# do i need resistor for 50-70 leds?

#### qingtian28

hey there, i'm not an familiar with electronic stuff..
i'm planning to make my name out of 50-70 LEDs just for fun.
i'm planning to use a 9V battery(the square type) to lit up all the 50-70 LEDs.
So,
1) do i need a resistor for all my leds?
2) if possible, can teach me how to solder the circuit? so tht i doesnt need to use resistor

thanks!!

#### john monks

If all LEDs are identical they can all be connected in parallel with only one resistor in series with the battery. But realistically some LEDs may be brighter than others.
It would be better to have one resistor per LED but that's a lot of work.
What color LEDs do you intend to use. This effects the forward voltage drop and consequently the value of the resistor or resistors?

#### duke37

With 9V you may be able to get three leds and a resistor in a string. 20 strings = 60 leds.

20 strings at 20mA = 0.4A. How long will the battery last?

#### qingtian28

If all LEDs are identical they can all be connected in parallel with only one resistor in series with the battery. But realistically some LEDs may be brighter than others.
It would be better to have one resistor per LED but that's a lot of work.
What color LEDs do you intend to use. This effects the forward voltage drop and consequently the value of the resistor or resistors?

red and green

#### qingtian28

With 9V you may be able to get three leds and a resistor in a string. 20 strings = 60 leds.

20 strings at 20mA = 0.4A. How long will the battery last?

i not very sure how long does it last. but i dont plan to on it 24/7 ..

#### BobK

That's a good thing,since you would have to change the battery approximately 168 times.
In other words, it's going to last about an hour.

Bob

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#### qingtian28

haha.. thanks bob .. but how should i solder them?
i was thinking of soldering all the +ve LED leg together and then solder the +ve supply in to 1 of the LED leg since all of them are connected to each other.
Am i right or wrong? plz correct me.

#### duke37

If you connect all the leds in parallel as you suggest, you will use three times the current that three leds (and resistor) in a string will use.
To make string, connect the + of one led to the - of another. Repeat and add a resistor, calculated to give 10 to 20mA. The current now goes through three leds so does three times as much work.