I am new to this electronics thing but I have an idea for a guitar pedal that I want to build.
The idea is that you would plug your guitar into this pedal and it would output to four guitar amps. Let's name the four outputs 1,2,3, and 4. So the plan is that only one output is outputting the signal from the guitar to one amp. So each output would be only passing a signal at a user select-able duration (using a potentiometer). With my little understanding I think it should be possible, I don't know what kind of relay I would need, and whether or not I would need to have external power for the relays. I have seen some projects that are using relays to switch audio but most of those are multiple inputs single output not single input multiple outputs. I apologize if this is not enough information or if I seems that I making other people do my job for me. I just need help getting started.
Not too sure of the purpose for this but if you are using relays it should be fine.
Most relays can not operate with the 5V @ 30 - 40mA from an arduino output pin so you need a transistor to boost the output to switch the relay. If it is a 5V relay then you can use the 5V output, otherwise you need an external power source to switch the relay. Do not forget that each relay needs a reverse biased diode across them.
You might be better off getting a relay shield.
Yes you can do this with Arduino if you prefer. I don't like relay's anymore because they are mechanical and fail. Also large and more expensive. I like using SCR's or Triacs now. I'm Thinking Triac in this case.
What I would do is write an array in Arduino that would contain the 4 output pins and write the software to index the array every time I hit the pedal (AKA switch). I would use the 4 output pins to run the 4 Relays, Triacs, SCRs, or Opto Isolators. Which ever I preferred. I'm sure you can get help with the coding in the Arduino and Programming section and Help with the electronics parts here. It's a process for sure. Especially when your new. But I promise the rush you get when your first project is finished and working is awesome.
Note that you need capacitors to couple the signals into and out of its switch lines. If you use a two-stage binary counter (e.g. a 74HC93 which is actually a 4-stage) you can drive that from an adjustable clock - you do not need a LMC555 but it is pretty easy to use, there are simpler ways - and you have your device all there.
In this application, relays are probably a good choice.
If you can [u]Blink[/u] an LED, you can turn a relay on & off.
If you understand the Blink Example, you should be able to write a sketch that blinks/sequences more than one LED, or turns on a series of relays in sequence.
Relays have a spec for the coil side (input/activation side) and for the contact (switch) side. If you want to drive the relay directly from the Arduino, the coil should be rated for 5V (exactly) and 40mA or less.
The contact side is not critical for a guitar signal. The voltage & amperage ratings for the contacts are the maximum allowed. The signal fro a guitar (or pedal) is one volt or so at a few microamps. The contacts of any relay can handle much more than that.
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...Although mechanical relays seem clunky and outdated, they can be very reliable. Where I work, we build a unit with 16 relays. I've worked here 15 years and I don't remember ever seeing a unit returned with a bad relay. There's an older product that was built back before I started, and I have seen some of those come back with "bad" relays. But, that's usually not the reason it was returned and usually the relay is just a little out-of-spec (resistance too high) rather than a dead relay.
DVDdoug:
In this application, relays are probably a good choice.
If you can [u]Blink[/u] an LED, you can turn a relay on & off.
If you understand the Blink Example, you should be able to write a sketch that blinks/sequences more than one LED, or turns on a series of relays in sequence.
That is what I was thinking, but instead of power it would pass through a audio signal. Which I guess has a current and voltage. Should I have a external power source if I get something like this? It says each relay needs 20mA, so for 4 of them means 80mA. 40mA over the 40mA maximum that you said.
As Mike points out, if you only ever operate one relay at a time, then you need only enough current for one relay.
Also, that board uses opto-isolators so when it says "each one needs 15-20mA Driver Current", it means that current is taken from the relay supply (which should really be a separate supply to an Arduino), not from the control input which you will observe requires only about 3 mA to operate:
Nevertheless, I repeat my previous points - a relay board is probably inappropriate for this function, and consumes a quite unnecessary amount of current (20 mA at least). The 74HC4052 or MAX4619 is designed for the exact purpose you requested, consumes almost no power and is vastly smaller, so should be your only choice.
And as I have also pointed out, a few CMOS ICs instead of an Arduino would perform the whole task with a running time of weeks on a pack of four AA cells.
Is that bad? Just switch it on and let it run through. Or a MOSFET if not 
So many options. So little time.