Wednesday, September 15, 2010

LED is still on

This is getting a little repetitive, I know.

Thing is if I don't mention it, people will think it's dead.


  1. snowbiker4:25 AM

    I went back and found your original led post. 3D cells, 510ohm resistor, red led. If I recall, red leds voltage drop is typically around 1.7V. Supply is 4.5V, so you're dropping 2.8V across the 510ohm resistor. That's 5.5mA draw. I just looked up on wikipedia the typical capacity of a D cell, and, guessing yours are Alkaline cells, I got 12A-hours. Dividing by the current that's over 2000 hours. Just shy of 100 days, even.

    And that's for a hypothetical battery that puts out 4.5V for its full 12A-hour life, and then suddenly drops to 0V. But of course they don't work that way, and will be constantly losing voltage and the LED just a little dimmer every day. As the battery voltage drops, the voltage drop over the resistor drops so the current drops and presents an even lighter load to the batteries, prolonging the glow. A function approximating the typical Alkaline battery discharge curve and some calculus would be needed to calculate the brightness-vs-time curve but my money's on still fairly bright at 5000 hours.

  2. So that LED lasts a good while then right? Is the light output adequate?

  3. Hi snowbiker

    Thanks for the calculations. Very interesting.

    That's a pretty brave guess though.

    We shall see...



  4. Hi Dennis

    The light is surprisingly bright for such a tiny device, as bright as many other LED's I have scattered about doing various things.

    Because the light is red, it's not practical for reading or anything.