What is on the board
Under the glass is a 15-way socket, a row of eight holes, one transistor with two resistors that switches the backlight, two capacitors and two solder jumpers. That is the whole circuit, and it is the same on all six boards.
One circuit
Step the part control through the board. There are seven things on it, and every one of them is on all six boards. The panel's own socket is the only part that depends on the glass, and the board layout around it is different from size to size because the outline is.
The two sockets and the row
The panel socket takes the orange tail of the glass. The panel arrives seated in it, and the glass sits on the face opposite the connectors.
The 15-way socket, U31, is a 1.0 mm FPC connector for a cable to a
breakout. Its eight live contacts are the same eight nets as the row of eight
holes. The previous article has them
side by side.
| Board | Outline, mm | Panel socket | Backlight switch |
|---|---|---|---|
| 0.96″ | 29.2 × 25.0 | U32, 8 ways, 0.5 mm | AO3401, high side |
| 1.8″ | 35.0 × 56.7 | U7, 12 ways, 0.5 mm | BSS138, low side |
| 2.0″ | 36.0 × 64.0 | FPC1, 14 ways, 0.5 mm | BSS138, low side |
| 2.4″ | 44.0 × 75.0 | U32, 10 ways, 1.0 mm | BSS138, low side |
| 2.8″ | 48.6 × 79.8 | FPC1, 12 ways, 0.5 mm | BSS138, low side |
| 3.5″ | 54.6 × 95.6 | FPC2, 12 ways, 0.5 mm | BSS138, low side |
The 3D model of the 0.96 inch shows an 8-pin header fitted to the row; the other five show bare holes. What ships in a box is not something this book can check, so look at yours before you decide whether you have soldering to do.
The backlight switch
The backlight is a lamp that takes more than a pin should drive, so a transistor switches it. On five boards that is an N-channel BSS138 between the lamp and ground, with a 2.2 Ω resistor in the lamp's path, and BL goes to its gate. On the 0.96 it is a P-channel AO3401 between 3V3 and the lamp.
Each has a 10 kΩ resistor from the gate to the rail that turns it off. That resistor is the reason the lamp is dark until BL is driven, and it is the reason a floating BL is not a lit lamp. The backlight article draws both circuits and lets you drive them.
Two capacitors, and nothing else
A 10 µF and a 100 nF sit across 3V3 and ground. There is no regulator, no level shifter, no protection diode and no pull-up on any of the other lines. What the host drives is what the panel's controller sees.
Two jumpers and four holes
Two pairs of pads, marked with the two nets each would join, ship open. Each one changes what a hole does, so they have their own article.
Four plain mounting holes finish the board, one in each corner: 4.8 mm on the five larger boards and 3.0 mm on the 0.96 inch. Nothing connects to them.
When it does not work
No. The BOM has none, and the net list shows the 3V3 hole going straight to the panel's supply pins, the lamp's switch and two capacitors. Whatever you put on 3V3 is what the panel gets, which is why the 3.3 V rule is absolute, and why the signal lines have no protection either.
A BSS138 N-channel transistor on the lamp's low side on the five larger boards, and an AO3401 P-channel one on the lamp's high side on the 0.96 inch. Both sit in series with a 2.2 Ω resistor and the lamp, and both have a 10 kΩ resistor on the gate so the lamp is off until BL is driven. The 0.96's lamp is wired for the high-side part, which is why its polarity is the other way round.
Usually not. A 10 µF and a 100 nF already sit across 3V3 and ground on every board. If the picture glitches when the lamp comes on or the whole host resets, the problem is the supply feeding the board or a long thin wire to it, and a bigger capacitor on your own supply is the place to look.
They are not in the net list, so they are not wired to anything. They are 4.8 mm on the five larger boards and 3.0 mm on the 0.96 inch, one in each corner, and the boards are 1.6 mm thick. Use plastic screws or standoffs if you are not sure what is plated.
The six boards place the same parts in different spots, and the 3D model of the 0.96 inch shows an 8-pin header fitted where the other five show bare holes. The circuit is the same; the layout is not. Go by the silkscreen names beside each part and not by where a part is on another board.
What each pair of pads ties together, why both ship open, and the one mistake they allow.
The two solder jumpers →Edit this page — content/books/lcd/what-is-on-the-board.mdx
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