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  • 11 months later...

I know it's a bit late, but better late then never :P
use 3mm leds, and no, they do not take very much power...using a 9V is a bit overkill, but you won't notice it on the leds as they draw a very low amount of current...using a 9V battery would mean that your circuit would last a very very very long time.  a small watch battery is enough to power many small 3mm leds...look for the specs on the current draw on the leds and check out the amount of mAh that a small watch battery will provide...with these two numbers, you can calculate how long they will last.

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Most watch batteries are only 1.5V when brand new. Most red LEDs are 2V and other colours need more.
I checked the spec's of a pretty big lithium coin cell with a good power rating. It is spec'd for a load current of only 0.19mA for a watch. It might not power an LED for very long. 

The Christmas LED pins have a custom IC that blinks them to save the battery's life. If you have 5 LEDs at 5mA each (not very bright) operating continuously then the small battery will be dead in less than one minute.

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A red LED needs about 2V no matter what is its size. A blue or white LED needs about 3.5V. A small one makes less light than a normal size one and a low current would make it even dimmer.

Look at my Ultra-Bright Chaser projects. My circuit blinks each LED for a very short amount of time to save power. I changed the battery in one that I made one year ago. If the LEDs were on continuously then the battery would last only about 6 hours.

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okay, found some specs that you two might find interesting...
http://www.vishay.com/leds/miniled/
if you look at the "low profile" Red, Yellow, and orange they are all 1.8V @ 2mA

Now a CR2016 Lithium Ion coin style watch battery is 3V...and delivers approx. 90mAh (according to http://www.gregsearle.com/rex_batteries.html)

Or you could use 2 - AG3 (Silver Oxide) coin batteries (1.55V @ 45mAh each) which would bring you to 3.1V @ 90mAh

So, IF I'm using one L.E.D. (scale appropriately for more L.E.D.'s) at 1.8V @ 2mA, how long would the 2 - AG3 3.1V @ 90mAh cells last?

Would this be the correct calculation?:
L.E.D.: 1.8V * 2mA = 3.6mW
2 - AG3 cells: 3.1V * 90mAh = 279mWh

so take one divide the other and get 77.5 hours? and that would be on continuously?

So, there's the numbers you do the math...maybe those custom christmas pin flashers have some caps or ic's in them to increase the battery life, I don't know for sure.

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if you look at the "low profile" Red, Yellow, and orange they are all 1.8V @ 2mA

No they aren't. They have a range of forward voltage from who knows what (maybe 1.6V?) to 2.2V. You get what they make. If the moon was full when they were made then they might be 1.8V.
Look at how dim they are. My normal size LEDs are 100 times brighter.

Now a CR2016 Lithium Ion coin style watch battery is 3V...and delivers approx. 90mAh

Energizer's CR2016 datasheet shows it supplying only 0.1mA for 900 hours and it ends at 2.0V.
It shows 12 pulses a day of 6.8mA for 2 seconds long for each pulse, and it drops to 2.0V after about 83mAh.
It might supply 2mA for a few hours.
How long does the battery in a keychain LED light last if it is on continuously? Half an hour?
How about a laser pointer?

So, IF I'm using one L.E.D. (scale appropriately for more L.E.D.'s) at 1.8V @ 2mA, how long would the 2 - AG3 3.1V @ 90mAh cells last?

Why? It is too dim. The battery will last a few hours at 2mA.

Would this be the correct calculation?:
L.E.D.: 1.8V * 2mA = 3.6mW
2 - AG3 cells: 3.1V * 90mAh = 279mWh

No. The mAh rating for a battery is at a very low current, more current reduces the mAh rating.
If the battery is perfect (impossible) then it would supply 2mA for 45 hours. I didn't look at its datasheet but it is probably rated at only 0.1mA for a watch.

maybe those custom christmas pin flashers have some caps or ic's in them to increase the battery life, I don't know for sure.

Of course they have an IC to make them flash.
A cap supplies the current for each flash then slowly recharges beween flashes. Each flash is for a very short duration so the average current is very low so the tiny battery can last for a reasonable amount of time.
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So, based on our conversation, I understand that you're saying it's impossible to have a nice bright christmas tree brooch that lasts more than a few hours without replacing the batteries? come on man...if you're not saying it's impossible...then how is it done? with 2V @20mA bright leds? a 12V car battery? :P

here's a little something that will bake ur noodle :P
http://www.flashingblinkylights.com/

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So, based on our conversation, I understand that you're saying it's impossible to have a nice bright christmas tree brooch that lasts more than a few hours without replacing the batteries? come on man...if you're not saying it's impossible...then how is it done? with 2V @20mA bright leds? a 12V car battery? :P

The Blinky Lights Company's ad doesn't say how big is its battery nor does it say how long the battery lasts.
Didn't you notice? The LEDs blink! Therefore they are turned off for 98% of the time, so the average battery current is low.

Each blink in my Chaser projects lasts for only 15ms to 30ms, then after a few chases a pause of 1 second. The LEDS are off for most of the time. The LEDs are ultra-bright and the battery lasts a long time.
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The Blinky Lights Company's ad doesn't say how big is its battery nor does it say how long the battery lasts

It says they use a CR927 which if you reference Wikipedia's handy dandy battery guide (http://en.wikipedia.org/wiki/List_of_battery_sizes) says that it's a 3V 30mAh cell (the blinky lights company says they use 2 in each brooch).

The LEDs blink! Therefore they are turned off for 98% of the time, so the average battery current is low.

My bad, I made the assumption that you knew we were talking about blinking brooches. But in reading back, I guess there wasn't a reference of that until now  :-[

So, what is the calculation used in determining how long your 2 CR927's (or other lithium type cell) will last? (based on 4 or 5 bright leds flashing 2% of the time each? so say in total it would be like 1 led on for 10% of the time).
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It says they use a CR927

I couldn't find a datasheet for the CR927 but found one for a CR1025 with the same capacity. It is rated at a current of only 0.064mA. At 400 hours the voltage has gradually dropped to 2.8V then a sharp dropoff after more time. That is the only load shown, for a watch.

So, what is the calculation used in determining how long your 2 CR927's (or other lithium type cell) will last? (based on 4 or 5 bright leds flashing 2% of the time each? so say in total it would be like 1 led on for 10% of the time).

The average current would be 0.68mA if the LED current is 6.8mA. If the big CR2016 battery is used then it can supply 6.8mA for 24 seconds per day. Its voltage drops to 2.0V which is too low for a blinky after about 83mAh so let us say at 60mAh the blinky would still be visible. Numbers crunching says the battery will last for 60/0.68= 88 hours. I don't believe it.

I think the battery might last for 80 hours if the blinky was turned on for two minutes then off with a pause of 2 hours then on again for two minutes, etc. Nobody knows how long the battery would last if it powers a blinky continuously.

Where are you going to get a circuit to make the blinking? If a PIC still works when the battery voltage drops to about 2.5V then its output current would be very low. Then each LED would need a driver transistor, each transistor would need a base resistor, each LED would need a current-limiting resistor and the circuit would need a supply bypass capacitor. You would need to carry it for your grandmother.
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