[Noisebridge-discuss] driving multiple LEDs with minimal batteries

Dr. Jesus j at hug.gs
Tue Jan 4 22:26:19 UTC 2011

On Tue, Jan 4, 2011 at 12:06 PM, meredith scheff <satiredun at gmail.com> wrote:
> I'd like to do a soft circuit scarf or three, but I'm always running up
> against the problem of power. I usually use fairly low power LEDs
> (<2v) driven by a 9v battery or one of sparkfun's LiPos.
> I've heard tell of somehow being able to power more, but I'm still learning
> this EE stuff. Could some kind person point me in the right direction?

You want to wire them up in parallel:

(+) -|>|- (-)
(+) -|>|- (-)
(+) -|>|- (-)

Not series:

(+) -|>|-  -|>|- -|>|- (-)

If you have too many LEDs on the same battery it won't work because
they will draw too much power.  How many is too many depends on the
LEDs.  If you hook them up directly to the battery, they may draw more
current than they're rated for, which is bad for the LEDs and may
cause the lipo battery to catch fire.

The cheap and easy way to make sure they don't draw too much power is
to put a resistor in series with the LED to limit the current.

(+) -/\/\/\-|>|- (-)
(+) -/\/\/\-|>|- (-)
(+) -/\/\/\-|>|- (-)

The resistor value in ohms is (battery volts)-(LED voltage drop) /
(the LED current you want in amps).  If you want 20 milliamps through
a single 2 volt LED and you're using a LiPo battery:

(4 volts - 2 volts) / 0.02 amps = 100 ohms

The LiPo battery voltage is only 4 volts when it's fully charged.
When it begins discharging, it drops to about 3.7 for most of its
discharge curve and then to 2.7 right at the very end.  Even though
the "right" number is 3.7 volts for most of the time the battery is
discharging, use 4 volts in your calculations to avoid using too
little resistance and putting too much current through the LED.

If you have too many LEDs in the circuit, the battery will try to
supply too much current.  If the battery is unregulated it might get
hot and catch fire.

The resistor "throws away" the extra energy going to the LED in the
form of heat, but a resistor is really cheap and you can put lots of
them in your circuit easily.  To make the battery last longer, you
need to build or buy a constant-current regulator or a switching
regulator, which is harder and a little more expensive.

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