# LED Circuit help.

Discussion in 'The Projects Forum' started by stephenstavros, Nov 27, 2014.

1. ### stephenstavros Thread Starter New Member

Nov 27, 2014
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0
Hello All.
First off I know nothing about electrics so please ignore my stupidity!

I'm making an A4 21st birthday card, and on the inside I'd like to have a showbiz style mirror with LEDs instead of lightbulbs.

I'm going to drill small holes through the plastic mirror so that the legs of the LEDs poke through but the bulbs stay on top. I'm making the card out of thick mount board and it will have an area behind the mirror to house the circuits and batteries.

My question is:- How many LEDs can I run off one 9v battery (I'd like 24 in total) and is this as simple as daisy chaining the legs together, connecting a resistor at one end and the battery at the other with a switch somewhere in between?

Thanks for reading and any help will be greatly appreciated.

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2. ### #12 Expert

Nov 30, 2010
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Depends on the color of the LEDs. If white@3V each, 2 per series string, 10 ma each (half the maximum power allowed for the LED) , 12 strings, 120 ma. It will work, but the 9V battery will struggle at that rate and fail in a few hours.

If green @ 2.2V each, 3 per string, 8 strings @ 10 ma; about 5 hours to death.

3. ### shteii01 AAC Fanatic!

Feb 19, 2010
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Let us assume that each led requires 2 volts to turn On. 24*2 volts=48 volts. If you put all led in series, then you will need 48 volts to turn them all On. You only have 9 volts. Therefore putting led in series is not going to work.

4. ### shteii01 AAC Fanatic!

Feb 19, 2010
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It is showbiz/artist make up mirror with the lights all around it... which means they need all white led.

5. ### djsfantasi AAC Fanatic!

Apr 11, 2010
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LEDs have a property called forward voltage. A typical value for a yellow LED is around 2.0VDC. Twenty four of them in series (as you described) would require 48 volts. Your 9 volt battery isn't sufficient.

You get around this by creating series of 3-4 LEDs with a resistor and then wiring these in parallel. You'd need 6 or 8 of these series wired in parallel.

The resistor is calculated by the number in series times their forward voltage. Call this Vs for voltage, series. Then subtract Vs from 9 volts and divide the result by the current requirement of the LED.

By the way, had you considered using surface mount LEDs? They can be soldered to and used just like a standard LED.

6. ### #12 Expert

Nov 30, 2010
16,663
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I thought so. I also thought this application might need a rather short life as a birthday card...possibly only a few minutes.

7. ### shteii01 AAC Fanatic!

Feb 19, 2010
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I generally agree. I would have done what you proposed (several chains in parallel), but I was thinking two 9 volt batteries in parallel, I think one 9 volt battery is just too little. And like you said, it has to work for a hour or a little more.

8. ### stephenstavros Thread Starter New Member

Nov 27, 2014
4
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Thanks for all the replies.
So will it work if use 16 LEDs and 4 9V batteries (4 LEDs per batterys)?
If so do I just need 4 LEDs 1 resistor and 1 battery all joined up to make the circuit?

Thanks.

9. ### shteii01 AAC Fanatic!

Feb 19, 2010
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We don't know the specifications of the led. There are two things needed to be known for led: forward voltage, operating current. Since none of these numbers are provided, asking us if configuration X or configuration Y will work is a waste of your and our time.

10. ### stephenstavros Thread Starter New Member

Nov 27, 2014
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Apr 5, 2008
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Hello,

The given leds have a rather small viewing angle (25-30 degrees).
This will be like a set of spots around your mirror.

Bertus

12. ### shteii01 AAC Fanatic!

Feb 19, 2010
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4 led per battery will not work. Each led has forward voltage of 3.1-3.4 volts. Let us take the best case scenario, forward voltage is 3.1 volts. 4 led times 3.1 volts equals to 12.4 volts. Your one battery is only 9 volts. How do you plan to squeeze 12.4 volts out of 9 volt battery?

13. ### shteii01 AAC Fanatic!

Feb 19, 2010
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This is what #12 proposed. I assumed that each led has: Forward Voltage of 3.4 volts, Forward Current of 20 mA.

14. ### #12 Expert

Nov 30, 2010
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12,000 mcd each? Times 24? At the distance of a birthday card? That's like 288 birthday candles at 20 inches. Ouch on the eyes!
That's one reason I recommended running them at half power.

Try them at full power (post #13). If the average attention span of a Birthday card is 40 seconds, that will work (electrically) but you might be temporarily blinded in less than 40 seconds.

15. ### blocco a spirale AAC Fanatic!

Jun 18, 2008
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I would use 3v lithium cells (e.g. CR2016 @ 1.6mm thick and 90mAh) as these will fit within a flat card and you can connect as many as you like in series to get the voltage required. e.g. You could connect 10 cells in series to power 3 strings of 8 white LEDs with a 150R resistor per string.

Last edited: Nov 27, 2014
16. ### #12 Expert

Nov 30, 2010
16,663
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Good idea. I have a (1) LED UV leak detector that runs on a coin cell for hours.

17. ### stephenstavros Thread Starter New Member

Nov 27, 2014
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Thank you all very much.