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

pushpender007

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

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

pushpender007

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


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

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.

CrossRoads

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


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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