Jan 16, 2012

Various prototypes leading to 8bi8

I've always been fascinated with LEDs and toys that use LEDs. In a previous post I revisted my first custom fabricated PCB project that used 3 5x7 5mm LED bi colour modules. After that I set out to make a 8x8 version. Here are some revisions I went through. This youtube video pretty much covers most of this post:

Rev1 populated base pcb and bare top pcb (Notice the wire re-enforcing the micro b connector.)
Revision 1:
I first built a small 8x8 LED backpack for a set of small 20mmx20mm red LED modules I bought from ebay. In that first design I used a micro controller and 6 0.1" headers to connect to base board that would contain a battery and wireless transceiver. the battery was going to be charged by a usb lipo charging circuit. and the wireless would enable the device to talk with other devices. lets just say the design was far to complicated and never really took off. I did however get the LED "daughter board" working from incoming SPI signals and independently, the only issue with that was the lack of user input (buttons) and the absence of a battery or power switch.

Rev2 populated base board and bare screen pcb
Revision 2:
After seeing this I decided to design my new revision with this capability, to use an AVRs internal ADC to sample voltage outputted by the LEDs allowing a 2D matrix input. This design still used the small 20x20mm red LED devices. it featured 6 0.1" headers on the back to again connect to a base board. this was mainly due to small size not being big enough to house a proper battery. The base board was redesigned to incorporate a USB enabled AVR IC. This also never really took off the ground. Probably because I'm more of a hardware guy than a software guy and high level libraries such as USB scare me, or at least give me nightmares. Just like the first I was able to get the display board functioning independently to the base board. However just like the first revision he display board had no extra room for user input or a battery pack only the 6 pin interface connector. The light input does work however since the display is very very small compared to a standard 3mm LED display or even a huge 5mm LED display it is very hard to aim a laser pointer at an individual pixel.

8bi8 and the larger 16bi8 version
Revision 3??: (8bi8 0.9)
This technically shouldn't be called a revision from the last two as it was practically a whole new project. In fact I haven't called it revision 3 I labeled it as the 8bi8 as it's a 8x8 bi-colour LED toy. The basic design was to use a MSP430 combines with a coin cell battery and a few buttons. basically it was a reproduction of the original design without the interface header and featuring on-board battery and inputs. For this design I was very very lazy and decided to give eagle auto router a try. This re-ignited my frustration with auto routers but did finally yield a result. After waiting ~30 days for my boards to arrive (a standard time from laen's PCB service to Australia). I discovered after soldering up a whole prototype that half of the display connection had been reversed. I managed to fix up the dodgy wiring with an even dodgier software job. The frame rate was serverly reduced due to this hack job and development on this platform stopped and development into a new revision began.

8bi8 1.0
After the experience with the first 8b8 revision I decided to spend more time designing this board. adding in current sinking mosfet from the shift register outputs in order to maintain a reasonable constant brightness across screen refreshes and maybe being able to push more current than the standard 74hct595 IC (current supply to LEDs is probably limited more by the coin cell) This revision will be covered fully in a later blog post.


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