Firmware engineer Evan Grove has done the only logical thing to do when you get your hands on an LED scoreboard display from the 1990s: reverse engineer it to make connect it to the Internet of Things (IoT).
"Coming in at 11 inches tall and 6 inches wide, this gigantic seven-segment display seems like a great toy starting point for my high-octane IoT project," Grove writes of the project, in a blog post brought to our attention by Adafruit. "I love science/electronics surplus stores, because oddities like this are abundant and cheap.
"On EBay, you might pay upwards of $50 for this thing. From a surplus shop? $4. The shop had several of these, but I didn’t want to buy more than one until I had determined that these are actually worth buying. That was a mistake — I should have bought all of them."
Built by Daktronics in the mid-1990s and likely removed from service in a basketball scoreboard, the impressive display isn't your standard seven-segment LED: there are 98 individual 5mm through-hole LEDs, arranged to make each of the seven segments. On the back is a nine-pin connector, which Grove surmised was used to drive each segment individually.
"Nine pins gives us one common pin, seven pins for each segment, and an extra pin full of fun and surprises," he writes. "Maybe all those LEDs draw so much current that it needs a second common pin? Perhaps that pin does nothing, and it would be used for a decimal point on a different model?"
Spotting that the two chips on the board's face were just resistor packs, and by following the traces on the board by shining a light through it, Grove was able to figure out that each segment was made up of two pairs of seven LEDs each and two resistors. Doing the math came out at about 5W to power everything — and the assumption of an 18V supply voltage. By throwing a 12V input into the board, Grove was able to figure out the polarity of the LEDs — and set about designing a driver.
"Even at the guesstimated max current draw, we will easily be able to power this thing with cheap, tried-and-true 2N3904 transistors," Grove says. "With [a specific] combination of resistors and transistor, the circuit should switch from 'off' to 'on' somewhere around 0.6-0.8V. That’s not a standard logic level, but it does mean that this circuit should accept input from any microcontroller running on 5V, 3.3V, or 1.8V logic."
To upgrade the build still further, Grove added a 74HC273 dual D-latch into the mix — producing a custom driver board which can control all seven segments and which can be linked to an Espressif ESP32-based Internet of Things project Grove had already been working on. "That meant integrating my display driver into the Zephyr RTOS," he explains. "Zephyr’s
auxdisplay driver API fits the 7-segment display reasonably well, so that’s where I implemented the driver."
The full project write-up, including schematics, is available on Grove's blog, along with a note of regret: "Someone knew better than I did," he says of his decision not to buy more than one panel, "because by the time I had another chance to buy the shop’s remaining stock, they were sold out."