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I am trying to turn an LED on and off repeatedly with a square wave.

Yesterday, I've checked the following circuit works:

enter image description here

(Please note that, the figure has one voltage source Vdd. But I've connected a square wave source to the resistor Rg)

But, the most of LED driving circuits have a form of the following:

enter image description here

I am curious that what roll a BJT does in this circuit. From this article, it has something to do with a constant current source. Then, what is an advantage if there is a constant current source?

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The npn Transistor is used to control the current flow through the LED. The resistor Rs can be chosen using the formula below:

ILED = 0.7 V / Rs

Note that this type of current limiter is far from perfect. You actually have to expect base voltages of 0.5...0.9 V, and they will vary greatly with temperature and depend on the exact type of transistor used. Consider this as a mere starting point for digging deeper into the subject of transistor current sources...

For more detail you can refer the below link http://www.physics.unlv.edu/~bill/PHYS483/current_lim.pdf

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  • \$\begingroup\$ So, it makes a current through the LED and the Rs more stable and steady. It does not make the LED turn on and off as fast as a frequency of a square wave, right? And I think it seems not to be necessary. If it is necessary, I have to order a proper BJT... \$\endgroup\$
    – Jeon
    Commented Jun 23, 2015 at 6:53
  • \$\begingroup\$ @zebonaut correct :-) \$\endgroup\$
    – Sanjeev Kumar
    Commented Jun 23, 2015 at 6:58
  • \$\begingroup\$ @Jeon you are correct it does not make led to Turn On or Off. \$\endgroup\$
    – Sanjeev Kumar
    Commented Jun 23, 2015 at 7:00

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