Custom servo controller on NXT tractor

[mattallen37]

I got 8063 for my birthday, and I wanted to use the large wheels for an NXT RC vehicle of some kind. So, I decided to make a tractor to begin with.
Here is a picture of the tractor with the trailer from the set. Sorry for the low quality.

Around the same time, I was toying with the idea of using a PICAXE 20X2 as a servo interface for the NXT. I started working on some different concepts on a breadboard, and came up with satisfying results. I tweaked the code a little, and then soldered together the controller. If anyone is interested, I can post the (sloppy) code for the PICAXE and the NXT. I haven't drawn up a schematic yet, but it is just about as basic as it could be. The only external resistor I used was a 100k pulldown on the rx programming pin to keep it from floating. The I2C pullups are internal; all I had to do was add a line of code to enable them. I am not really sure how strong/weak the pullups are, but they seem to work well with the NXT. The PICAXE itself and the program support 6 servos, but due to my lack of header pins and perfboard, I only soldered it up for three servos. I only use one for this project anyhow.

To power the servo, I use a standard (new style) Lego 9v battery box. I am using rechargeable AA batteries, so the normal voltage would be around 7.2v. That is too high for the servo, so I modified a paper-clip to take the place of one of the batteries (and put it in the horribly tight AA space). I also have a bridge rectifier between the battery box and the servo controller, ensuring proper polarity (it also drops the voltage a little). In the end, I measure 6.1v at the servo pins.

Here is a picture of the servo controller.

Here is a picture of what the tractor looks like now (other than a minor change to the blade).

[mattallen37]

Here is a picture of the gearing.

I have previously broken a 12 tooth gear in a differential by apply power from two NXT motors. I didn't want to deal with that hassle again, so I geared it up 5:3. Being a tractor, I wanted it to be powerful, and go slow. I get the power from two NXT motors, and I get slow speeds by gearing down from the differential 3:1. The overall gearing (according to my calculations) is 1:0.55..., so about 2:1. Also note though, that with the huge wheels, it is a fairly good speed for driving.

I also built it with a driving ring, so I can manually lock the differential (make it a live axle).

[skaltura]

Excellent

Indeed a coincidence that you posted your servo controller shortly after mine.
Thanks for the comment on my project by the way...

Instead of using a breadboard for my project, I used Proteus to simulate the project on the computer. Then, when it works the way it should, you can use the same software to design a PCB for it.

[mattallen37]

I just wrote a blog post about the controller. Included is a schematic and PICAXE program.