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Topic: Using resistors on all output pins connected to transistors? (Read 891 times) previous topic - next topic

Hello,

I am going to build this http://www.instructables.com/id/DIY-TiX-Clock/step4/The-Final-product-and-files/ (Tix Clock) using the Arduino Uno instead of the ATMEGA16.  I also will not be incorporating push-buttons.  My question is when I am running this off of my arduino, do I need resistors on the three output pins that go to the three bases of the row transistors?

The output pins that control the columns won't need resistors because they are going into a LED driver transistor IC.

Thanks

dhenry

Quote
do I need resistors on the three output pins that go to the three bases of the row transistors?


You don't need to: the particular arrangement is putting the led on the emitter so it serves as part of the load.

In general, you want to. But you don't have to.

Grumpy_Mike

That is true but what a crap way of switching LEDs. Switched like that they do not get the full 5V but a maximum of 4.3V. What a very poor way to draw that diagram.
I always amazed at how crap instructables continue to be.

retrolefty

#3
Dec 11, 2012, 04:42 pm Last Edit: Dec 11, 2012, 04:56 pm by retrolefty Reason: 1
No. If the leds in each column are indeed being current controlled by a current sinking constant current driver, then the switching transistor does not, and in fact should not have a series base resistor installed. The collector emitter junction of the transistor will be forced into full saturation and will have minimum voltage drop, but even if there was voltage drop the constant current sinking output drivers would compensate and maintain proper led current at all times. That is how my 5x5x5 led cube is wired up using this schematic:

http://picprojects.org.uk/projects/lc/Cube555Csch.pdf

Edit: My above my not be correct for your circuit because even though you state:

Quote
The output pins that control the columns won't need resistors because they are going into a LED driver transistor IC.

I think that is an inaccurate statement as the ICs you are using for current sinking output are not of the constant current type, but rather just Darlington driver outputs. The output shift registers used in my schematic above do use constant current output drivers to control led current, your circuit appears not to and I don't see how you are managing led current correctly. And just adding a base resistor to the switching transistor will not solve the problem of led brightness will vary depending on how many leds you might have commanded on in any given level. That seems to be a flawed design in my opinion that is just relying on the scanning speed duty cycle to keep peak led current from destroying the leds. 

Lefty

Ok, so how should I go about hooking up these LEDs?  What would make the most sense?

retrolefty


Ok, so how should I go about hooking up these LEDs?  What would make the most sense?


At a minimum you need current limiting resistors wired between each output driver pin to each vertical column of leds to set the desired current value you want your leds to operate at.

Lefty

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