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2 LED connect to common ground?

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dog812

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If i have 2 strings of LED's that are hooked to diffferent switches. To turn one on then flick the switch and turn the other on.
Can i use a common ground for both strings?
Or will it screw the LED's up that are switched off while the other string is on?

Here is my lil diagram to help.

+ (0) - 0 +

+ = positive
- = negative
0 = led
(0) = powered / lit up led

Thanks in advance.
 
You have 2 strings of LEDs and 2 switches.
Each string is turned on and off with its switch.
The switch can be in the positive side or in the negative side.
What do you want to do?
 
Well actually it is one switch. a on - off - on , so one side "on" then over to the other side on...
But the space is very minimal, so i want to run the least amount of wires as possible.. so i want to try to only put three wires through..
So i want to use the ground as a comon wire for both sets of lights..
Just wondering if it will work..
or will the current from the one side.. go through the ground and up the other side and wreck it or something??
 
yes 3 position switch.. on - off - on, so either side will have a different string of leds on the switch,..,
All i need to know is if the ground can be a common ground for both strings without screwing it up..
here take a look at what i am making, you ll understand more..
it is called the dual. LED Hula Hoop
 
The on-off-on switch can have one string of lights connected to one ON terminal and the other string of lights connected to the other ON terminal. The middle terminal connects to the positive supply or to ground depending how the ON terminals are connected to the correct polarity of the LEDs or to the current-limiting resistors.

So the LEDs can have a common ground or have a common positive supply.
 
You can do it this way or that way.
 

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Well actually it is one switch. a on - off - on , so one side "on" then over to the other side on...
But the space is very minimal, so i want to run the least amount of wires as possible.. so i want to try to only put three wires through..
So i want to use the ground as a comon wire for both sets of lights..
Just wondering if it will work..
or will the current from the one side.. go through the ground and up the other side and wreck it or something??

It will work fine......they are diodes!;)
 
Hi Dog,
Your very simple circuit won't work:
1) Red LEDs need 2V, blue LEDs need 3.5V. But you have only a single 1.5V battery cell so the LEDs will never light.
2) If the battery voltage is high enough then the LEDs will immediately burn out because nothing limits the current. Our circuits had a current-limiting resistor in series with the LEDs.
3) Your LEDs are in parallel so the one with the lowest voltage (they are all slightly different) will hog all the current and will quickly burn out. Then the next lowest voltage LED will burn out, then the next.
We connected our LEDs in series so that the current in each LED is the same. But then the battery voltage must be higher.
 
Well i am going to use resistors, and a 3.7 v lithium batt... but to keep the diagram simple i just drew it up like that.
The main question .. all the way back to the first post.. was can i have a common ground between the 2? like in my drawing.
 
The voltage from the lithium battery is almost the same as the voltage needed by blue LEDs. Then you will have bright blue when the battery is charged (determined by the value of the current-limiting resistor) and you see the brightness dim as the battery voltage runs down.
You won't see the red LEDs dim as much.

A lithium battery becomes dangerous if it is discharged below 3V. Then special care must be used when charging it so it doesn't catch on fire. Your charger should do the special care.

Most lithium rechargeable battery powered products have a circuit that disconnects the battery when it discharges to 3V.
 
The common ground is fine. Only one side of the LEDs are lighted at a time because that is how the switch does it.
 
Ok i understand..
About the parralell and series thing..
I am now running 4.8 volt batteries (4XAA rechargeable)
with the proper resistors on each led.

Is that better than running them in series?
If i run them in series.. once one burns out or dies.. wont the whole string not work?
 
Hi,

Yes, if they're connected in series and one burns out, then the whole string won't light up. the best way, using the 4 x AA batteries is to connect the LEDs in paralell and use a separate resistor for each LED. In order to run them in series, you would need amuch higher voltage to make them light up. It is fine to use common ground for both strings. one string being on will not affect the other string.

Hope this answers your Q.

Neil
 
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