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Topic: Shift registers ,Resistors (Read 1 time)previous topic - next topic

pushpender007

Nov 12, 2013, 02:10 pm
I was reading this article to understand about shift register, http://arduino.cc/en/Tutorial/ShiftOut. I saw it is using 8 resistors at anode of every Red LED.

Rather than using 8 resistors, can I use one resistors at combined cathode of 8 LEDs?

fungus

#1
Nov 12, 2013, 02:36 pm
No.
No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

pushpender007

#2
Nov 12, 2013, 03:24 pm
Can I know the reason please? I thought resistor can be at any end input or output.  Like I use one resistor which I combine LED's cathode to grnd with different input coming from arduino ports directly?

fungus

#3
Nov 12, 2013, 04:18 pm

Can I know the reason please? I thought resistor can be at any end input or output.

Yes, but if you replace 8 resistors with one resistor then the circuit isn't equivalent.

With only 1 resistor the LED brightness will depend on how many LEDs are lit up.

No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

pushpender007

#4
Nov 12, 2013, 04:29 pm
I also got to read this through google. But did not understand it well. I am new into this. Little more deeper understanding of it will be very helpful. As I guess LED brightness is dependent on the current flowing through in it. How current will be reduced in case of single resistor and not in case of multiple resistor.

#5
Nov 12, 2013, 04:49 pm
With 1 resistor per LED, there is 1 path for current flow.
The amount of current, say from a 5V source, is
(5V - Vf)/resistor = current
If Vf, the voltage drop across the LED, is 2.7V, and the resistor is 120 ohm, then:
(5V - 2.7V)/120 = 19.2mA.

Now, if you have many LEDs in parallel, the current will still be the same because the resistor is still the same and the voltage drop across the LEDs is pretty equal; so the current gets shared over several LEDs.
1 LED: 19.2mA
2 LED: 8.5mA
3 LED: 6.x mA
:
8 LED: 2.xmA

So you can see how the brightness will very depending on how many LEDs are selected to be on.
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.

fungus

#6
Nov 12, 2013, 06:30 pm

I also got to read this through google. But did not understand it well. I am new into this. Little more deeper understanding of it will be very helpful.

What does Ohm's law say ?

It says that more current won't go through if you turn on more LEDs.

No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

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