Temperature Table

I did some additional calculations to work out the raw value relationship with temperature. Rereading the book helped. He had me add a 2200 ohm resistor in order to mimic the R25 value of the LEGO sensor (12.2 instead of 10 kohm). So the R25 value I should be using in the equation is 12.2 -- which affects the results dramatically. Less dramatically, the beta value for the resistor I received was 3907, not 3750, so I fixed that, too. Not relying on the LEGO temperature calculation, I should be able to much more accurately do the math to calculate the correct temperature.

In that vein, I went ahead and further modified the calculations to affect reality. Instead of assuming an R25 of 12.2 kohms, I simply subtracted the 2200 ohm resistor from the measured value, and used that lower resistance to calculate the temperature with beta of 3907.

It's Alive!

I finally plugged in the new board, since I could be reasonably sure the epoxy was mostly set. Of course, I still had to do some more trimming of the epoxy near the negative terminal to get it fit. But I was rewarded with a well-functioning NXT, tethered to whatever power outlet it was connected to.

This got me thinking, in order to not only allow for portability, but also accommodate short power outages, it would be nice to hook up a rechargeable battery pack in parallel with the NXT. There would be a trickle charge while it was running off of the wall power, but if I had to unplug it to move it, or the power was lost to the apartment, it could run on its own for a while. Neat idea, no? In the meantime, I need to set up a battery pack that I can plug into the board to run it away from the AC/DC converter without installing all of the batteries in the back.


As you can see, I didn't bother with any tabs or clasps. This saves a lot on complexity, and the extra Lego pieces simply serve as additional points from which to attach structure.

Copper Top

Today was the most productive day I've had, short of the day the NXT arrived and I build the default robot. All right, this was probably even more productive than that.

The first task at hand was to make the 9 V power supply. Philo's instructional website was my essential guide to get me through this.

My saw did a fair job of cutting the copper board, although the cut was not as straight as I would have liked. Fortunately, because the initial strip I cut was longer than it needed to be, I just trimmed the most problematic side off. A little filing helped straighten everything out and ensure a tight fit. Because I had not planned very far ahead, the plate ended up being much smaller than I had expected.

One observation: with a double-sided copper coated plate, the lower plate can end up with a harsh finish, as the saw tries to finish the job. Unfortunately, I had put the side that I wanted to keep nice on the bottom, so I learned the lesson the hard way.

Because the 9 V power supply plug was too thick to fit through the gap in the NXT, I had a decision to make regarding how to weld the power jack to the board. Do I want to extend the receptacle past the plate, or shave down the power supply plug until it could fit? My inclination was to extend the power port out, but that would require clipping some of the plastic on my $250 NXT. Now, this plastic seemed extraneous to me, but I was reluctant to modify such an expensive piece of equipment without knowing all possible consequences.

The power supply, on the other hand, only cost $25, so I decided to take my chances modifying it. Using one of my increasingly useful files, I shaved down the rubber of the plug until it fit snuggly through the gap in the NXT. This allowed me to keep the power port within the boundaries of the plate, per Philo's diagram.


One thing that happened consistently throughout this project is that I was consistently surprised by how little space there was, and how small my margin of error was. In this vein, I had to break down the fuse holder a little; the plastic part made it both too tall and long to fit.

The cathode on the TVS wasn't labeled terribly well, so I ended up measuring the resistance in both directions. I merely set it up so that there would be infinite resistance from the positive (spring) post to the negative.

The posts were carefully epoxied into place, and I left them to set overnight; in the meantime, I obviously couldn't test to see that everything fit.



A quick rundown of the supplies:

• 9 V transient voltage suppressor: $0.35
  
• 9 V DC power supply: $25.50
  
• Copper board:
  
• 2.1 mm x 5.6 mm power port:
  
• Battery holder:
  
• 3A fuse:
  
• Fuse holder:
  
• Spare wire: free, around the house
  
• Sense of accomplishment: Priceless
  

And the tools needed:

• Saw
  
• Soldering iron (and solder)
  
• Files
  
• Utility knife
  
• Strong scissors