Eight steps of brightness

Eight levels, unevenly spaced

Brightness on this chip is not a voltage or a current — it is how much of each digit's turn the segments are actually lit for. The datasheet calls it the pulse width and gives eight of them: 1, 2, 4, 10, 11, 12, 13 and 14 sixteenths.

Those numbers are worth looking at twice. Levels 0, 1 and 2 are a quarter of the range between them, levels 3 to 7 share the rest, and the jump from level 2 to level 3 is bigger than the whole distance from 3 to 7.

Which is exactly backwards from how the control feels. Somebody turning brightness up a step at a time sees almost nothing happen between 3 and 7, and then finds the only real dimming available is down at the bottom of the range.

For a bedside clock, level 0 or 1 is the night setting. For a display that has to be read across a lit room, anything from 3 up, and the difference between 3 and 7 is mostly how much current it draws.

It is sent with the digits, not on its own

setBrightness() only changes a variable inside the library. The value reaches the chip as part of the next write, tacked on to the end of the digits.

display.setBrightness(1);
// nothing has changed on the display yet
display.showNumberDec(value);   // now it is dim

This trips people up in setup(), where a setBrightness after the last showNumberDec appears to be ignored. It is not ignored, it is queued, and it will appear the next time anything is written — possibly minutes later, which makes it look intermittent.

Off is a separate bit

display.setBrightness(4, false);   // dark
display.setBrightness(4, true);    // and back, same number still there

The second argument is the display-on bit, and the digits survive it. The chip keeps its four bytes; only the output is switched off.

That is the clean way to blink a whole display — an alarm, a warning, a value that has gone stale — without re-sending anything or losing what was on it. It is also lower effort than blanking and rewriting: two bytes on the wire instead of seven.

And brightness is current

Each step up is more time with the LEDs conducting, and a panel showing 8888 at level 7 is the most current this board will ever draw. One display on a USB port is nothing to worry about. Six of them at level 7, powered through an Uno's 5V pin, is worth measuring before you trust it — more than one display picks that up.