out the holes in the board. Then I soldered my new wires to the board and clipped off the excess. Polarity doesn't matter here.

Next, I reinserted the PC board into the case and stuffed a loop of the wire into the strain relief hole.

I sparingly placed some clear two-part epoxy into the strain relief hole as well as on the six blue tabs on the bottom side of the case. Then I reattached the gray bottom piece. I used a small C-clamp to hold the halves together until the epoxy set.

Here's the final assembly. You can see where I had to cut into the four corners to get the two halves apart when I was disassembling the sensor.

I proceeded to repair another six or seven connectors and several more light sensors in the set and learned a few lessons along the way.

1. It's best to use an ohmmeter to check the connections as you place the wires on each end and before closing up the backs on both ends. That way if you have a bad connection, you'll find it right away and won't have to guess which end is bad when you inevitably have to redo one.

2. When you press the wires down onto the barbs, you have to be very careful not to bend or flatten the barbs. Also, the barb must penetrate the insulation of the wire and contact the stranded wire inside. But the wire is very thin, and it's easy to cut completely through it with the barb. This takes some practice and patience to get right.

3. You'll also want to do a final continuity check after everything is completely back together. I discovered that it's possible to bend the barbs and move them out of contact with the wires when reinserting the back of the connector. This can happen even if you get a good continuity test before replacing the back.

4. You do need to pay attention to polarity when making the connector cables, but it's pretty easy. With the connectors on your bench as shown here...

...just lay the wire flat and connect the bottom wire to the bottom connectors in each end. Likewise, use the top wire for the top two connectors.

The real key to this repair is patience and gentleness. Go slowly, work carefully, and you'll be rewarded with a set of Lego RCX connectors and sensors that will last another 20 years!

And it turns out I had a fun time building a little RoverBot and programming it up to follow an oval track. Take a look here for a demo. I guess the little kid in me still lives...

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