1. Using amount of current much less than 10-20 milliamps to power indicator lamp LEDs in battery powered equipment can allow increased battery life, and/or smaller battery size.
2. In line powered equipment, reducing LED current from 20 milliamps to 2
mA or less can save a customer 11 cents per LED per year assuming 5 volt
supply rail voltage, 80% efficiency of power supply and 11 cents per KWH
The costliness of lower efficiency LEDs increases with supply rail voltage.
3. LEDs and dropping resistors do produce heat. Reducing current through LEDs and dropping resistors by 8-20 milliamps sometimes means lowering the temperature of the entire product by .5 to 1 degree C, occaisionally 2 degrees C. By the "usual rule of thumb" for relationship between failure rate and temperature, making a product run 1 degree C cooler decreases rate of heat-affected failures by about 7%.
Such LEDs are "only a little dim" at 2 milliamps. Many of these do reasonably well at 4-5 milliamps, while being only a little brighter at higher currents due to efficiency decreasing as current increases past a couple milliamps or so. These are also notable for low cost, because this technology was in mass production as far back as the late 1970's.
To get such LEDs from Digi-Key: Go to their website www.digikey.com and type LED into the "search box" and click "Go". On the reulting page, find "Optoelectronics" and click the subheading "LEDs - <75mA, Discrete".
Next step is to click "Red" in the first selection column (color) and then click the button "Apply Filters".
Next step is to go to "Wavelength", the last column, scroll down to the longest wavelength numbers, and while holding down the "Shift" key on your keyboard, click both 697 and 700 nm (or everything in the range of 690 to 700 nm inclusive).
Narrow the choice down from there. I find typical is to want LEDs with wire leads, meaning clicking "Through Hole" for "Mounting Type", "Radial" for "Package/Case", and also click the "In Stock" checkbox under these selection columns, then click "Apply Filters".
Next that I would do is go to the "Viewing Angle" column and while holding down the "Shift" key, click all viewing angles 60 degrees or greater. In addition, there is a "Packaging" column, and for hobbyists and prototype developers there is usually one good choice among the 2 that remain - "Bulk".
Before clicking "Apply Filters", I would also use the "click-with-Shift" trick to rule out any that can be ruled out, since this "loop" ends with a list of items as soon as you get it down to no more than 20 items. For example, I try this to exclude sizes smaller than 3 mm and larger than 5 mm, and to exclude brightnesses less than 1 mcd.
As of late 2/16/2009, that gets the list down to 3 items, all of which are Panasonic items that are round and of size 3-3.2 mm. My favorite one of these, which I use, is Panasonic LN28RPX, with Digi-Key catalog number of P563-ND.
Panasonic LN28RPX is a red tinted diffused 3 mm / T1 LED. Although it is rated only 2 millicandela at 20 mA, I find it only mildly dim at 2 mA and quite reasonable at 5 mA.
I retry the above search with minor differences in use of search terms and I turn up 3 very low cost LEDs of this type by Lumex:
Lumex part #SSL-LX3044HD, 3 mm / T1, rated 6 mcd at 20 mA
Lumex part #SSL-LX5063HD, 5 mm / T1-3/4, rated 5 mcd at 20 mA
Lumex part #SSL-LX5093HD, 5 mm / T1-3/4, rated 5 mcd at 20 mA
An older wide-angle "flat-top" "Fresnel lens" LED of this chemistry has part number of TLR-147 or TLR147. That one in my experience has served well at 3-5 mA and may remain available at a few Radio Shack stores.
More-modern brighter and more efficient LEDs with InGaAlP chemistry have similar wavelength and color specifications. The older technology of "high efficiency red" is highly available at low cost and in my experience tends to often require 3 milliamps to be "usefully bright".
Specific LEDs of this type available from Digi-Key, round 3 mm (T1) and round 5 mm (T1-3/4) with "through hole" "radial" leads, viewing angle at least 60 degrees and lower cost include:
3 mm / T1 ones:
Lumex part number SSL-LX3052ID
Lumex part number SSL-LX3044LID, notably lower cost
Lite-On part number LTL-10223W, noted as lacking a flange
Lite-On part number LTL-4221N, strangely costing more with inferior specs
Lite-On part number LTL-1CHE, lower cost
Kingbright part number WP132XID
Kingbright part number WP424IDT, 100 degree wide viewing angle flat-top
5 mm / "T1-3/4" ones: Lumex part numbers SSL-LX5063ID, SSL-LX5093ID and SSL-LX5093LID
Kingbright part number WP483IDT, 100 degree wide viewing angle flat-top
Please keep in mind that flat-top / cylindrical versions with wider viewing angles can easily require about 3-5 mA to achieve "good" brightness, and other versions with viewing angle closer to 60 degrees can easily appear dim if current is less than 2-3 milliamps or so. This particular LED chemistry tends to have efficiency maximized when current is at least 5-10 milliamps, and efficiency usually decreases significantly when current is reduced to around or under 4 millimaps.
Digi-Key has some offerings by Avago and Kingbright in this area. (through-hole, radial, test current 2 mA, nominal wavelength in the 568 to 635 nm range). It appears to me that Kingbright makes the "viewing angle" of these somewhat on the narrow side. I have only tested in this area one and it was the 5 mm yellow one by Avago, (HLMP-4719) and found it noticeably dim at 2 mA, but I found it "up to par" at 3.5 mA. High efficiency red ones (nominal wavelength 623 to 635 nm) should do well at 2 mA.
The Avago 2 mA ones have a rated maximum current of 7 mA.
Some of these are even rated for performance at 1 or 2 mA. Ones characterized for performance at 2 mA should do very well at 1 mA.
Ones rated for performance at either 1 or 2 mA and that are diffused red tinted 3 mm or 5 mm types include:
Avago HLMP-K150 and HLMP-D150 (60-65 degree viewing angle)
Kingbright WP7104LSRD and WP7113LSRD (30-40 degree viewing angle, and with impressively low cost)
Panasonic LN28RALXU does do well at low currents, does somewhat well at 1 mA, is characterized at 5 mA (3.8 mcd at 5 mA) and has a 70 degree viewing angle.
Not rated at low current but also likely good are Lumex SSL-LX5093SRD (60 degree 5 mm), SSL-LX2573SRD (110 degree rectangular), and SSL-LX25783SRD (a larger rectangular 110 degree one). The Lumex ones have fairly low cost.
All of these have maximum continuous current of 30 mA, including the ones with ratings at lower currents as low as 1 mA.
I mention a few others in my "Short List".
For my favorite general-purpose through-hole green ones, go here.
A green one good for viewing at greater distances when aimed horizontally is Cree C566C-GFS-CV0Z0792. Its rated viewing angle is 70 by 35 degrees. Make the wider spread horizontal. These need about 2 mA to be visible at short distances in direct sunlight.
A green one good for long distance viewing at a distance in a particular general direction, such as in a hallway or a specific distant viewing location, are Cree C503B-GCS-CY0C0791 (with lead stops) and C503B-GCN-CY0C0791 (without lead stops). Its rated viewing angle is 30 degrees. These need only about 1-1.25 mA to be visible at shorter distances in direct sunlight.
CAUTION - InGaN LEDs are static-sensitive.
Duty cycle should be what achieves suitable brightness with a suitable peak current, can be anywhere from 1.5 to 25%, and is typically 2.5-6.25%.
UPDATE 3/17/2009: Some (maybe most) recent Nichia green low power LEDs such as at least some lots of NSPG520AS and NSPG320CS have efficiency remaining so high at currents as low as .06-.1 milliamp that they have only minor benefit from pulsing.
My Page For Low Current LED Pulsing Circuits
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