The PWM pad

This is the newer board, the one with the PWM pad. The earlier version of this display had five signal pins and no PWM row, and no way to change its brightness. The digit, the shift register and the chain behave the same on both; this chapter is about the newer board only. If your back has no PWM pin and no boxed options, skip to the chain.

It ships open

On the back, beside the PWM pin of the IN edge, two small pads sit side by side. This is SB1, a solder jumper. One pad is on the logic rail, the 3V3/5V pin; the other is on the PWM pin. The silkscreen draws a line from the rail down to the pad and prints a box underneath.

The pad ships open: nothing joins the two pads. That is read from the manufacturing files, not guessed from the part's name in the net list, which is "NC" and would suggest the opposite. The paste layer over the two pads is two separate polygons, no copper region spans them, and the board's own box says to short the pad for full brightness, which would be pointless if it were already shorted.

In the box, in the board's own order: use PWM to adjust brightness, drive the pin high or low to switch the digit on and off, or short the pad for full brightness.

Three ways, and the one nobody chose

Start with the first button, which is the board as it comes out of the bag. The pad is open and nothing is on PWM, so the pin floats, a 10 k resistor holds the dimmer switch off, and the digit is dark. A new module that lights nothing is usually this, not a fault.

The three ways are all ways of settling that pin:

• Short the pad. A blob of solder across the two pads ties the PWM pin to the logic rail, so the switch is always on. Nothing to wire. Do not also drive the pin from your board.
• Drive it high or low. Any output pin gives you on and off, and nothing between.
• Give it a PWM signal. The fraction of each period that the signal is high is the fraction of the time the digit is lit. The next two pages are about this.

A bridge is only solder, so it can be undone with braid or a pump.

In a chain

The PWM pin is one wire across the board, and so is the logic rail, and a chain's headers join both of them from module to module. That cuts two ways.

One PWM wire from your board dims every module in the chain together. You do not wire one per digit.

And a pad bridged on any module ties the whole chain's PWM wire to the rail. Every module is then at full brightness, and a PWM output wired to the chain is trying to drive a wire that is held high. Choose one: leave every pad open and drive PWM from your board, or bridge one pad and wire nothing to PWM.