Hi All -
My daughter asked if I could build her a small timer - very simple. Just three 7-segment LED displays and a button - push the button and it wakes up and counts down, then sounds a beeper, and turns itself off.
Simple. Sure, no problem, take no time at all. I'll use a 2313 since I have a pile of them.
Of all things to be a little stuck on, I'm trying to decide how to power the damn thing. It will run off batteries, so I want it to last as long as possible. The LED displays do suck juice when they're on, but they won't be on for more than a couple of minutes when in use, then it shuts itself down. I'll use an NPN in the VCC line - the button will short that to turn the 2313 on, and the first thing that will do is shove VCC to the base to keep it on. To turn off, set it LO, and let a pull-down keep it LO.
As I see it, powering a small battery device devolves down to these initial choices ...
1) Use a 1/2AA 6V battery and run directly off that. Simple, no regulator, fewer parts. Small battery though, don't know how long it would last. And they're expensive.
2) 9V battery. Now need to regulate down to 5V. Could use a simple cheap 7805-type, but that wastes a lot of power. Could use a small switching regulator, like a MAX727 (got a bunch of those too :) More efficient (how much more ?) but needs some more parts.
3) One (or two) 1.5V AA battery with a boost regulator.
What do you think ?