View Full Version : Led Flasher ith 555

06-16-2010, 08:26 AM
i hv tried to make a led flasher with 555ic but,somehow it is only glowing continuously means no flashing is there

plss help me in this matter i have attached the 555 cct.

06-16-2010, 09:44 AM
Connect pin number 4 to pin 8 (VCC)
Connect a .1uF capacitor from pin 5 to ground.

06-16-2010, 02:15 PM
hey allbits u mean connecting 5 pin to (-)ve teminal of battery

06-16-2010, 02:23 PM
yep, by means of a 0.1uF capacitor.

06-16-2010, 02:28 PM
hey thanks, this is the first thing which i made on perfboard

06-16-2010, 02:31 PM
hey Rao how would i understand this circuit, i have just made it, but don't know how it works

06-16-2010, 02:41 PM
What you made is called an "Astable Multivibrator"

Please go through this link. http://www.visionics.a.se/html/curriculum/Experiments/555%20Astable/555%20Timer1.html

06-16-2010, 07:00 PM
hey thanks, but it still same as before

06-16-2010, 08:48 PM
:) astable multivibrator makes continuous oscillation without the need for external timer pulse with frequency equal to the formula

f = 1/T = 1.45/ ((Ra + 2*Rb)*C)

Where Ra = resistor between pins 8 and 7
Rb = resistor between pins 7 and 2,6
C = capacitor between 2,6 and 1 or gnd.

so in your case, Ra = 1K, Rb = 1K and C = 10 to 330uF, ..

Now do the math and find out the frequency or Time period and see how it adds up to your LED... :)

06-17-2010, 03:25 PM
thanks Rao ,but then what is need of .1uf capacitor b/w pin 5 and 1

06-17-2010, 04:54 PM
Pin no 5 is the control voltage Input. It simply means you can change the output frequency by varying the voltage at pin 5, apart from your RC network. It is an OPTION. you do that kind of things for applications like in freq modulation.

You do not need to use it in your application. You can leave it unconnected. But when you leave it unconnected, noise at pin 5 can attack your output frequency. This effect is negligible. But then, to avoid noise, you bypass the pin to ground through a capacitor. A Capacitor 'grounds' the noise voltages at the pin, so you get a clean output.

The datasheet says .01uf Disk cap. .1 will work too.

06-24-2010, 01:01 AM
hey Rao how would i understand this circuit, i have just made it, but don't know how it works

Never use an IC without reading its datasheet.