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?
No.
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?
Pannu:
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.
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.
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.
Pannu:
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.