Cable options to power Uno and Max7219

I am working on a finish-line gate project for toy cars.

My questions is about power cable options.

As the final project with be played with by a child, I want the powering options to be simple.

My initial idea is to use a USB charging wall-brick (from an iPad) to supply 5V @1A.

I need to power an Uno and a separate Max7219 8-digit 7-seg display module board. I want the USB supply to power the both Uno and 7219 board directly (ie, the 7219 is not powered by the Uno’s 5V rail, but rather by the charging brick to assure adequate current).

Here my problem: I only went to use one charging brick, so at some point I need to split the power cable to a USB-B for the Uno and Pins/wire to attach to the Vcc header pin on the 7219 module.

Any ideas on how to split the supply cable or the power? I can always hack something off a USB cable, but the wires in there are really fragile and I’m not sure it would hold up in a play environment. Another idea is to land the USB power on a protoboard and split it off that way. In addition, I have been searching for an off-the-shelf solution but have not had luck finding an items that Y-splits to USB on one side and pigtails/bare wire on the other.

Would love to hear ideas about potential options or examples.

Thanks!

If you don't mind soldering, I'd tap into the pins of the USB connector on the Arduino and run power/ground over to the 7219 module directly, before you reach the 500ma fuse.

Use a USB to header cable.


Or

Feed cable +5v into the Arduino 5v female header pin, GND into the header GND pin.

Take 5v from the IOREF pin (which is 5v) over to your display.

If you need upload a sketch later, unplug this cable and just use the USB on board connector.

You may be able to power the I/O from the Arduino power header directly without any problem, i.e. if the current draw isn’t too much; measure the total current and let us know what it is.


Show us a good schematic of your circuit.
Show us a good image of your wiring.
Give links to components.
Posting images:


Suggest you buy one of these and add it to your tool box. (measure USB voltage and current).

If you are powering the Arduino from a stable +5v power source.....

Cant you just use the VIN pin to get access to the direct +5v power source?

  • update:
    might want to check schematic a bit.. I believe there might be a diode that drops voltage a bit perhaps?

As far as current goes.. isnt that one of the benfits of using the MAX72xx chipset?

It 'multiplexes' (or is it charlie plex? I always forget).. anywho..

so really only 1 led is on at a time.. but blinked so fast.. the eye thinks all correct leds are on at the same time?

limiting current to minimal need?

blh64:
If you don’t mind soldering, I’d tap into the pins of the USB connector on the Arduino and run power/ground over to the 7219 module directly, before you reach the 500ma fuse.

Thanks! Soldering not a problem… but if I go this route, I would only need to tap the + pin on the USB jack, as I could just link the 7219 back to Arduino ground via the header pins, correct?

xl97:
Cant you just use the VIN pin to get access to the direct +5v power source?

I understood from my research that the VIN could only be used if I was supplying power through the barrel jack. But if using the USB jack, there is nothing there (could be wrong about that, though).

xl97:
As far as current goes.. isnt that one of the benfits of using the MAX72xx chipset?

It 'multiplexes' (or is it charlie plex? I always forget).. anywho..

Pretty sure the 7219 multiplexes and lights a digit all at once, and per my reading of datasheet should budget 320mA as a worst case for each digit (pls correct me if I'm wrong on this).

So I think that puts the Uno close to the edge (~400mA when powered by USB?) but perhaps not over it. I was trying to design in a way that was bulletproof, but I thought I had read elsewhere in the forum that it was best to power the LED module directly.

larryd:
Use a USB to header cable.

Feed cable +5v into the Arduino 5v female header pin, GND into the header GND pin.

Take 5v from the IOREF pin (which is 5v) over to your display.

Just curious… How is this different from supplying power to the USB jack on the Arduino and then connecting the 5V pin over to the Max module VCC?

steve20016:
As the final project with be played with by a child, I want the powering options to be simple.

My initial idea is to use a USB charging wall-brick (from an iPad) to supply 5V @1A.

I'd be checking the power supply unit is an original isolated supply type and not an elcheapo chinese knock-off.

“ Just curious… How is this different from supplying power to the USB jack on the Arduino and then connecting the 5V pin over to the Max module VCC?”

It bypasses the Vcc steering transistor ‘T1’ and the on board 500mA fuse. (On an Arduino UNO)

“I'd be checking the power supply unit is an original isolated supply type and not an elcheapo chinese knock-off.”

Thanks for the suggestion. Yes, it’s an OEM charger from Apple. I only use OEM chargers!

So in thinking this project through some more, once I get proof of concept with my Uno, I might migrate over to a Nano for the final project.

Nano would have smaller footprint and profile without the headers, and I can easily land 5V on the pad without the header pins and keep a nice clean layout with wire management. (And sacrifice a basic USB 2.0 cable). I might land the power line first on a power distribution bus to provide a kind of strain relief in case the USB power cable gets tripped over in a play area.

I’ll report out the current draw I’m getting on the 7219 module once it arrives and
I get it up and running.

MAX7219 drives 8 LEDs on at a time for its multiplexing. So with Iset set to say 10mA, then the MAX7219 will be using around 80mA.

You can find USB B power splitter cable with various ends

https://www.amazon.com/UCEC-Splitter-Charge-Devices-Single/dp/B0179OXY9I/ref=asc_df_B0179OXY9I/?tag=hyprod-20&linkCode=df0&hvadid=309743296044&hvpos=1o4&hvnetw=g&hvrand=11877364175374183992&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9001868&hvtargid=pla-568844069426&psc=1

Use one for the Uno/Nano, and add breakout adapter like larryD showed for the MAX7219 board.

How about a laptop hub?
Laptop Hub

steve20016:
I’ll report out the current draw I’m getting on the 7219 module once it arrives and
I get it up and running.

Closing the loop here...

I measured about 140mA while the Max7219 module was operating during Display-Test mode.