You need a separate power supply for the panel. There is a connector for that
separate from the logic connector, typically a big one in the center of the
board. The board requires 5V (double check the polarity: what is printed
on the board is correct - I once got boards with supplied cables that had red
(suggesting +) and black (suggesting GND) reversed!). This power supply is
used to light the LEDs; plan for ~3.5 Ampere per 32x32 panel.
The connector on the RGB panels is called a Hub75 interface. Each panel typically has two ports, one is the input and the other is the output to chain additional panels. Usually an arrow shows which of the connectors is the input.
Here you see a Hub75 connector to be seen at the bottom of the RGB panel
board including the arrow indicating the input direction:
Other boards are very similar, but instead of zero-indexed color bits
R0, G0, B0, R1, G1, B1, they start the index with one and name these
R1, G1, B1, R2, G2, B2; the functionality is identical.
Throughout this document, we will use the one-index base, so we will call these
signals R1, G1, B1, R2, G2, B2 below.
The strobe signals is sometimes also called latch or lat. We'll call it
strobe here.
If you plug an IDC-cable into your RGB panel to the input connector, this is
how the signal positions are on the other end of the cable (imagine the LED
panels somewhere outside the picture on the left); note the notch on the right
side of the connector:
The RPi only has 3.3V logic output level, but many displays operated at 5V interprets these logic levels fine, just make sure to run a short cable to the board. If you do run into glitches or erratic pixels, consider some line-buffering, e.g. using the active adapter PCB. Since we only need output pins on the RPi, we don't need to worry about level conversion back.
For a single chain of LED-panels, we need 13 IO lines, which fit all in the header of the old Raspberry Pis. Newer Raspberry Pis with 40 pins have more GPIO lines which allows us to connect three parallel chains of RGB panels.
For reference, this is how the numbering on the Raspberry Pi looks like:
This is the same representation used in the table below, which helps for visual inspection.
For each of the up to three chains, you have to connect GND, strobe,
clock, OE-, A, B, C, D to all of these (the D line is needed
for 32x32 displays; 32x16 displays don't need it); you find the positions
below (there are more GND pins on the Raspberry Pi, but they are left out
for simplicity).
Then for each panel, there is a set of (R1, G1, B1, R2, G2, B2) that you have
to connect to the corresponding pins that are marked [1], [2] and [3] for
chain 1, 2, and 3 below.
If you only connect one panel or have one chain, connect it to [1] (:smile:); if you
use parallel chains, add the other [2] and [3].
To make things quicker to navigate visually, each chain is marked with a separate icon:
[1]=:smile:, [2]=:boom: and [3]=:droplet: ; signals that go to all
chains have all icons.
| Connection | Pin | Pin | Connection |
|---|---|---|---|
| - | 1 | 2 | - |
| 💧 [3] G1 | 3 | 4 | - |
| 💧 [3] B1 | 5 | 6 | GND 😄💥💧 |
| 😄💥💧 strobe | 7 | 8 | [3] R1 💧 |
| - | 9 | 10 | E 😄💥💧 (for 64 row matrix, 1:32) |
| 😄💥💧 clock | 11 | 12 | OE- 😄💥💧 |
| 😄 [1] G1 | 13 | 14 | - |
| 😄💥💧 A | 15 | 16 | B 😄💥💧 |
| - | 17 | 18 | C 😄💥💧 |
| 😄 [1] B2 | 19 | 20 | - |
| 😄 [1] G2 | 21 | 22 | D 😄💥💧 (for 32 row matrix, 1:16) |
| 😄 [1] R1 | 23 | 24 | [1] R2 😄 |
| - | 25 | 26 | [1] B1 😄 |
| - | 27 | 28 | - |
| 💥 [2] G1 | 29 | 30 | - |
| 💥 [2] B1 | 31 | 32 | [2] R1 💥 |
| 💥 [2] G2 | 33 | 34 | - |
| 💥 [2] R2 | 35 | 36 | [3] G2 💧 |
| 💧[3] R2 | 37 | 38 | [2] B2 💥 |
| - | 39 | 40 | [3] B2 💧 |
In the adapter/ directory, there are some boards that make the wiring task simpler.
You connect the Pi to the first element in the chain of panels. Each panel has an output connector, that you then can connect to the next panel. Thus you can create a larger panel. Here a schematic view, below in the 'Power' section, you can see a real-live panel with three chains of 5 panels each seen from the back.
These displays suck a lot of current. At 5V, when all LEDs are on (full white), my 32x32 LED panel draws about 3.4A. For an outdoor panel that is very bright, that can be twice as much. That means, you need a beefy power supply to drive these panels; a 2A USB charger or similar is not enough for a 32x32 panel; it might be for a 16x32.
If you connect multiple boards together, you needs a power supply that can keep up with 3.5A / panel. Good are old PC power supplies that often provide > 20A on the 5V rail. Or you can get a dedicated 5V high current switching power supply for these kind of applications (check eBay).
The current draw is pretty spiky. Due to the PWM of the LEDs, there are very short peaks of a couple of 100ns to about 1ms of full current draw. Often, the power cable can't support these very short spikes due to inherent inductance. This can result in 'noisy' outputs, with random pixels not behaving as they should. A low ESR capacitor close to the input is good in these cases.
On some displays, the quality of the output quickly gets erratic
when voltage drops below 4.5V. Some even need a little bit higher voltage around
5.5V to work reliably. Also, tweak with the --led-slowdown-gpio flag.
When you connect these boards to a power source, the following are good guidelines:
-
Have fairly thick cables connecting the power to the board. Plan not to loose more than 50mV from the source to the LED matrix. So that would be 50mV / 3.5A = 14 mΩ. For both supply wires, so 7mΩ each trace. A 1mm² copper cable has about 17.5mΩ/meter, so you'd need a 2.5mm² copper cable per meter and panel. Multiply by meter and number of panels to get the needed cross-section. (For Americans: that would be ~13 gauge wire for 3 ft and one panel)
-
While a star configuration for the cabeling would be optimal (each panel gets an individual wire from the power supply), it is typically sufficient using aluminum mounting brackets or bars as part of your power solution. With aluminum of 1mm² specific resistivity of about 28mΩ/meter, you'd need a cross sectional area of about 4mm² per panel and meter.
In the following example you see the structural aluminum bars in the middle (covered in colored vinyl) dualing as power bars. The 60A/5V power supply is connected to the center bolts (display uses about 42A all LEDs on):
-
Often these boards come with cables that have connectors crimped on. Some cheap cables are typically too thin; you might want to clip them close to the connector solder your proper, thick cable to it.
-
It is good to buffer the current spikes directly at the panel. The most spikes happen while PWM-ing a single line. So let's say we want to buffer the energy to power a single line without dropping more than 50mV. We use 3.5A which is 3.5Joule/second. We do about 140Hz refresh rate and divide that in 16 lines, so we need 3.5 Joule/140/16 = ~1.6mJoule in the time period to display one line. We want to get the energy out of the voltage drop of 50mV; so with W = 1/2CU², we can calculate the capacitance needed: C = 2 * 1.6mJoule / ((5V)² - (5V - 50mV)²) = ~6400µF. So, 2 x 3300µF low-ESR capacitors in parallel directly at the board are a good choice (two, because lower parallel ESR; also fits easier under board). (In reality, we need of course less, as the highest ripple comes with 50% duty cyle thus half the current; also the input is recharching all the time. But: as engineer plan for maximum and then some; in the picture above I am using 1x3300uF per panel and it works fine).
Now welcome your over-engineered power solution :)