Here’s the umptieth LED board:
It’s been hooked up to SPI and FTDI connectors, and the two 8×8 LED matrices have been tied to an ATmega168 and an A6276 chip, described as a “16-Bit Serial Input, Constant-Current Latched LED Driver”. The voltage regulator and the I2C/power pins on the right haven’t been hooked up yet.
This shows the backside, which is surprisingly robust once done, but a huge pain to build:
I use wire-wrap wire because its insulation can stand the soldering heat without problems and because the wire bonds easily with solder. For anything but one-offs, pcb’s are obviously a much better way to go.
Here’s the unit hooked up via a USB/FTDI board by ModernDevice:
Here’s a better view with one LED actually turned on:
The brightness and contrast are actually far better than in the above picture.
The design is made for scanning across columns while providing 8 bits of on/off data for the current column via 8 output pins on the ATmega. There’s a 16 MHz resonator on board, so this thing is fully compatible with the Arduino IDE.
Here’s a sketch which makes a single led turn on and meander across all positions:
This was used to verify that each dot is accessible and that there are no short circuits. The analogWrite() call is used to vary the display brightness, by controlling all LED driver outputs via PWM. Another useful property of this driver chip is that it needs only one resistor to limit the current through each of the leds.
Update: Silly me, with constant current on the columns, I have to cycle the rows of course. Will need to add an ULN2981 driver to allow the row pins from the ATmega to drive up to 16 leds at once. With this change the LEDs will cycle at 125 i.s.o. 62.5 Hz and can be slightly brighter.