I'm designing a very simple led light timer based on the Pomodoro time management technique. For those of you unfamiliar, you work for a set amount of time, take a short break, and repeat.
My end result is to have a 5v USB powered device that lights a green LED for 25 minutes, then switches to a red LED for 5 minutes over and over again. This does not at all need to be accurate...+/- 5-7% isn't going to kill me. Ideally, it'd be small enough of a device, like one of the smaller Altoid tins, to sit on my desk unobtrusively. Though an Arduino-controlled Nixie tube countdown timer does have its charm....
I've found and re-read this q&a, as a start but there are so many options that I'm not sure where I need to start.
From my understanding of the 555, it pulses (I'm thinking of it in astable mode), and triggers something. I'd originally thought I'd need a decade counter to keep TRACK of the pulses, but the page noted above seems to indicate that I can control the time between triggers with resistor and capacitor combinations. Of course, that suggests I'll need some formula to determine proper capacitance and resistance...which I've been unable to find.
Other solutions suggest relays, which I've not worked with yet...so I don't know what I don't know about relays in this solution.
Can someone point me in a good starting direction, or perhaps recommend which schematic on the above page would be most appropriate for my purposes, if any?