Firmware engineer Evan Grove has performed the one logical factor to do while you get your fingers on an LED scoreboard show from the Nineteen Nineties: reverse engineer it to make join it to the Web of Issues (IoT).
“Coming in at 11 inches tall and 6 inches broad, this gigantic seven-segment show looks as if an excellent toy place to begin for my high-octane IoT challenge,” Grove writes of the challenge, in a weblog put up dropped at our consideration by Adafruit. “I really like science/electronics surplus shops, as a result of oddities like this are ample and low-cost.
“On EBay, you would possibly pay upwards of $50 for this factor. From a surplus store? $4. The store had a number of of those, however I didn’t need to purchase multiple till I had decided that these are literally value shopping for. That was a mistake — I ought to have purchased all of them.”
Constructed by Daktronics within the mid-Nineteen Nineties and certain faraway from service in a basketball scoreboard, the spectacular show is not your customary seven-segment LED: there are 98 particular person 5mm through-hole LEDs, organized to make every of the seven segments. On the again is a nine-pin connector, which Grove surmised was used to drive every phase individually.
“9 pins offers us one widespread pin, seven pins for every phase, and an additional pin filled with enjoyable and surprises,” he writes. “Perhaps all these LEDs draw a lot present that it wants a second widespread pin? Maybe that pin does nothing, and it might be used for a decimal level on a distinct mannequin?”
Recognizing that the 2 chips on the board’s face had been simply resistor packs, and by following the traces on the board by shining a light-weight by it, Grove was in a position to determine that every phase was made up of two pairs of seven LEDs every and two resistors. Doing the mathematics got here out at about 5W to energy all the things — and the belief of an 18V provide voltage. By throwing a 12V enter into the board, Grove was in a position to determine the polarity of the LEDs — and set about designing a driver.
“Even on the guesstimated max present draw, we are going to simply have the ability to energy this factor with low-cost, tried-and-true 2N3904 transistors,” Grove says. “With [a specific] mixture of resistors and transistor, the circuit ought to change from ‘off’ to ‘on’ someplace round 0.6-0.8V. That’s not an ordinary logic degree, nevertheless it does imply that this circuit ought to settle for enter from any microcontroller operating on 5V, 3.3V, or 1.8V logic.”
To improve the construct nonetheless additional, Grove added a 74HC273 twin D-latch into the combo — producing a customized driver board which might management all seven segments and which might be linked to an Espressif ESP32-based Web of Issues challenge Grove had already been engaged on. “That meant integrating my show driver into the Zephyr RTOS,” he explains. “Zephyr’s auxdisplay driver API matches the 7-segment show fairly properly, in order that’s the place I applied the motive force.”
The complete challenge write-up, together with schematics, is obtainable on Grove’s weblog, together with a be aware of remorse: “Somebody knew higher than I did,” he says of his determination to not purchase multiple panel, “as a result of by the point I had one other likelihood to purchase the store’s remaining inventory, they had been offered out.”