Advice on connecting a 'sinking' 5V signal as an input to an Uno.

Hi all,

I will give you a brief outline of what I’m trying to achieve and would really welcome your knowledge and advice. I have a ‘smattering’ of electronics knowledge - basically enough knowledge to get me in trouble but not enough to get me back out of it - so I’m hoping you can help.

My current project is a home arcade machine. I’m hoping to adorn the sides with addressable LEDs (WS2811 LEDs to be exact). (See photo)

(If you think this will look ‘tacky’ then you are in full agreement with my daughter and most of my other family members… I personally believe you can’t have too many LEDs… :slight_smile: )

These are 12VDC and, because they are addressable, they take three inputs - a 12VDC, 0V and a ‘data’ input which I’m hoping to send out from my Arduino Uno (or a.n.other arduino type device if the Uno isn’t up to the job). From my research, it seems that the ‘data’ to the addressable LEDs is just a PWM output and that there are various software libraries available for this so I’m not too worried about this part yet!

Now… on to the problem…

The control panel uses RGB LED buttons. These are going to be controlled by a circuit board made by Ultimarc called an IPAC Ultimate. This device allows the control non-addressable LEDs (both RGB and single on/off ones). (see photo) - (it’s handy because it can be run from my PC using some software called “LEDBlinky” that is designed to be used with arcade games and will make the various LEDs cohesive with what’s happening on the arcade machine)

Is it possible to use the ‘single on/off’ LED outputs (which appear to “sink” 5VDC) from the IPAC Ultimate board to switch inputs on my Arduino Uno ?

With my limited knowledge of electronics I was thinking of a design something like the fag-packet drawing (see photo) but, I have no idea if this will work? And, whether I need other things along the way (resistors etc?). Also, whether I would need to link the two ‘commons’ of the boards? Or even link the 5VDC some way?? (although that last idea doesn’t sound like a good one??)

As I said at the start of this post, please don’t assume any knowledge on my part. If you have any ideas on how I can get this to work then it would be very much appreciated.

Thank you :slight_smile:

That board's outputs are probably constant current drive, so if they have enough compliance you can use them
to drive logic inputs so long as you enable pull-ups.

You need to common all the grounds.

Addressable LED strips use a special protocol, there are libraries for driving them.

Hi Mark

Thanks for getting back to me so quickly!

It's great news that this might actually be do-able!

The specs of the IPAC Ultimates outputs states:
Can be used with single-color or RGB LEDs (RGB use 3 channels)
Constant current negates the need for resistors for each LED
Can be used with LEDs with inbuilt resistors if required
6 X 1 amp MOSFET drivers which are suitable for LED strips and other high-power LED applications.

I presume that's what you mean when you said the boards outputs will be "constant current drive" ?

I'll make sure I common the grounds.

Thanks again for your help :slight_smile:

Yes - constant current outputs include a shunt resistor for monitoring the current, so won't pull all the way
down to 0V, but will normally be a volt or so higher, but that's good enough for a logic circuit normally.

You might want to check that the internal pullups work with those outputs - if there's too much leakage current
when they are off, if might read wrong, but an external 4k7 pullup resistor would probably cure that.

The leakage is unlikely to be huge, otherwise the LEDs wouldn't go dark when they should, but large MOSFETs
tend to have a bit of leakage current that might interact with too large a pullup resistor.

Mark, thank you very much for the info. That’s great. I’ll get something connected up and do some testing.

All being well, I’ll have my daughter shaking her head in despair before you know it!!!

Cheers :slight_smile: