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Thursday, July 6, 2017

Can LED lights run for 25 years? 10 years?

Some people are working on circuits to make a dim LED light run for a year, or even 25 years. The initial attempts are to provide "marker lights". These lights are not light enough to use to walk around in the dark or read by, but they are perfect for marking tent pegs, camp sites, or trails at, for example, concerts or camp sites. They can also be used to mark latrines which are often some distance from camp sites, and be used in astronomy to possibly read text (although the lights are not very bright).

Tritium gas tubes last about 10 years, but they are expensive and not available legally in the US for hobbyists. Also there are some fakes sold on Ebay that only last a few months. In the US they can only be sold as part of a watch, gun sight or compass.  Ebay does not appear to regulate plain tritium vials sold to the US.

First a quick review of how batteries work.
  • An item, like an LED, needs a minimum amount of volts and amps (or milliamps) coming from the battery to make it work. If the minimum amount of both are not provided, the device does not work.
  • Volts is one measurement of how much power a battery puts out. As the battery is used up, the amount of volts it can provide goes down. So an LED might require 3 volts to work, or about two 1.5v AA batteries. As the battery is used up the volts go down, and the LED might go completely off at 2.5vdc.
  • Amps is another measurement, amps get "used up" as an LED is powered on. An LED might require 20 milliamps (ma), and the battery can supply only so many milliamps. Once those are used up, the battery no longer powers the LED light. But also, as the battery is used, the volts also go down, and the amps it can provide also goes down. Eventually the battery can only provide 10ma, which does not light the LED. 
  • Typical LEDs currently use 20-50ma (milliamps) but these projects are an attempt to use micro amps (a much smaller unit), instead of milliamps.
I have personally used voltage boosters to get more power out of AA and other batteries. The battery life is improved around 30%. Link here. But I don't use low-brightness LEDs. I have used 14,000mcd to 1 watt LEDs in my designs.

Here are some examples of current projects from Hackaday.io.

TritiLED is a circuit used to power an LED for 1 or more years before the battery needs changing. It is not a tritium vial, it is a low-powered LED with a special circuit.

ATTiny45 Everled. This runs at 10 microamps (ua) and has run for 2+  years on a single 3v coin cell. In this case the ATTiny45 is pulsed on for just one microsecond, which lights the LED, then the power is turned off. However the flickering is so fast that the human eye does not notice the LED turning off.

Decade Flashlight. Estimated run time is 10 years on 2 AA batteries. 3 boards for $3.05 on Oshpark but you must also pay shipping, buy the parts, and assemble it. You must also program a PIC microcontroller.

Quarter century lamp. Will work for 25 years on 2 AA batteries which provide 3.6vdc. Lithium Iron Disulfide batteries also seem to only lose 5% of their energy after 20 years of storage so the author might use these in his device.

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