LED schematic

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A LED, any LED, consumes more energy then a watch battery can provide for more then a short period of time. For short, intermittend use I saw two of those batteries in series with a switch and a white LED. No electronics whatshowever. It serves very well as a keyring light.

petrus bitbyter

Traditionally LEDs have been driven from a fixed voltage source with a current limiting resistor in series, this is important as LEDs have a fixed forward voltage drop which should not be exceeded (between 1.75V & 2.2V for red, green or yellow and about 3.4V for blue or white), this obviously means a single 1.5V button cell will not do, the voltage source needs to be sufficiently higher than the LED forward voltage that some voltage can be dropped across the resistor to produce (Vs-Vled)/R=I which could be in the range 2 to 20mA depending on the rated specification of the LED.

It is however increasingly becoming common practice to use a transistor or chip voltage converters to drive a LED from a single cell, some types pulse drive the LED so it can be driven harder for correspondingly narrow pulses - persistence of vision gives the illusion of higher light output for a given battery drain.

A couple of months ago Elektor Electronics magazine published an article on a single-cell LED driver chip, accompanied by a free cover mount ready built PCB, google for Elektor and see if the back issue is still available. The chip is not much bigger than a pin head and only needs a couple of small components added to make an LED driver not much bigger than the button cell.

Yes, you can do this easily. Get a bettery, a switch, a resistor(or a few from 100ohms to 10kohms(2 of each say since they are cheap)), and a few LED's. You can probably get everything for 10$ or so if you shop around or maybe 10000$ at Radio Sucks.

Battery + ---- LED ---- Resistor ---- - Battery

You just make a path from the + side of the batter to the - side of the batter(closed loop) and put the components along the path. If it doesn't work the first time then flip the battery around(because direction does matter with an LED) or the LED. The larger value the resistor the dimmer the LED will be but it will draw less current and hence last longer.

You then put your switch in the path too and it doesn't matter where. Do not hook up the LED without a resistor though as it will burn out very quickly.

Once you do this and get it working you'll have some idea. Not sure about the watch battery though as they are pretty low power sources and will be drained quickly but depending on how much current you draw.

Probably best way to get started is just experiment and ask questions. Its not as hard as it looks.

I do strongly suggest a switch so the LED can be turned off when not needed to conserve batteries.

Not necessarily. If the battery is equal or less than LED forward voltage, resistor is not needed. The formula for finding optimal resistor is: Vsource - V_LED / I_LED

Vsource is the battery, V_LED is the LED forward voltage, and I_LED is the current in Amp (multiple mA by 1000). In most cases 20mA is enough for normal use without blinding people. Take a red LED for example, off a 3v button battery:

Since you may end up with non standard resistor value, it usually works to round up to nearest standard resistor value (like 68) or even higher to be conservative (like 82 or 100)

Blue and white LED can run off 3v batteries without resistor since their normal voltage is just a bit higher.