Friday, June 25, 2010

Arduino Common Knowledge, Part 1 of N: LEDs Without Resistors

Some time ago I read a book called "C++ Common Knowledge". It consisted of a series of short 2-3 page articles, each detailing some important, but often misunderstood detail of C++. Recently, I've answered a number of Arduino/Electronics questions that seem to have this same feeling to them. This is the first in a series of short articles I'm going to call "Arduino Common Knowledge".

Today, we'll start with an idea that seems to go against all safety warnings: you can blink an LED directly off an Arduino/AVR pin. Yep, no resistors. We've probably all experienced the fun of plugging an LED in between our 5V and ground rails, and watching it glow brightly for a second before exploding and smelling quite awful -- so it seems natural we'd always want to install a current-limiting resistor to stop this problem.

But here's the interesting part: your Arduino/AVR is a current-limiting device. The I/O pins can typically only source about 15-20mA of current. Thus, if we connect our LED between the pin and ground, and toggle the pin high, our LED glows nicely without exploding.

There are a few caveats to this though: you can't do this on every pin on the AVR at the same time, or even a large number of pins. The ATMEGA168 data sheet specifies that the Absolute Maximum DC current between Vcc and GND is 200.0mA. Since the AVR core and periphrasis such as the UART/SPI/ADC draw some current of their own, we clearly can't control a huge number of LEDs this way. Also note that if we leave the LEDs on for a long time, you'll probably notice the AVR getting a bit warm sourcing that much current.

Next Up: Pull-Up Resistors (and Why You Need Them)

Tuesday, June 22, 2010

Another Day of Printing (and Issy Building)

I got another day of solid printing out of the Makerbot. Finished printing Issy's legs, printed his head camera mount, and the first segment of the tail. The head camera mount is a fairly massive print, at 3.5" x 1.75" and an inch tall. The first attempt warped massively after the first 3 layers and had to be tossed. I got a much better raft on the second one, and it came out fine (some warpage, but not enough to ruin the print). I'm hoping to crank out the snout and second tail section tomorrow.

Unfortunately, no pictures. My camera is on vacation in Florida. But, here's a rendering of the final head/snout with the MS Lifecam cameras:

Sunday, June 13, 2010

Printing Grippers

The Makerbot has gotten a decent workout this past week, printing legs for Issy and (among other parts) a gripper for an upcoming bot. The third time really is the charm: with all the designs for this gripper and Issy's legs, the third design was the one I stuck with. Here's some pics of the aforementioned HS-55 based grippers I'm creating for a 2011 robowaiter entry:

Each gripper has two printed parts: the HS-55 mount and the C-bracket. The servo mount is the same part for both the left and right sides but I had to slightly tailor the C-brackets to get decent range of motion. Each HS-55 mounts to an AX-12 C-bracket.

The gripper fingers are made of 1/16" thick 5052, about 3/4" wide, mounted with two 2mm screws. Instead of a bearing, I'm using a 3/16" Chicago bolt on the non-driven end of the printed C-bracket. Preliminary tests show the grippers have no problem holding the robowaiter plate.


Thursday, June 10, 2010

Issy's New Legs

One of the key features desired for Issy3 is a set of feet with tactile feedback. I've played with FSR versions in the past, and they tend not to survive that long.

Two afternoons of prototyping and printing has yielded the first of Issy's new feet. I have one leg assembled, but I'll still have to get some different hardware tomorrow to tweak it completely.

The foot consists of three printed parts: the outer case, the foot pad, and a retainer insert inside the case. A screw connects throw the foot pad and the retainer, and a spring pushes the foot downward. There will be a little spring steel connector on top that will get touched by the screw -- acting as a simple switch. A vinyl footpad gives traction.

Each foot takes about an hour of printing on the Makerbot.