# Basic diode light

Discussion in 'Electronic Basics' started by Mastermech, Mar 10, 2005.

1. ### MastermechGuest

I want to use a few diode to light a shadowbox and am curious about wiring
it up.

I understand basic electronics etc but am curious about battery life plan
on using 9v, and if a resistor is actually needed, or can I run a few in
series and be done with it.

The resistor is needed if you want to have any say whatsoever in how
bright your LEDs are, not to mention to prevent you from accidentally
letting the magic smoke out. Check the data sheets for your LEDs,
they'll have a forward voltage drop listed. Figure out how many lights
you need, in series you have a total forward voltage drop of N * Vd.

At this point, if you don't care too much about repeatable amounts of
illumination, you can just take however much voltage you have left in
your 9V, divide by the amount of lighting current you want, and place a
resistor of that size in the series to set the current. This will work
quasi-well. The problem is that a 9V battery is really a 9V _nominal_
battery, and will usually start around 9.5V or so and make it down below
7 by the end of it's life. If you want more regulation over the
intensity than those numbers provide, and/or to run the battery farther
down before it dies, you're going to need to be more creative. Linear
regulated current sources are fairly simple to do (though substantially
more complicated than a string of LEDs, a resistor, and a battery) and
will give you better brightess constancy. To really use as much of the
battery as there is you'll need to go to a switching regulator, which is
even more complicated and which, judging from your initial post, you
will most likely not be able to get to work.

3. ### RayGuest

Depends on the forward voltage and current required of the diode. I am
guessing that you want to use a couple hi powered white LEDs. Typical
bright white LEDs use about 20mA of current and about 3.5V. Using a 9V
would limit you to only two diodes in series and you should use a
resistor of about 100 ohms; 2V/100ohms = 20mA

Battery live. I couldn't find specs on a regular 9V but rechargeables
are only good for 200mAh meaning about 10 hours. So I would guess a good
energizer would be good for about 3 days or so, could be as long as a
week.

Some coin battery powered LED flashlights don't need a resister because
the battery cannot supply enough current to burn up the LED.

You might want to consider rechargeable AA batteries. Energizer has them
rated at 2100mAh which should give you some long run time.

Purchase your LEDs from EBay. I got a pack of 50 for \$8 and \$5 shipping.
Radio Shack wanted \$2.50 each. I only wanted 10 so I have all kinds of
LED "flashlights" for the kids.

4. ### MastermechGuest

Maybe I should put a light sensor in, possible through a npn to turn on and
off.

Any ideas.

Thanks again

5. ### Guest

http://members.misty.com/don/ledd.html for the resistor. Your favorite
battery vendor site (Duracell has good tech info) to look up the rating
in amp-hours or milliamp-hours for a 9 V battery. If the battery is
rated at 1 amp-hour (1000 milliamp-hour) and you have an LED drawing
20 milliamps, the battery ought to last for about 1000/20 or 50 hours.
It will be brighter at the beginning and dimmer at the end.

Matt Roberds