Normally we don't discuss these kind of issues without more information. Not that we don't believe you but so far we have nothing. No Make or model for the mp3 player. No voltage measurements (you didn't say that you measured it, you simply said it it is such and such),
Let's break it down:
1
I'm building a system that uses an arduino to operate 3 relays on a relay shield.
(simple enough)
2
Two are used for momentary dry contact closures to start and reset a countdown clock
3
the other is used to turn a button light off for the duration of an external audio track played by a separate mp3 player
.
4
The trigger for the arduino is an analog (I'm assuming) output signal of 5V (100mA) generated by the mp3 player that lasts for the duration of the audio track.
5
The only thing I want to use this output signal for is to tell arduino when to trigger the relays.
6
So I only care if the signal is high or low.
7
I had been fussing with reading it into pin A0 using any values above 3V as high and 0 as low. But now I'm wondering if I can just read it into a digital pin to simply do a digitalRead to determine high or low.
8
I'm especially interested in little fussy coding snipets I need to include to make sure I'm not accidentally causing explosions and mass hysteria all over my psyche.
First things first.
We don't assume here (you know why)
We need to know exactly what kind of signal is coming from the mp3 player.
You have two options:
Option-1: Find the user manual and post the instructions describing the signal
Option-2: Put a meter on the end of the cable that is plugged into the mp3 player and measure the voltage and post that.
Second,
We're not going to write the code for you or give you snippets. You are perfectly capable of googling such things as
"arduino Analog read"
or "arduino digital read"
or "arduino digital write"
The ball's in your court. Do your homework. Get the info we need. Research the above syntax and then make an attempt to write the code and we'll go from there .