# Quick Easy Circuit Question (LED)

Discussion in 'LEDs and Optoelectronics' started by morrillb, Sep 27, 2012.

1. ### morrillb

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Sep 27, 2012
In connecting 4 LEDs in Series, what range of batteries can i use to power them?

I'm looking for pretty much the smallest battery possible and go from there. And how could i connect to them?

thanks

5mm LEDs

2. ### MrEE

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Apr 13, 2012
The voltage drop across an LED will depend on the type and color of the LED. See the following chart at
http://www.oksolar.com/led/led_color_chart.htm.
For standard red or green color you can use a 9V battery and have some room for a current limiting resistor.
You always need a current limiter with leds. As an example suppose each led is driven by 10mA and the voltage drop across each is 1.8V then the total voltage across 4 leds will be 4 x 1.8 = 7.2V now using ohms law, the resistor is calculated as (9V-7.2V)/10mA = 0.18 kohm or 180 ohms. Increase this value to 220, 270 or 330 ohms or even higher will reduce the brightness as needed.

3. ### morrillb

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Sep 27, 2012
thats pretty much what i figured would be the case, but are there any other light sources that require less voltage?

4. ### MrEE

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Apr 13, 2012
Not that I know of. What is your application?

5. ### morrillb

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Sep 27, 2012
Well I'm trying to incorporate lighting with a polymer hockey puck without adding too much weight or volume. It comes down to battery size/weight. If i can incorporate 4 LEDs with a central battery embedded into the puck that would be ideal.

6. ### morrillb

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Sep 27, 2012
Not only would a 9v brick be almost too big, but its geometry makes for the puck to experience uneven weights throughout and would most likely cause lots of waffling/bouncing. If there were 9v cyclinders the 3/4" and under in height that would be perfect as i could center it and series the LEDs circular and have even distribution, but i cant find anything of the sort.

7. ### BobK

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1,689
Jan 5, 2010
If you want minimum voltage, put the LEDs in parallel, not series. Then a 3V battery is enough (for red or green LEDs.) Use a seperate resistor for each LED.

Bob

8. ### morrillb

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Sep 27, 2012
will that reduce brightness or have any other effects?

9. ### MrEE

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Apr 13, 2012
No, the only effect is that now you have 4 resistors instead of 1. What is the minimum time you want the leds to be on without having to replace or recharge the battery?

10. ### morrillb

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Sep 27, 2012
Well I'm planning on making the battery easily removable, so I'd take it out after every use (approximately 1-2 hours of time on) and use the battery only when needed which would of course be during darkness.

Another question, are there terminals i can make/buy for easy connection to a coin/button cell battery or should i just use the two faces?

11. ### CocaCola

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Apr 7, 2012
There are coin cell sockets but if you want a quick and dirty get yourself some neodymium magnets (they even have some at Walmart in the craft area) and use them to hold the wire to each side of the battery... Make a little loop at the end of the wire so the magnet sits flatter... Or get creative and make little flat terminals the magnet can hold to the battery... Something like soldering the wire to a penny (or brass/copper washer) and then use the magnet to hold the penny against the battery... You can even solder directly to the magnet but the heat will weaken the battery and it's not always easy to get the solder to stick...

Some people just use rubber bands to hold the wires in place...

84
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Apr 13, 2012

7
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Sep 27, 2012
14. ### BobK

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Jan 5, 2010
If you are using a coin cell, even the large CR2032, you will not have to worry about taking it out after 2 Hours, it will already be dead!

Bob