November 2005, Issue 184

Large-Scale Electronic Display

CIRCUIT DESIGN

The BASIC Stamp 2 microcontroller uses 4 bits (P4 through P7) to select one of 16 outputs from a 4:16 master demultiplexer. Each of the 16 outputs selects from one of 16 slave demultiplexers, which are controlled by bits P0 through P3 on the microcontroller (see Figure 3).

The design can accommodate up to 256 digits. My scoreboard uses 25 digits, so I installed two slave demultiplexers. This allows for up to 32 digits. I could have used an even smaller master demultiplexer (e.g., 1:2) to minimize the footprint, but I was planning for an expansion. The slave demultiplexers’ output selects the appropriate SCR, which, when turned on, provides a ground path for the pulse.

After a digit is selected, the appropriate SSR applies a 12-VDC pulse. Bits P8 through P14 set the segments. Bit P15 resets them. The software controls the pulse duration (width), which varies with the sizes of the digits (see Table 1).

An RS-232 serial port connects the scoreboard and the laptop. Because of the low data rate (9,600 bps), the scorekeeper can operate approximately 75 yards away from the scoreboard without any communication problems. Pins 1 through 4 on the BASIC Stamp 2 microcontroller provide this mode of data transfer. You can use other methods  (e.g., radio frequency, infrared, RS-422/485) depending on distance between the scoreboard and the scorekeeper.

The ICs operate from a 5-VDC source. The digits require at least 12 VDC at the coils. The power supply for the digits should be adjustable between 12 and 20 VDC because of voltage drops across the steering diodes, SCRs, and signal path. I used a linear power supply capable of supplying 0 to 30 VDC and up to 5 A. I found 15 VDC to be adequate. You could use a couple of car batteries with voltage regulators if you want uninterrupted operation in case the main supply goes down. I added a small heatsink to the 5-V LM7805 regulator, but the SCRs operated well without any heatsinks.