I've got a HCMS-3972 LED display, and I'd like to play with it from Arduino following this tutorial. The diagram and schematics show a capacitor, but does not specify any details. The data sheet says the following:

The value of the capacitor depends on the series resistance from the ground back to the power supply and the range of frequencies that need to be suppressed. It is also advantageous to use the largest ground plane possible.

I'm a complete beginner and can't decipher what that means. How should I pick the capacitor to use for this project?

  • \$\begingroup\$ A 100nF ceramic would be a good start. \$\endgroup\$
    – Kartman
    Apr 18 '21 at 2:22
  • \$\begingroup\$ Thanks, care to explain why @Kartman ? \$\endgroup\$
    – rem
    Apr 18 '21 at 2:38
  • 1
    \$\begingroup\$ It’s s bit like asking what size wood is needed to hold up a roof - there’s a lot more information needed to give a precise answer. I have no idea of how you will actually wire it, with what size wire etc, etc. So, 100nF is my guess. For a more complete answer we’d have to delve into some physics and math, which you’re probably not interested in. \$\endgroup\$
    – Kartman
    Apr 18 '21 at 3:06

An old rule of thumb that I follow is 10-100 uF aluminum per board, and 100 nF ceramic per IC. If the display is close enough to the Arduino, you might consider it to be an extension of the Arduino and not a separate board and skip the aluminum.

More is usually better, but some power sources may not like too much capacitance, I would start on the low side of the aluminum range.

These old rules are for DIPs operating at a few megahertz. Modern high-frequency surface mount circuits have different rules.

The ceramics are for high-frequency glitches, so they should be close to the ICs.

Your display is not a typical IC, it is a power hungry monster, so I would probably use several 100 nF or a 1 uF ceramic.

The link shows the circuit being wired on a solderless breadboard, but be aware that some high-power and/or high frequency circuits are difficult or impossible to get to work on a solderless breadboard.

If your circuit isn't working, and you have a scope, look at the Vled and Vlogic at the display. If it looks noisy, add more caps. Unfortunately, evaluating noise can be difficult, a digital circuit will always have glitches.

As a beginner, you should follow tutorials as close as possible. We often see beginners wire circuits with really long wires and/or make significant changes to the circuit and expect it to work.

Picture: Yellow ceramic close to IC, black aluminum on rail.

enter image description here


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