Low power LED

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Hello!

I want to make a binary watch, but I've got some power issues.

A coin cell can hold up to 610mAh @ 3V. However if I try to power the LEDs (there will be 20 of them, so let's say 13 are on simultanously), and we power them with 3V, in series with a 1kOhm resistor.
That makes 13 * 5 mA = 60 mA only for the LEDs.

This will drain the battery in a few hours, not very good. LEDs should be very efficient, what am I doing wrong?

Cheers,

axos88

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LEDs are efficient. But....

Using a current limit resistor isn't. Neither is having them continuously on.

One way to improve efficiency is to use a small switch-mode power supply with a small (low value) current sense resistor. There are these things that are made exactly for powering LEDs. They take a voltage proportional to the current through them (or a few of them and adjust the voltage to maintain the correct current. Costs more, takes more space, but a lot more efficient (by around 30% or more).

Jim

 

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Like a lot of the watches of old that used LED's, you will probably need to put a button on the watch to show the time when needed and therefore not waste the coin cell.

Even of you manage to get them running at 2ma each with current limiting and time slices, you're looking at maximum (1000maH as a high value) of 38 hours or a bit longer.

At least as far as I know anyways

see a crummy site
http://smackaay.com/

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Thanks guys for the advice. :)

axos88

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High efficiency LEDs (e.g. 10 Cd at 20 mA) are visible even at currents in the 0.1 mA range.