My LED sign

I made an LED sign about 14 years ago. It consists of a 72×7 array of 5mm red LEDs. The LEDs are flangeless, which allows tight packing on standard perfboard.The array was easy to build: I just bent the pins over and soldered them together, adding electrical tape to isolate between rows and columns.

The case consists of quarter inch plywood with an acrylic front. My dad helped me with that, because it involved some tricky drilling lengthwise through plywood. I designed and built all the circuitry inside. Here's the sign with the front removed:

You can see spacers made from bits of hardwood flooring, and a bit of circuitry. The circuitry drives rows via transistors which have low saturation voltage. Here's a closeup:

The circuit boards simply slide out of the front. They consist of one board with the LED array and row circuitry, and another board with the microcontroller and column circuitry. The components face outward, and the wiring is in the centre. Here is the other side of the sandwich and the empty case:

The microcontroller controls the sign via a chain of 74HC374 octal D flip-flop chips. It outputs 8 bits, pulses the clock to latch data in the first chip and move data to the next chip, and repeats the process. While this is going on, the outputs of the last chip in the chain are disabled, which blanks the sign via the row drivers. To provide sufficient current for the rows, 7407 buffers are used. Current limiting is via the yellow DIP resistor arrays. Here is a block diagram:

The microcontroller is a Motorola MC68705P3S, from the 6805 family. I chose it because I had the chips, programmer and toolchain from a previous project. It is a very basic 8 bit NMOS microcontroller with 112 bytes of RAM and 1668 bytes of EPROM. Unlike modern microcontrollers such as the MSP430 family, it has very few registers: the accumulator, index register, program counter, stack pointer, and condition code register. Also, output levels are similar to TTL, so the first 74374 in the chain is a 74HCT374. Here's a closeup of the area around the microcontroller:

The sign works via a serial interface at 600 baud. That may seem ridiculously slow, but it's the perfect speed for receiving new columns for horizontal scrolling. A slow speed is also required due to the microcontroller's limitations. The external oscillator is 4 MHz, but 4 cycles correspond to one instruction cycle, and all instructions take several cycles. There is no UART or DMA hardware on the chip, and everything is done via a timer interrupt and assembler code. The timer interrupt runs at three times the baud rate, so when a start bit is detected, subsequent bits can be sampled in the middle third of the bit. The electrical interface to the serial port is via a MAX232 chip at the other end of the board.

Firmware functionality is quite basic. There is a command line interface with a few functions for direct human use and a few functions which work with raw binary data. The human interface consists of an interactive drawing mode, printing of text via the on-board character generator, sign clearing, lamp test, and returning the sign contents in human-readable form via the serial interface. Due to limited EPROM space, the on-board character generator just supports one font with uppercase letters and some symbols. Other functions support scrolling up and to the left, uploading arbitrary bytes, and uploading groups of columns. These functions allow uploading of raw sign contents, where a character generator running on a computer can provide additional flexibility.

On various occasions I spent time working on PC software for the sign. I first built an interface library. Then I built a server which accepts requests from clients, along with a library that clients use to connect to the server. This allowed multiple programs to use the sign at the same time. Finally, I added support for messages which appear at most and/or at least for a specified time. This should allow the sign to be used for notifications. However, I never used it for a sustained period, because such a large notification display isn't very practical.

Besides looping a short message, there is no support for standalone operation. That means the sign is not able to display various messages with effects one would see on a LED sign that's used for advertising. I don't regret this lack of functionality, because I don't think I'd find that useful. However, if I was building something like this now, I would definitely use a more powerful microcontroller which could support that and more. I thought of replacing the MC68705P3S, but that only makes sense if I have some usage in mind.

You can download the firmware from Dropbox if you're interested. I developed it using the Motorola Portable Cross Assembler (PASM) version 1.0 toolchain.