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Author Topic: Problems with voltage using a few leds  (Read 1570 times)
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Coin cells can only supply a few mA
Is this only a behavior of coin cells? What about AAA batteries?

All batteries do it, but coin cells do it a lot more than AAA, AA, etc.

3xAA batteries is a good way to power Arduino projects.

If your peripherals need exactly 5V then add a voltage booster to them.
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You need more current.
This is what happens:
- Every led you power needs current
- The more current you take the more the voltage goes down
- If you power too many leds your voltage will drop to 2.7v (and even less) making the ATmega chip to halt.

You can solve everything with a power supply such as this one (you will need to cut the connector and adapt it for the arduino):
http://www.amazon.com/Raspberry-Pi-Micro-Supply-Charger/dp/B00DZLSEVI/

Or this one if you don't want to get your hands dirty:
http://www.amazon.com/D-Link-Hi-Speed-7-Port-Powered-DUB-H7/dp/B00008VFAF/
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Or an expensive one that will plug into the barrel jack connector.
http://www.dipmicro.com/store/DCA-0510

3-AAA battery pack can connect to the 5V & Gnd header pins; 5V here is after the reverse polarity protection diode, do not get the pins swapped. Also a good idea to add a diode (1N4001 or similar) from 5V pin to the Vin pin to avoid back-powering the 5V regulator.
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Thank you for all the answers. In my case, I´ve chose this kind of battery because it´s a Lilypad and my project is for sewing in fabric. Sorry for don´t tell you previously. I thought that I´ve already told you. I´m trying now with the sparkfun power supply which converts 1,5 volt (AAA) in 5 V. The behavior is better but even so It´s not I expected.

I don´t understand (in the link attached bellow) how she can handle this amount of leds without using transistors and with the same power supply I´m now trying.

http://web.media.mit.edu/~leah/LilyPad/build/turn_signal_jacket.html
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I don´t understand (in the link attached bellow) how she can handle this amount of leds without using transistors and with the same power supply I´m now trying.
I agree. The LEDs are just an LED and current limit resistor, that many in parallel would seem to be drawing way too much current thru an IO pin.
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I don´t understand (in the link attached bellow) how she can handle this amount of leds without using transistors and with the same power supply I´m now trying.
I agree. The LEDs are just an LED and current limit resistor, that many in parallel would seem to be drawing way too much current thru an IO pin.

The LEDs could be multiplexed (only one LED at a time).

OTOH I wouldn't have thought it was possible to power a Mega328 using a CR2032 (even without LEDs).


« Last Edit: December 12, 2013, 10:09:45 am by fungus » Logged

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It doesn't say button cell, shows a AAA battery & Lilypad power supply.
Does show 8 or 10 LEDs connected in parallel to two IO pins, presumably 1 high and 1 low to turn the LEDs on. No multiplexing there.
LED modules are simple LED/resistor, 100 ohm for the one I looked at, implies near 20mA current draw: (5V - 3.2V)/100 = 18.5mA.
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It doesn't say button cell, shows a AAA battery & Lilypad power supply.

Presumably that's one of these: https://www.sparkfun.com/products/11259

I looked up a datasheet for the CR2032: https://www.adafruit.com/datasheets/maxell_cr2032_datasheet.pdf

It has nominal discharge of 0.4mA. It can manage about 2mA but not much more. How are they using them to power Lilypads?
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LED modules are simple LED/resistor, 100 ohm for the one I looked at, implies near 20mA current draw: (5V - 3.2V)/100 = 18.5mA.
Sorry for being insistent. I know that this set of 8 leds are considered as one led due to being connected in parallel. But if you connect in parallel you keep the drop voltage between those leds, but the more leds in parallel you add the more current you need, right? And each pin only provides at maximum of 40 mA, so i don´t see how she makes it without transistor...
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Sorry for being insistent. I know that this set of 8 leds are considered as one led due to being connected in parallel. But if you connect in parallel you keep the drop voltage between those leds, but the more leds in parallel you add the more current you need, right? And each pin only provides at maximum of 40 mA, so i don´t see how she makes it without transistor...

There's no way they can be running at 20mA, no. She's probably overloading the I/O pin.

But... Lilypad wire has resistance so it's hard to know exactly what's happening.
.
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OTOH I wouldn't have thought it was possible to power a Mega328 using a CR2032 (even without LEDs).

However see this:





OK, it's an AtTiny85, but that would draw about the same when asleep.

I used that as a "burglar deterrent" that flashes an LED every two seconds, for a millisecond.
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@Nick Gammon
In your first picture above, don't you mean 56 ohm resistor?
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Whoops! I'll fix the graphic.
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New graphic uploaded (you may have to refresh your browser). The old one had the resistor marked as 580Ω, now it is 56Ω.

Thanks for that. I must have done that late in the day. smiley
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Thanks for that. I must have done that late in the day. smiley

No worries.
Been there, done that, replaced the component.  smiley
Just in my case I was repairing a SMPS and replaced the 100K resistor feeding the controller chip with the rectified mains, with a 100 ohm one.
I'm sure you can guess what the outcome was.
I won't repeat the expletives I shouted, but suffice to say I wasn't very impressed with myself.

EDIT:

BTW, how long does that flasher circuit work for with that battery?
« Last Edit: December 12, 2013, 04:49:44 pm by UnoDueTre » Logged

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