Electronic Circuits and Projects Forum

You could use a button battery, as long as it's able to supply enough current?, but bear in mind it won't last very long, they only have a very small capacity. Google for "joule thief" for circuits.

There is no such thing as a "1.5V " LED. It might be 1.3V and burn out with a 1.5V supply or it might be 1.8V and not light with a 1.5V supply.

LEDs are driven with current, not voltage. Usually a current-limiting resistor is in series with an LED (or the internal resistance of a tiny battery) and the supply voltage is higher than the LED's max voltage rating.

I sketched the voltage stepup circuit for my solar garden light and added a flashing circuit to it. It will work with a 1.5V LED or a 3.5V LED when the battery voltage is less than 1.0V.

EDIT: Added a transistor to isolate the voltage stepup circuit from the flasher circuit and to make the LED blink briefly instead of pause briefly.

This is a great example that current drives a LED, not voltage:

http://www.youtube.com/watch?v=roPJosBqsXA

Most LED projects use 3x of the 1.5v button batts because 1.5v isn't enough to light common LEDs. 2x (3v) can light red ones but there's not much headroom voltage for a ballast resistor to stabilize the current.

The 3V lithium "coin" batteries get used a lot without resistors in flashlights, because the battery's output resistance is itself high enough to limit the LED current.

That is why they are also in computers as backup memory, since it has high enough ESR.

Originally Posted by Krumlink

That is why they are also in computers as backup memory, since it has high enough ESR.

? A backup memory has no use for a high ESR. Very few apps actually desire a high ISR, the tiny LED flashlight thing is a very unusual case.

Lithium coins are good for holding backup memory because the app can tolerate high ESR batts ok.

The main reason Lithium batteries are used for memory backup is their extremely low self discharge rate. Since the CMOS memory chips draw next to nothing while in standby, it is really the "shelf life" of the battery that determines how long it will last in this application.

Too bad the old LM3909 isn't made anymore. Radio Shack used to sell the IC. Here's a site that built it without the IC.
http://home.cogeco.ca/~rpaisley4/LM3909.html

BlueRoom, that's a really good example for noobs. Who needs Eagle anyway?