Modifying The Lego Pathfinder

Once I had Pathfinder up and running I wanted to experiment with some simple modifications. My primary goal was to see how quick and easy it would be to prototype ideas. Since Pathfinder was already built and the software was working, why not try modifying it?

I setup a few basic and totally arbitrary design parameters. The modified Pathfinder should-

  • Have a rigid body – something I could pick up and turn over in my hand without having it fall apart.
  • The motor/wheel drive ratio should be setup so that the new robot moves slower. Pathfinder, running at full speed, tended to be a real spitfire. Fun, but a little difficult to control and keep out of trouble.
  • At least one contact sensor so that it could take some action when it bumped into obstacles.

With that in mind, I started picking through my Lego parts boxes, and put together some simple designs. After about 30 minutes of trial and error, I ended up with this-


Pathfinder’s bottom chassis was disassembled. The motors were mounted directly to the RCX block. A simple meshed gear mechanism was constructed to gear down the drive speed. The wheels were changed to a larger diameter – probably nullifying the gear ratio change. One button was placed towards the front of the bottom chassis. The contact switch was added so that it extends slightly in front of the robot.

The software was then modified to take advantage of the contact switch. At the same time modifications were made to lower the motor drive speeds.


This is the bottom view including the cross bracing added to strengthen the chassis.


Gear alignment was a breeze since the standard Lego dimensions take care of most of it. This is something I definitely want to keep in mind as I design other non-Lego robots.


The axial spacing was also very easy to setup perfectly since all the Lego part sizes are standardized.


The contact sensor was mounted low and extended slightly to the front of the robot.


Unfortunately the contact switch by itself doesn’t seem to be very effective. It has a very small travel, and small contact area. If the robot has any speed at all, it will run into an obstacle pretty hard before the switch actuates. In actual practice I will have to provide some sort of additional mechanism.

Things I didn’t like:

While you are constructing your robot it’s very easy to accidently depress buttons on the RCX brick. This happened to me a couple of times. It’s kind of shocking to be pressing some blocks in place and suddenly have the wheels start turning in your hand. The first time it happened I almost dropped the robot.


  • I’m very impressed with how easy it is to prototype concepts using Mindstorms.
  • There’s a lot to be learned from the Lego standardization – lots of ideas that can be utilized in other non-Lego based designs.


One thought on “Modifying The Lego Pathfinder

  1. Hello .

    Sir, please help me find some maps to make a pathfinder robot,which follows a white line .

    thanks a lot .

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