The two solder jumpers

Which two

On the connector face, near the middle, there are two small pairs of pads each labelled with a pair of names. The labels say which two nets that jumper would tie together.

The reference printed beside the pair (CS, GND, SB3, BL or 3V3) is different from board to board, so go by the two net names. The 0.96 inch is the only one whose backlight jumper goes to GND, because its lamp is switched on the high side and a low BL turns it on.

What bridging each one does

CS to GND. CS is held low, so the controller is selected all the time. That is fine with one display on the bus. It means you can leave the CS hole unwired, and it means a second device on the same bus would hear every byte the display does.

BL to the rail. BL is held at the level that turns this board's lamp on, so the lamp is on all the time at full. You can leave the BL hole unwired, and there is no dimming. Without it the lamp is off until BL is driven, because a 10 kΩ resistor holds its transistor off.

Why they ship open

Open leaves both pads for the host to use, whatever the host is. Closed would have decided for everyone: a lamp that cannot dim and a select that cannot be shared. A bridge is easy to add and a little work to take away, so the board starts in the state nothing has to be undone from.

The one mistake

Bridge a jumper and the pad is no longer yours to drive. Two things drive it by default:

• begin() sets the CS pin high as it brings the bus up, and drives it for every transfer after that.
• begin() attaches PWM to the BL pin and writes full brightness to it, and every backlight() call after that writes a duty cycle.

With the CS jumper bridged, the first one is a GPIO driving high into ground straight away. With the BL jumper bridged, the pin and the rail agree while the lamp is at full and fight the moment the sketch dims it, because the pin then drives the other way for part of every cycle. Tell the library the pin does not exist, before begin():

LB_Wiring pins = LB_WIRING;   // the defaults for the board you picked
pins.cs = -1;                 // CS jumper bridged: the pad is tied to GND
pins.backlight = -1;          // BL jumper bridged: the lamp is always on
display.setWiring(pins);      // before begin()
display.begin();

With backlight at -1 the library's backlight() does nothing, and printInfo() says the pin is not driven. That is the point.

Bridging, and undoing it

The two pads are 1.27 mm apart centre to centre, with about a seventh of a millimetre of mask between them. Flux on both, a clean fine tip, a touch of solder, and the blob wets across the gap by itself. A meter on continuity confirms it.

To undo it, solder wick with a little flux and the iron on top for a second or two lifts the blob. Check with the meter that the pads read open. Nothing on the board was cut to make them, so there is nothing to repair.