What is on the board

In the order the signal meets them

The circuit board has two faces. The one with the white printing faces outward, and it carries the holes you wire and one solder pad, the PWM pad. Every other part is on the other face, the one the display sits on, in the space under it. The drawing below is the printed face as if the board were glass: parts where they really sit, small ones drawn bigger so a thumb can find them. Press one. To see the real thing, open the module in 3D and lift the display off.

The header. Your six signal wires and two power wires arrive at the IN holes. The headers are not fitted; the next article is about them.

U1, the 74HC595. A shift register: three wires in, eight outputs. It takes the data, clock and latch, and holds a byte, one bit for each segment. It runs from the 3V3/5V pin directly, so it is the part the logic rail feeds. Chapter 3 is about what it does with the bits.

U2, the ULN2803A. Eight drivers in one chip, each a switch to GND that closes when its input is high. U1's eight outputs go into U2's eight inputs, so that U1's first output (QA) switches segment A's channel and its last (QH) the point's. It is what carries the segments' current, which U1 could not.

The resistors. Between each output of U2 and the display sits one resistor: 75 ohm for each of segments A to G, 510 ohm for the point. They set how hard a segment is driven, and the point gets the larger one. The warning on the back is about the ten pads, which are on the display's side of them.

The display. Its ten pins are soldered to the ten pads along the top and bottom edges. Two of them, the anode pins, are one wire, and each of the others is one segment. The display is on the other face of the board, and these pads are where its pins come through. They are not for you to wire to.

The dimmer, in two paragraphs

The common anode is not tied to the 12 V supply. Between them sits Q2, an AO3407A P-channel MOSFET: its source is the 12 V and its drain is the anode. R9, 5.1 k, pulls Q2's gate up to 12 V, so Q2 is off unless something pulls the gate down. That something is Q1, an S8050 NPN transistor: when it conducts it pulls the gate low and Q2 turns on.

Q1 is driven from the PWM pad through R11, 220 ohm, and D1, a Schottky diode, in series. R12, 10 k, holds Q1's base to GND, so a PWM pin left floating means Q1 off, Q2 off and the whole digit dark. A pair of capacitors sits on each rail, 0.1 uF and 10 uF, to keep it steady. Chapter 4 follows this branch, in how the dimmer works.

What is not on the board

The net list the board was made from has two more parts, D2 and R10. They are not in the bill of materials, so they are not fitted, and nothing in this book depends on them. The four header positions are not fitted either: H1 and H7 on the IN edge have male footprints, U3 and U14 on the OUT edge have female ones, and the strips and sockets are loose in the bag.

The ten pads are on the board and they are not for wiring. They are the display's own pins, and a voltage applied to one goes straight to an LED with no resistor in front of it.