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I'm completely new to electronics but want to build a blue light led array 8x9 with a 12V input and have no idea how to do this on a protoboard! If someone could help with a schematic it would be great, i think the 12V would light 3 of the leds if I did a paralell circuit but not sure?

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  • \$\begingroup\$ There are lots and lots of ways. Do you have any requirements at least? \$\endgroup\$ – Ignacio Vazquez-Abrams Mar 21 '15 at 1:07
  • \$\begingroup\$ Do you need individual control of the LEDs? or is turning them all on and off together good enough? \$\endgroup\$ – Gorloth Mar 21 '15 at 1:16
  • \$\begingroup\$ all on at the same time is what I want? \$\endgroup\$ – Lucio89 Mar 21 '15 at 1:26
  • \$\begingroup\$ easiest way on a perfboard would be best \$\endgroup\$ – Lucio89 Mar 21 '15 at 1:27
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If you want them all on at the same time you just need to make multiple legs of 3 or 4 LEDs in series, depending on what LEDs you buy (the sum of their forward-voltages can't exceed 12V) and each leg has to have a resistor to control the current.

The equation to calculate the value of the resistor in series is pretty simple. First find the forward-voltage (Vf) of the LED and the typical forward-current(If) of the LED. It can be found in the datasheet or provided by the seller. Then, take your source voltage (Vs) and subtract the Vf of each LED in the series. Next, take that number and divide it by the desired current (If). The result will be the value of your resistor.

If you have 3 LEDs, the equation would be:

(Vs-Vf-Vf-Vf)/Ic = R

Keep in mind that the total voltage for the LEDs has to be lower than the source voltage.

As an example, imagine you use this LEDs. The Vf is 3.4V, so you can use 3 of them in 12V source. The max current is 30mA, but using max current will reduce the life of your LED, so let's use If=25mA.

Now, to find the value of your resistor.

(Vs-Vf-Vf-Vf)/Ic = (12 - 3.4 - 3.4 - 3.4) / 0.025 = 72 Ohms.

There are no 72 Ohm resistors, so using the 75 Ohm should be fine.

schematic

simulate this circuit – Schematic created using CircuitLab

Just remember, the numbers in your equation will depend on what LEDs you buy.

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