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Understand ElectronicsWhile it isn't necessary to know anything about electronics to build a stompbox - many have built stompboxes without knowing a thing about it - understanding the fundamentals about electronics is something that's bound to help you one day.
ResistorsThis might be the simplest component to understand. Resistors resist, the futility of resistance can, and will, be debated. In theory (but not in practice) a battery can output an infinite amount of electrical current, so that if you were to connect the positive side to the negative side all the charge in the battery would instantly be gone. We'll come back to why this is different in theory and practice.
To prevent this from happening our devices limits the current they allow to flow through them. Our devices have a resistance that limits the current. Devices have a current consumption that's limited by an array of components, each with their own resistance.
A resistor is a component who's primary function is to limit the current flowing through a particular part of the device. The resistance of resistors is measured in Ohms. Combining resistors with other components we can achieve complex functions.
The simplest circuit, using a resistor, that we'll be looking at here is simply replacing the wire between the positive and negative side of the previously mentioned battery with a resistor. Let's say we have a 9 volt battery and connect a 10 Ohm resistor between it's poles. We've now limited the current from a theoretical infinite amount of Amperes to a more manageable 0.9 Amperes.
How did we come to that conclusion? Well, the current (measured in Amperes) that flows through a resistor (measured in Ohms) is a function of the voltage applied to it and it's resistance. The current in Amperes is equal to the volt applied divided by the resistance in Ohms. In our example 9 Volts divided by 10 Ohms equals 0.9 Amperes.
So what about our battery, why won't infinite current flow when connecting a wire from plus to minus? Well for two reasons. The first and most likely the biggest reason is that the battery has an internal resistance that is not zero, more likely one or two Ohms.
Links for more helpPrimer for understanding transistors
Geofex, don't leave until you've read it all
How to select led resistors scroll down to R.G's answer (numbered list). By the way, he's the guy that responsible for the Geofex site.
About trace width when designing PCBs
Another thread about trace width