Re: Want something...
didibreakit said:
I was just wondering, how may LED's could you run on a triple A battery? Any ideas? Any idea how long the battery would last?
The simple answer is none! That's because a LED needs a certain minimum voltage before it will start to emit light and the required voltage is greater than the 1.5V produced by the AAA battery; you'll need at least a couple of batteries in series, but ...
Now a more complicated answer. You can use a circuit (called a DC-DC converter) that, when powered by the battery, produces an output at a higher voltage. About 3-4V would be enough to work with most LEDs (green, yellow and red at least). A DC-DC converter is not 100% efficient so some of the battery power is wasted in the voltage conversion process - let's say it's 80% efficient.
Now we have 80% of the battery capacity to power some LEDs. A fairly conservative guess for the initial capacity of an AAA battery is somewhere in the 1000 mAh range. Another way of putting that is to say the battery can supply 1000 milliamps for one hour. As a very conservative estimate, let's say a LED needs 10 mA to be bright enough. We can supply 800 mA for an hour (remember 20% of the capacity is lost in the converter) so we expect to supply 10 mA for 80 hours, that is the battery will last roughly 80 hours with one LED. Two LEDs would take 20 mA so the battery would last 40 hours in that case. You could power four LEDs (40 mA) for almost an entire day (20 hours) without replacing the battery.
By the way, battery capacity is not quite as simple as I've made it seem; if you left four 10 mA LEDs on continuously then you might get 20 hours of light, but if you only switched them on for a couple of hours per day you might get more than 20 hours light in total. Batteries tend to recover some of their capacity after a rest (the wonders of chemistry).
Modern LEDs are quite bright even when supplied with very little current so the figures I've just given really are on the conservative side. If your LEDs actually take 2 mA rather than the assumed 10 mA all you need to do is increase the estimated times by 5. Another way to get longer battery life is by flashing the LEDs using a repetitive on-off sequence with a low duty-cycle, i.e. a high peak current for a small amount of time and no current at all for the rest of the time. If the repetition is rate is high enough you would never know they were flashing.
David
P.S. Here's a little project for you - it demonstrates how to get 5V from a 1.5V battery:
**broken link removed**