Re: Putting Red LED's In Osram Dot it light

On Wed, 20 Feb 2008 02:29:30 -0800 (PST), "Dave.H"
<the1930s@xxxxxxxxxxxxxx> wrote:

I want to put 16,000 mcd red LEDs in an Osram Dot it light for working
outside when I don't want my night vision affected, the light in
question has 3 white LED's and a resistor (red-yellow-black-gold-
brown) voltage measured from the pins of the LED's is 3.122 VDC. How
would I go about installing these 2.0 volt red LED's? Do I just
change the resistor, if so what value? Unit is powered by 3 AAA
batteries.

Dave,

It sounds like the white LEDs are in parallel, drawing (theoretically)
about 20 mA per LED.

If you wanted to put your red LEDs in there at the same current draw
and wired in parallel, you'd use something like a 43 ohm 1/4 watt
resistor. If the LEDs are too bright, use a larger value resistor and
gain increased battery life. (56 ohms, approx. 15 mA. 82 ohms,
approx. 10 mA. 150 ohms, approx. 5 mA. 820 ohms, approx. 1 mA.)

If you find that one LED is much brighter than the other two with your
LEDs wired in parallel, use three individual resistors, one for each
LED, to balance the currents. 120 ohms per LED will give you approx.
20 mA per LED. Larger value resistors will lower the current.

Here's how to calculate the resistors and currents:

R = (Batt voltage - LED voltage) / current

Current = (Batt voltage - LED voltage) / R

You want to limit the current through each LED to a maximum of 20
milliamps. High-brightness LEDs put out a decent amount of light at 5
milliamps, or even 1 milliamp. For an outdoor night light, 1 milliamp
might be enough if all you want to do is look at a chart at close
range, or something like that. Experiment to find the current level
that meets your needs. (You might even find that a single LED
provides adequate brightness.)

If you have room in that housing to add a miniature switch, you could
connect two of the LEDs, biased for fairly high current, switched from
the existing power switch. Use your extra switch for a single
LED/resistor combo, biased for very low current. That would give you
a choice between dim and bright light levels, depending upon your
needs.

Have fun!

Tom

.

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