Solder it before you plug it

The order matters

Press Build it and watch the third step. That is the one that costs people an evening.

The panels go in first because you cannot get an iron to their pads once the header is standing in the way. The header goes in second. The diffusion sheet goes in last, and only if this is the display that gets it, because it is loose film sitting on top of the digits rather than something fixed to the board.

Why the header is not optional

The main README of the Segment-Display repository says it in as many words, and it is worth repeating because it is the single biggest source of support mail on these boards: a friction fit through an unsoldered hole is not a connection.

A pin resting in a plated hole touches in a few places, through whatever oxide is on both surfaces, with a resistance in the tens of ohms that changes as anything flexes. On a 5 V logic line that is enough to turn a HIGH into something the chip reads as neither high nor low. What you see is:

• digits that come and go,
• segments lighting that you never asked for,
• a display that works until you nudge the desk,
• and, most misleading of all, a display that works perfectly for ten minutes and then does not.

Every one of those reads like a wiring mistake or a library problem, and people spend the evening in the sketch. A 25 to 40 W iron and two minutes fixes it.

The joints you are making

Per display: twenty-four joints for the two panels, five for the header, twenty-nine in total. All of them are through-hole on 2.54 mm pitch, with a generous annular ring, on a board with no ground plane pulling heat away. This is the easiest soldering in the shop.

Two things worth getting right:

Sit the panel flat before you solder more than one pin. Tack one corner pin, check the panel is square against the board and not standing off it, then do the rest. A panel soldered at an angle cannot be straightened without desoldering twelve joints, and inside a round glass tube the angle is very visible.

Solder the header from the side the pins do not stick out. The five pads sit on the edge of the board and are half-castellated — the hole is cut through the edge — so solder will flow round to the outside and give you a joint you can inspect from either face.

Then the glass

The finished board slides down the tube with the digits facing out through the glass and the pad end towards the cork. There is nothing holding it in place except the fit, which is deliberate: it comes back out.

The cork is the strain relief. Run the five leads out past it rather than through it, push the cork home, and the wires cannot pull on the joints you have just made.