How do these LEDs light up

[cheece2001]

I am new here and this may be an easy answer for people here. I have trouble understanding this.

There are 4 LEDs hooked up comeing from the IC and they are to light up according to what happens within the IC.

But I don't understand this. All I see is them lighiting up all the time since they are hoocked up to the 3.3V as shown in the diagram. Won't the LED be on full blast at all times? How is the IC to control wheather one turns ON of off when ther are all connected to the 3.3V.

Can someone explain me this?



In the data sheet under these pins, it says.. LED1 current sink input, LED2 ... etc

 

[barry]

The LEDs have 2 pins; just because one of them is connected to +3.3 does not mean it's on all the time. What is the other pin connected to? There needs to be a current path, and that is provided by the IC. Probably that IC output is an open-collector (basically, a switch) which means when it's "on", current will flow from the power supply, through the LED to ground. When the output is off, it's like the switch is open and no current flows.

 

[vinodstanur]

You have connected +ve of all LEDs to 3.3V and each negative pin to IC pins. So when ever an IC pin becomes LOW (means it internally connect to ground), that particular LED light up. If the IC cannot source current, (ie it can only sink current), then the only way to glow an LED using that IC (i/o pin) is by connecting the -ve of LED to IC pin and +ve to supply voltage.
But be careful, if the internal resistance of IC is very low, it can damage the LED because for a normal LED, 3.3V can damage it easily. Use a resistor, say 100 ohm in series with each LED. U can design the resistor value by considering a maximuim current of around 10mA through an LED.

 

[rfredel]

Hello cheece2001,

without more informations about the LEDs and the IC it is very difficult to give you advise.

Normaly LEDs need a resistor to limit the current. 3,3V will be a little bit less, because the LEDs have a forward voltage between 2,0V and 2,5V. The transistors inside the IC will have a "LOW"-level of 0,7V for example. So you have a too low voltage for light of the LED.

Regards

Rainer