IC 555 in astable mode

[shanky25]

I just connected IC 555 in astable mode.... my doubt is.. can i connect the output pulse train directly to an LED.. and ground the other terminal of the led so that it can be used as an LED blinker?

 

[fingaz]

Hi,

You can either connect the anode (+) of the led to a positive supply, and the cathode to pin 3 (so it will ground it) or connect the anode of the led to pin 3 and cathode of the led to ground.

Both ways will flash an led, but remember to use a current limiting resistor on the + side of the led

 

[eblc1388]

You will also need to include a 330 ohm current limiting resistor in series With the LED.

Without it, the LED, 555 or both could be damaged.

 

[shanky25]

@ fingaz

thanks a lot man.. i got it.. actually first i did it in the second way!! (ie connecting the anode to pin 3 and grounded the cathode).. this didn't help me.. anyways thanks once again!

 

[Dean Huster]

Most devices, such as the 555 and all TTL outputs, can "sink" more current than they can "source" and as such, it's usually better to connect the LED's anode to the positive supply and the cathode to the IC output through the proper value current limiting resistor. It's a little easier on the output electronics.

In logic cases, that may often "invert" the desired output indication, so an inverter between the device output and the LED would be in order.

Dean

 

[audioguru]

Hi Dean,
The datasheets for the 555 show that it can sink 200mA and it can source 200mA. At such a high current the voltage loss (typical loss of 2.5V) is the same.

The Cmos 555 has much less output current.