Seven segments and a dot

Eight lights, seven of them bars

The display on this board has eight lights in it. Seven are bars arranged around the outline of an 8, and the eighth is the round dot at the bottom right. Nothing joins them. Each one can be on or off on its own, and the shapes we call digits are the ten combinations that happen to look like numbers.

They have names, the same on every seven-segment display: A across the top, B and C down the right, D across the bottom, E and F up the left, G through the middle, and DP for the point.

Press the letters and you will find what the ten digits cost. A 1 is two bars. An 8 is all seven. A 6 and a 5 differ by one bar, E, which is why a single dead segment can turn one into the other.

Why eight and not ten

Eight lights fit in one byte, one bit each, and a byte is the unit everything behind the display moves in. The shift register under the display holds eight bits, shiftOut() sends eight bits, and the table in the sketch is ten bytes long because there are ten digits.

Bit 0 is segment A, bit 6 is G, and bit 7 is the point. The chain of ideas in this handbook is short: you write a number, your sketch turns it into a byte, the byte goes down one wire, and the lights it names come on.

What the board already knows

Nothing. It does not know what a 3 is. It switches eight lights on and off in whatever pattern arrives, and every pattern is equally valid to it, including the 246 that are not digits.

That is more often a feature than a nuisance. A minus sign is segment G on its own. A degree symbol is A, B, F and G. Two modules showing E and 0 make a perfectly good error message.