Using an LED signal as an input.


This is probably a stupid question.. .. But if you don't ask and all that ....

Long story short.

Using an Arduino Pro mini, I have built my son a 'Bat Computer' display. Flashing LED's mostly. But even with a shift register, I ran out of inputs. Using the Analogue as input and even RX and TX digital lines.

But after putting it on the wall, I realise it really needs a sound element.

So. My question. As the pro mini is 'full'. Could I use a second pro mini to control an MP3 module. Using a wire soldered to one of the first pro mini led's to initiate a signal so that the second pro min knows when a particular led has been activated?

I can't see a reason why not. But I'd rather ask those who have more experience than I have before I start building this only to find there's some reason it wouldn't work.

Like this.. |500x267



Yes, you can look for a signal from another arduino. But, you should look at the other side of the resistor for the signal. Check the signal straight from the arduino output (before going through the resistor).

Be sure to provide common grounds between the two arduinos. If you feel like a little extra security, add a 1k resistor between arduinos.

Thank you for that.

So 2nd arduino pickup BEFORE the resistor and a 1k resistor between the 1st and second arduino's. Common grounds. No problem.

What does the 1k do?



I just feel a bit safer if I don't hook two different systems together directly, so I like a 1k (or so) to help isolate the two systems in case the power/ground gets disrupted. It may just be my cautions.

No problem. Makes sense.

Thank you.


As the pro mini is 'full'.

So I would use a port expander or a shift register to give you more input/output. Using a second processor is a very silly solution and causes more problems than it solves.

retrofiesta: But even with a shift register, I ran out of inputs.

I'm with Mike - that make no sense.

retrofiesta: As the pro mini is 'full'.

We have another classic "XY problem" here. :grinning:

Shift registers allow almost unlimited expansion as they are "chained". In addition, they simplify design an assembly as within reason, their positions can be staggered so that each spills to 8 (or 16, or whatever) inputs or outputs in one place, and the next to another of the same number, and the next ... and so on, distributing the connections over a practical separation.

You would also be using shift registers for outputs. Basically, about five Arduino pins controls any number of digital inputs and outputs.

A MAX7219 can control 64 separate LEDs, which need not all be of the same colour. These also can be expanded using just three pins for up to a dozen or so such chips.

I suspect you are reluctant to alter the present elements of your design due to affection and personal self-satisfaction. "Let it go!" If it needs a re-design, well it needs one! Re-designing it in a methodical fashion will permit you to continue to improve it until you run out of code and RAM space - and that may similarly be a reason to improve your coding at some point.

So - if you really want to do it properly, detail your project here.