Pages: [1]   Go Down
Author Topic: LEDs in Parallel  (Read 664 times)
0 Members and 1 Guest are viewing this topic.
Offline Offline
Full Member
***
Karma: 0
Posts: 139
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

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.


* Picture 4.png (52.38 KB, 600x426 - viewed 41 times.)
Logged

"The old Europe"
Offline Offline
Edison Member
*
Karma: 1
Posts: 2005
Bootloaders suck!
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

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.

Do some google-ing and report back ;-)
« Last Edit: February 08, 2012, 05:04:42 pm by madworm » Logged

• Upload doesn't work? Do a loop-back test.
• There's absolutely NO excuse for not having an ISP!
• Your AVR needs a brain surgery? Use the online FUSE calculator.
My projects: RGB LED matrix, RGB LED ring, various ATtiny gadgets...
• Microsoft is not the answer. It is the question, and the answer is NO!

Colorado
Offline Offline
Edison Member
*
Karma: 47
Posts: 1562
Reviving dead brain cells with Arduinos.
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

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

Offline Offline
Full Member
***
Karma: 0
Posts: 139
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

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?


* Picture 2.png (23.02 KB, 431x586 - viewed 21 times.)
Logged

Cumming, GA
Offline Offline
Edison Member
*
Karma: 20
Posts: 1643
Ultimate DIY: Arduino
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

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.
Logged

Toronto, Canada
Offline Offline
Edison Member
*
Karma: 2
Posts: 1234
"Keep it R.E.I.L. - "Research, Experiment, Investigate and Learn"
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

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. 
Logged

Global Moderator
Boston area, metrowest
Online Online
Brattain Member
*****
Karma: 515
Posts: 26266
Author of "Arduino for Teens". Available for Design & Build services. Now with Unlimited Eagle board sizes!
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

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
Logged

Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

Global Moderator
Boston area, metrowest
Online Online
Brattain Member
*****
Karma: 515
Posts: 26266
Author of "Arduino for Teens". Available for Design & Build services. Now with Unlimited Eagle board sizes!
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

Example


* transistor_leds.jpg (24.45 KB, 960x720 - viewed 22 times.)
« Last Edit: February 09, 2012, 01:14:10 am by CrossRoads » Logged

Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

Offline Offline
Full Member
***
Karma: 0
Posts: 139
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

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?
Logged

0
Offline Offline
Jr. Member
**
Karma: 0
Posts: 78
Arduino rocks
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

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.
Logged

Manchester (England England)
Offline Offline
Brattain Member
*****
Karma: 596
Posts: 33288
Solder is electric glue
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

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.
Logged

Offline Offline
Full Member
***
Karma: 0
Posts: 139
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

Thanks. That makes a lot more sense.
Logged

Pages: [1]   Go Up
Jump to: