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I'm going to start a project to make an array of LEDs using the shift registers 74HC595, I've seen many tutorials and it's not really difficult using the Arduino plataform.

The situation is as follows: in the tutorials they put a resistance between the outputs of the 74HC595 (Qx) and the LEDs , but in each tutorial the resistance varies (between 200 and 575 Ohms).

Then the question would be: if feed with 5V to 74HC595 (the same power as the Arduino) and the resistance that I put between its outputs (Qx) and the LEDs is lower (let's say 200 Ohms), the LEDs will light brighter than if I do it with resistors with higher values (let's say 500 Ohms)?

  • What does the 74HC595 datasheet say about maximum current per output and maximum current for the entire IC? The Texas Instruments datasheet uses 560 ohms per LED on the Typical Application page. – VE7JRO Aug 27 '18 at 19:43
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    @VE7JRO, the OP is asking, using a lot of words, if a 200 ohm series resistor will produce brighter LED output when compared to a 500 ohm series resistor. – jsotola Aug 27 '18 at 19:47
  • I know the Ohm´s law, I just asking for experience, I Live in Cuba and is very dificult to me order pieces (when I buy them I have to wait almost 4 month to get them); so, when I order I try to order "things" that I would know that will work, I ask for this reason. Sorry – k.Cyborg Aug 27 '18 at 19:48
  • They say 25mA per output. – k.Cyborg Aug 27 '18 at 19:51
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    no need to apologize .... then you should be asking about what @VE7JRO mentioned in the above comment .... can the 74HC595 handle the load when all the LEDs are on? – jsotola Aug 27 '18 at 19:53
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You are correct that a lower resistance would offer higher brightness. The more current through the LED, the brighter it will be. The current is calculated by dividing the voltage across the resistor by resistance value. The voltage across the resistor would be your 5V - the voltage drop across the LED. this often varies by color. A RED LED might have a 1.2V drop, and GREEN or BLUE would be higher. So to get the same relative brightness in different color LEDS, you will often see a lower resistance used, to get the same current.

Lets say you have a 20mA Red LED, with a 1.2V drop. So (5 - 1.2)/.020 = 190 ohms. If the LED required 10mA, that would be 380 ohms.

Now you DO need to look at the data sheet for the chip driving the LED, and make sure it is up to the task. Can a pin source or sink 10mA? 20mA?

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