# Driver circuit for 8 LED in series

Joined Oct 10, 2015
15
I am looking for a driver circuit or any simple circuit to glow 8 green LED in series/parallel using 9 Volt battery ..

i have attached the LED datasheet

its forward Current 20 mA.
forward Voltage 3.1V

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#### Alec_t

Joined Sep 17, 2013
14,313
The simplest circuit would be 4 parallel strings, each string comprising 2 LEDs in series with a 150 Ohm 1/4 W resistor.
LED current would reduce as the battery voltage dropped. A more complex circuit would use a respective constant-current 20mA source for each string.
Either way, the total current would be ~80mA, so a little 9V battery wouldn't last long.

Joined Oct 10, 2015
15
thanks for the prompt reply is there any circuit to improve battery life for the above mention design 4 parallell strings and 2 LED in series

#### mcgyvr

Joined Oct 15, 2009
5,394
thanks for the prompt reply is there any circuit to improve battery life for the above mention design 4 parallell strings and 2 LED in series
No unless you are willing to sacrifice brightness..
If so just change the resistor value to change the string current.
Drive them at 5mA instead of 20

#### spinnaker

Joined Oct 29, 2009
7,830
thanks for the prompt reply is there any circuit to improve battery life for the above mention design 4 parallell strings and 2 LED in series
Take a look at a buckpuck. I use one from this company but with a larger array than you have,. They work very well. You might be able to find a smaller device. It would be the easiest way to go.

Joined Oct 10, 2015
15
thanks to every one.

kindly find the attachment and tell me if i need to improve the circuit.

#### Dodgydave

Joined Jun 22, 2012
11,302
Looks good. 7.5 V with approx 19mA led current, but the Lm317 needs 3V minimum drop input to output.

I would have made the 240R resistor 120R, and used 610R instead of 1K2, but it still gives same result.

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#### Alec_t

Joined Sep 17, 2013
14,313
To get maximum efficiency a DC-DC boost converter could be used to raise the 9V to > 25V, so that the 8 LEDs could be connected in series and draw about 20mA total.

Joined Oct 10, 2015
15
thank a million sir,one more help how to include 8 photo darlington in same circuit op560a.pdf

is it possible to use all 8 PD in same circuit or if using 9v battery is not suitable i will go for 12v adapter

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#### Dodgydave

Joined Jun 22, 2012
11,302
Your lm317 needs a minimum of 3V drop to make it work, if you want to input 9V, your maximum output is 6V for it to keep up.

#### Alec_t

Joined Sep 17, 2013
14,313
is it possible to use all 8 PD in same circuit
The circuit would depend on what you want to do with them.

Joined Oct 10, 2015
15
is it possible to make this circuit? by merging 9v to 7805 and adding 8 photodarlington in parallel with 10k resistor

#### spinnaker

Joined Oct 29, 2009
7,830
is it possible to make this circuit? by merging 9v to 7805 and adding 8 photodarlington in parallel with 10k resistor

You do not want to use a 7805 if you want efficiency. You want a buck regulator. The 7805 is a linear regulator. And you really want to regulate current and not voltage.

#### Colin55

Joined Aug 27, 2015
519

#### spinnaker

Joined Oct 29, 2009
7,830
Nice. Do you have that in a traditional schematic? Might be easier to follow. How many wraps on the bolt? Or what value coil can be used in it's place?

#### Alec_t

Joined Sep 17, 2013
14,313
is it possible to make this circuit? by merging 9v to 7805 and adding 8 photodarlington in parallel with 10k resistor
No, because that circuit doesn't show any photodarlingtons . Where did you want to connect them and what would they be driving?

#### Dodgydave

Joined Jun 22, 2012
11,302
is it possible to make this circuit? by merging 9v to 7805 and adding 8 photodarlington in parallel with 10k resistor
that circuit is better with the 7805, are you trying to make a torch?

Joined Oct 10, 2015
15

please consider the npn darlington as photo darlington op560a

is it possible to to add both circuit? and can it be driven by 9volt battery?View attachment 92802

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Joined Oct 10, 2015
15
[that circuit is better with the 7805, are you trying to make a torch?]

no this have to connect with opticfiber oneside led and the other side photodarlington

#### Dodgydave

Joined Jun 22, 2012
11,302
I wouldn't use a Lm317, better to use the 7805, it will power all the circuit both leds and receivers,