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Topic: LEDs in Parallel (Read 811 times) previous topic - next topic

Ben1234

Hi,
  I am attempting to control 10-20 LEDs on a single digital input and was wondering if either of the schematics in the photo would work or if they would just destroy an Arduino Uno. Thanks.

madworm

#1
Feb 08, 2012, 10:52 pm Last Edit: Feb 08, 2012, 11:04 pm by madworm Reason: 1
The right drawing is less incomplete.

A few observations:

a) you still need a base resistor for each transistor (can't say which, as I don't know which transistor / LED you use)
b) you still need a form of current limiting (series resistor for low power leds)
c) if you use NPN transistors, they should be on the GND side of the LEDs.

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KirAsh4

... and you're also shorting your ground with the digital pin (the very last line at the very top of the LED stack.)

Ben1234

Thanks for the notice. I hadn't realized I had that shorting on the top of the stack.

As for resistors let's just say all the LEDs are blue and I had them in parallel. Could I get away with one resistor for each LED? Although a number of people were commenting saying that the current would drop and some of the LEDs would not light up at the end? I found this online at http://led.linear1.org/led.wiz 
Any thoughts?

pwillard

1 - Yes, every LED needs a resistor.
2 - With 5V being supplied, 39 Ohms is low.  My calculation says you don't want lower than 75 Ohm and if you want lower current draw for the whole array, use 100 to 220 ohms.
3 - You can drive this with a single arduino pin and a Power transistor, like TIP122 with a 1k Ohm resistor between the pin and the base.  Emitter to GND and Collector to the resistor bank/cathode end of the LED's.

Techone

The last schematic, with 20 led's connect to a 39 ohms... is about 10 mA each, if the led drop is 1.2 V.  Therefore 10 mA time 20 = 200 mA total curent. With a 220 ohms, you have about 2 mA each led. 20 time 2 mA = 40 mA. Now, that a better total current. 

CrossRoads

Blue LEDs typically have a higher Vforward - like 3 to 3.5V
So
(Vsource - Vforward - Vce-transistor)/20ma = Resistor
(5-3.35-0.7)/.02 = 40 ohm
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

CrossRoads

#7
Feb 09, 2012, 07:11 am Last Edit: Feb 09, 2012, 07:14 am by CrossRoads Reason: 1
Example
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Ben1234

Can you please explain the bottom of the schematics. This is still all a little confusing but greatly appreciated. I understand the top but are the arrows at the bottom going to ground? If so then what is the 35ma resistor connected to? Or do you have a wire in 5v, one in say Digital pin 1, and one going to ground?

Fat D

A transistor has three pins: The collector, where the current goes in (NPN), the emitter, where the current goes out (NPN again) and the base, which controls how much current gets through (on both types). The 150 Ohms resistor goes to the Arduino digital pin to allow the Arduino to control the transistor. The emitter goes to ground, because that is where all the current has to go eventually. Leaving the collector free to be in the path between the LEDs and ground.

Grumpy_Mike

Quote
35ma resistor connected to

35mA is the current going through the base of the transistor, that is the connector on the side of the bar. The emitter is the connection with the arrow and the collector is the other one on the symbol. It is going to an output pin of an arduino.

Ben1234

Thanks. That makes a lot more sense.

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