Help me to explain the LM3915 operation.

[ryeeson]

The data sheet can be found with the link bellow.
http://www.datasheetcatalog.com/datasheets_pdf/L/M/3/9/LM3915.shtml

bassiclly i understand the LM3915 basic operation about what is the LM3915 does. What i dont understand is, how does the voltage/current flowing out to the LM3915's pin as shown in the diagram bellow..

[audioguru]

Hi Ryeeson,
It uses controlled LED drive current from the comparators so that the output transistors supply a constant current to the LEDs, so that current-limiting resistors aren't needed. The output transistors don't saturate, they operate as variable resistors.
Look at how constant is the output current. The LED supply voltage increases from 1V to 25V but the LED current changes only from 10.6mA to 11.1mA.

[ryeeson]

Thanks again Audioguru, I have Email you for another question.

I still remember you said

[audioguru]

Since the ouput pins are connected directly to the LEDs and the forward voltage of an LED is about 2V, then the output voltage can't go down to a logic low voltage of about 0.5V, but is about 3.0V when the circuit has a 5V supply.
The datasheet shows how a PNP transistor can detect current in an output like this:

[ryeeson]

Hello,

The transistor will detect if there is a current flow through the LED right? so if there is a current, then the +5V will be the output at the transistor right?

Thanks...

[audioguru]

The transistor will detect if there is a current flow through the LED right? so if there is a current, then the +5V will be the output at the transistor right?

Correct. ;D