# Beginner question?

Discussion in 'General Electronics Discussion' started by bobtheturtle, Oct 11, 2014.

1. ### bobtheturtle

6
0
Oct 11, 2014
Hi, I am planning on creating a string of led lights (like Christmas/Halloween lights) I have a 9v battery and i want to string together 30 white 5mm led lights. I will be using 1/4watt resistor on each light. Will the lights be to dim, to birght, or not work at all?

Any help would be appreciated. Thanks for your time

2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,497
2,838
Jan 21, 2010
Go to the resource section and look at the resource describing how to use LEDs. That will answer this question in great detail as well as pointing you to web sites that can do some of the calculations for you.

The short answer is that you should probably connect pairs of the LEDs in series and then connect a resistor in series with each of these. All of the resistor/LED/LED strings should be connected in parallel (like the rungs of a ladder).

Assuming you want the 9V battery to last a while, you might want the LEDs to run at between 1mA and 5mA (you should experiment first to make sure this is bright enough) For 1mA the resistors should be about 2.2 kΩ. for 5mA you would use about 560Ω and for 10mA 220Ω.

Remember that the total current will be 15 times this value, so if you use 560Ω resistors, the total current will be 15*5 mA = 75mA. A normal 9V battery will last about 6 hours, but the LEDs will get dimmer and dimmer with time. Even after 6 hours there may be a bit of a glow.

abuhafss, KrisBlueNZ and hevans1944 like this.
3. ### bobtheturtle

6
0
Oct 11, 2014
Thank you Steve, I read the entire article and really learned a lot.

I have an led light attached to a resistor but when I touch the 9v battery leads to the circuit, one of two things happen, the light doesn't turn on or the light goes on for a second and then turns off. Could this be caused by the leads on the 9v battery being to small?

4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,497
2,838
Jan 21, 2010
Well... If you connect the 9V battery around the wrong way you could easily destroy the LED.

Going on for a second then off can happen if you have no resistor or the resistor is connected the wrong way (i.e. it is across the LED instead of in series with it). In this case the LED is going to be very bright until it burns out.

Either way you end up with what is called a Dark Emitting Diode (DED)

5. ### donkey

1,295
56
Feb 26, 2011
could be bad connection too. also I see you are using a 1/4watt resistor..... what value?

6
0
Oct 11, 2014
7. ### donkey

1,295
56
Feb 26, 2011
how many you tried?

6
0
Oct 11, 2014

9. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,497
2,838
Jan 21, 2010
Are you sure you're not connecting everything in parallel?

Is the LED very bright before it fails?

10. ### bobtheturtle

6
0
Oct 11, 2014
So I have been playing with this string of lights for hours, trying to figure out how to light all of them up with one 9v battery but I cant figure it out. I can only get 3 of the light to light up. They all work but only three at a time. Any ideas?

I'm using a 1/4 watt resistor for all 8 leds. (they are all white I just colored some of the leds red)

Every light is a 5mm white
• 5mm (T-1 3/4) LEDs and some 1/4-watt resistors for making connections.
• In water clear casing/lens. Emits focused, ultra bright white light.
• Great for electronic and electrical experiments
• Forward Voltage: 3.0-3.2V

11. ### Bluejets

4,871
1,017
Oct 5, 2014
The last detail should give you a clue...... "forward voltage : 3-3.2V"
Each LED needs this voltage level........

12. ### bobtheturtle

6
0
Oct 11, 2014
haha wow Thanks Blue Jet. If that's the case then how do you light up 30 leds in a string?
Im really new to this whole thing so bare with me.

My thought process:

If I buy white leds with 1 FV than i should be able to get 9 leds to light up with one 9v battery.

Note: I prefer to have all 30 leds to be lit by one battery.

13. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,497
2,838
Jan 21, 2010
Go to the resource section and look at the resource describing how to use LEDs. That will answer this question in great detail as well as pointing you to web sites that can do some of the calculations for you.

The short answer is that you should probably connect pairs of the LEDs in series and then connect a resistor in series with each of these. All of the resistor/LED/LED strings should be connected in parallel (like the rungs of a ladder).

Assuming you want the 9V battery to last a while, you might want the LEDs to run at between 1mA and 5mA (you should experiment first to make sure this is bright enough) For 1mA the resistors should be about 2.2 kΩ. for 5mA you would use about 560Ω and for 10mA 220Ω.

Remember that the total current will be 15 times this value, so if you use 560Ω resistors, the total current will be 15*5 mA = 75mA. A normal 9V battery will last about 6 hours, but the LEDs will get dimmer and dimmer with time. Even after 6 hours there may be a bit of a glow.

14. ### Bluejets

4,871
1,017
Oct 5, 2014
Use 1 resistor (the 220R that you have) and 2 LEDs in series.
Make up 15 sets of these in parallel.
With the resistor you have, each leg will be limited to approx 13mA. .......... 9v-(3v+3v) = 3v ....... 3v/220R = 13mA.
Times 15, close to 200mA.
BUT......run that on a little 9v (216) battery either won't work or only last a very short time.