Building The Rocket Doctors Recommend Most (Part 3)

Time to shift attention to the LEDs.

I have 25 bright white LEDs I bought from Adafruit; they’re clear, not diffuse, which may be a mistake. On the other hand the bottles may diffuse the light enough. On Arduino Day I bought (way cheap) four Arduino compatible boards from SparkFun including two Arduino Pro Minis (a 5V and a 3.3V), which are small enough to fit in this rocket. And the 3.3V one can run off a battery small enough to fit, too. From SparkFun I also got a ULN2803 8-channel Darlington driver and some snap-off right angle male headers.

I had a 3.3V Arduino Pro Mini already, in fact, but I’d soldered straight male headers to it to connect an FTDI board (this one), with which to interface to a computer for uploading programs. Excuse me, sketches. But for this rocket, those headers won’t fit. Hence buying another Arduino Pro Mini and right angle headers.

I started by prototyping a 3 by 3 LED matrix on a protoboard, with one column selected at a time by moving a jumper between inputs on the ULN2803, and any number of rows selected by jumpers on the current limiting resistors. The resistors are only 15 ohms, which will give 20 mA current for a 3.3 V supply voltage (and assuming 3.0 V for the LEDs’ forward voltage, which the spec sheet says is “typical”.) They work, so far. And here’s a column lit up.IMG_2580

That all seemed good, so the next step was to connect an Arduino. I used the old one since the header angle doesn’t matter here. Once that was in place I went to find the FTDI board.

Which I couldn’t.

I’d just had it a night or two previously, I knew it! And now it’d disappeared. That’s the trouble with tiny electronics. It goes missing so easily.

About two days later, I moved my Apple Magic Trackpad, and guess what was under it?IMG_2595No, not the quarter.

OK, with the FTDI found, I could program the Arduino. I used a variant of the sketch given here, debugged it (needs to shift tempData somewhere inside that loop, don’t you think?), added a second data array to alternate with for some blinky action, and got it working.

blinkWe’re cooking! The 6 by 3 array should present no problems. Other than successfully stuffing it inside a rocket, I mean.




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