November 2005, Issue 184

Large-Scale Electronic Display

CONTROL ELECTRONICS

I used a standard operational design for this project. A Parallax BASIC Stamp 2 microcontroller receives commands via a laptop computer’s serial port. The microcontroller interprets the command and sends pulses to the selected digit.

The scoreboard consists of 25 digits. The six digits on the left side are 9" high. The other digits are all 6". These were what I had in stock.

Digits require 12-VDC pulses at a few hundred milliamps peak, but only for a few milliseconds. Thus, the signals from the microcontroller’s output bits must be suitably amplified to deliver this power to the coils. Each digit requires a driver circuit to set, or flip, the segment into view. A similar circuit must reset, or flop, the segment out of view. A smaller design would feature a single driver circuit with a multiplexer to perform both set and reset functions. I opted for two separate drivers.

Additionally, each digit would normally require seven drivers to set segments A through G and another seven to reset each segment. However, you can save components by paralleling all seven reset segments and resetting all of the segments of a digit simultaneously. Before setting a digit to a desired value, you must reset all the segments. Only one reset driver circuit is required.

Digits are multiplexed so each segment of a digit is wired in parallel with the corresponding segment of all the other digits (i.e., all segments A are together). Because of the center-tapped coil arrangement and the paralleling of the reset wires, there’s the possibility of alternate routes for pulses. Thus, steering diodes must be placed in series with each coil to prevent multi-digit segments from changing simultaneously.

Each half of the coil also requires a standard shunt diode to prevent induced reverse currents from causing damage. Figure 1 is the diode wiring arrangement for a single segment.

A digit is selected by closing a combination of two switches (see Figure 2a). Switch S1 is actually a DC/DC solid-state relay (SSR) that routes the 12-VDC power to the coil. Switch S2 is a silicon-controlled rectifier (SCR), triggered by the output of a 4:16 demultiplexer commanded by the microcontroller.

Eight SSRs are required, seven for segments A through G and one for the resets. Twenty-five SCRs are required, one for each digit. Using these components minimizes the parts count, but it costs more than using power transistors or FETs. Figure 2b is a less expensive arrangement that requires more parts.