How to connect a ADAFRUIT16x32 RGB LED Matrix Panel to a Raspberry Pi 4

Question

I am having difficulty. I assume I have connected the 16x32 RGB LED Matrix Panel to a Raspberry Pi 4 I have but It never displays. I have followed all the tutorials on ADAFRUIT but they only confirm the pin out connection to a Pi3. has anyone ever got this to work?

shabaz · Answer

As Clem says, it's very likely that the logic levels are incompatible. A lot of these 5V display boards use older logic ICs with non-3.3V-compatible inputs, and you'll either get nothing on the display, or stuff that looks like it sometimes works with corruption.

There's also a high chance of power related issues, or software issues, if you're trying with a different version Pi. So, it's not surprising it isn't working. But, without photos showing your setup, including power source, and the output you got when installing and running the code (since you mention it did give you issues) it isn't possible to troubleshoot those aspects easily.

Problemchild · Answer

Definitely a 3.3-5v voltage level problem. I've not used their display but many others and they are all 5V using TTL shift registers for the most part and often CMOS so the 3.3v logic will be out of spec. Notice on the LED hat they use a separate 5V input and a 5V levelshifter. a bunch of transistors would do it as well up to you ...fortunately the levelshifting is all one way so easy enough!!

shabaz · Answer

Hi Leo,

It's great you've got the level converters, and if you're getting some output, then there's also a chance that the driver or interface software is likely to be functioning on the Pi 4 too.

However, that Adafruit level converter circuit is bad.. they ought to have 100nF capacitors close to the supply pins on the chip. I couldn't see them on your photo either, so it is worth adding them if they are missing.

The 2A PSU may not be enough (check the specs for the display boards, in case it specifies the recommended minimum) but since you're only trying to illuminate a few lines, this hopefully isn't an issue.. your multimeter is reporting a good value.

shabaz · Answer

Yes, with a short path, i.e. point-to-point on top or the underside. Like this:

shabaz · Answer

Also, the wiring is extremely long, and that can have issues at the speed at which the Pi's interface will be controlling that display panel. Hopefully the capacitors can help, but also a few hours spent shortening and tidying the wiring could be useful. The jumper cables are being used in an unusual way, they don't need to be soldered onto the proto board. Instead, it could be better to get a length of insulated wire and strip, twist and solder it, because then the wiring can be of the right length. If you need jumper sockets/plugs at the display end, then you could cut one end off from the jumper wire (or buy the jumper sockets and manually crimp them to your wire - that needs some more equipment though, although it is possible to get away with soldering the crimp terminals to the wire). If you need to extend a wire, then rather than chaining the jumper wires, another technique is to cut the jumper wire in half, then solder a wire, and then put heatshrink over it (needs a hot air tool, but a candle can be used). Anyway, the above is just some tips for prototyping slightly better, if the capacitors do not help. Here is an example of wiring onto a proto board, you can see all the on-board wiring is short, and any off-board wiring is reasonably tidy (the example here is using screw terminals and also header pins and jumper wires, but you could also directly solder off-board wiring too, it is just less maintainable, and might need some glue or tying down for strain relief). The 100nF capacitors are blue. Here's another example using breadboard but the concepts are similar. When using a breadboard, solid core wire can sometimes be easier. Again the 100nF capacitors are there, in blue. Also you can see some jumper wires, that were made by using crimps, and then putting heatshrink sleeving around them. This way the off-board wiring can be made to the desired length.

clem57 · Answer

Good voltage of 5 V. It also suggested to supply around 2 amps externally and not from the Pi. Also in terms of software, the GPIO pins need to act as a PWM switch on and off at a frequency or duty cycle to make the LED's "glow". The faster the switching, the brighter the "glow". An Arduino has hardware and CPU dedicated to do this whereas the Raspberry Pi relies on software OS and is less deterministic. Keep in mind that these displays are designed to be driven by FPGAs or other high speed processors: they do not have built in PWM control of any kind. Instead, you're supposed to redraw the screen over and over to 'manually' PWM the whole thing. On a 16 MHz arduino, we managed to squeeze 12-bit color (4096 colors) with 20% CPU usage but this display would really shine if driven by any FPGA, CPLD, Propeller, XMOS or other high speed multi-core controller. The good news is that the display is pre-white balanced with nice uniformity so if you turn on all the LEDs its not a particularly tinted white. The above to me says the Raspberry Pi not very well suited although may be workable. If the Raspberry Pi gets busy CPU wise, the GPIO pins may not be driven at the desired rate due to interference. I have not examined the demo software to understand how it works. Clem

clem57 · Answer

harness68 What software did you try? Do you have a digital meter to check the voltage changes to the panel? I have done simple LED's with no problem. My suggestion is check for the obvious before going too deep. Clem