Seven segments and a dot

Eight lights, seven of them bars

The display in your hand 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 just the ten combinations that happen to look like numbers.

They have names, and the names are the same on every seven-segment display ever made: 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 actually cost. A 1 is two bars. An 8 is all seven. A 6 and a 5 differ by exactly one bar, which is why a single broken segment turns one into the other and is worth knowing when a display starts showing the wrong number.

Why eight and not ten

Eight is not an accident. Eight lights fit in one byte, one bit each, and a byte is the unit that everything downstream of here moves in: the shift register on the back of the module holds eight bits, shiftOut() sends eight bits, and the table in the sketch is ten bytes long because there are ten digits.

So 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 eight bars light.

What the board already knows

Nothing. The module 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 a feature more often than it is 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 is a perfectly good error message.