Potentiometer Ground Noise With MAX7219

I think I see that you didn't solder the pins to the ProMicro.

The header pins? Those are soldered on.

So here is another problem. Things are connected by long wavy bits of wire, and through a "solderless breadboard". :worried:

Page 10 of the datasheet recommends "a 10μF electrolytic and a 0.1μF ceramic capacitor between V+ and GND as close to the device as possible."

This is of course purely for stability of the MAX7219. The MAX7219 is switching pulses of up to 320 mA (in proportion to how many segments/ rows are lit, not the brightness setting because the brightness is implemented by PWM) so you do have a significant impost on the supply rail. A capacitor of ten times that - 100μF - soldered directly to the MAX module will be a start. :+1:

I'd love to have the confidence to solder my projects together without testing them out on a breadboard, but alas, finding problems like this one is why I start with this.


Tom... :smiley: :+1: :coffee: :australia:

The caps came in the mail at last. I've tried a 0.1uF and a 0.47uF between the wipers and ground but I didn't see any noticeable difference. Putting a cap between the positive and negative of the MAX7219 did help quite a bit though, with 1000uF having the greatest effect. The output still has a little wiggle even if I turn down the LED brightness, but it's plenty manageable via code.

I assume you are using the USB port for power for the MAX chip.

This means that the MAX chip current is flowing through the Pro MIcro PCB, this will propably be the cause of that last bit of wiggle.

If you power your project externally with 5V applied to the Pro Micro and the MAX with separate wires from the supply that would be my next step if you need less wiggle.

Tom... :smiley: :+1: :coffee: :australia:

Hi Tom! That would be ideal but not possible for this project, it's essential that it functions only over USB. Thank you though!

You can still power it externally and use USB.
In the configuration you have now, make sure the MAX IC gnd and power are the closest to the 5V and gnd pins of the Pro Mini.

Did you solder those pins in so solder appears on both sides of the PCB, those holes are plated through and the greater the surface contact the better and lower resistance the joint.

Tom... :smiley: :+1: :coffee: :australia: