Help! My Flashing LED Circuit Isn't Working

[Atriusbread]

I just purchased a breadboard and components because I love tinkering with things. It's not for a class or anything more of a hobby, but I just needed some advice on a simple hello world like blinking led circuit. I have gotten down the basics of lighting up leds, and my second project I wanted to create a flashing Led. However all that happens in my circuit is the Led lights up. I am using a 556 timer Ic. Could you please take a look at these videos and tell me what's wrong with my circuit.

http://s93.photobucket.com/albums/l75/Bouzy_Bouzy/?action=view&current=P1010110.flv

http://s93.photobucket.com/albums/l75/Bouzy_Bouzy/?action=view&current=P1010111.flv

The top resistor is a 470 ohm .5 watt and the bottom one is a 220k ohm .5 watt. I tried 2 different capacitors. ONe is a 10uF and one is a .1uF. They both have the same effect (I don't really understand how to figure out what capacitor I need for the circuit but these seem like they should work.)

 

[Redbelly98]

Welcome to PF!

Mostly, I'm guessing the LED is blinking on and off too quickly for you to notice. But offhand, I would expect the 10 uF and 220k combination to work, since RC = 2 seconds in that case.

Also, there should be 2 resistors in the circuit. Do you have a datasheet for the 556? That should show you how to set up the circuit, and how to calculate on & off times for a given choice of resistors and capacitor.

 

[dlgoff]

Just a note here. The 556 is a dual timer (two 555s in one chip). There's also a quad timer; the 558.

 

[Atriusbread]

Thanks for the welcome. Actually, there are two resistors there. I don't know if you missed it or what, but there is a 220k ohm and a 470 ohm resistor. Actually, I followed the data sheet for the timer. I am positive the way I have it set up should work which is why it is just so frustrating. Any ideas?

 

[Redbelly98]

Try replacing the 470 ohms with a 100k or another 220k resistor, and use the 10 uF capacaitor.

I am guessing that with the 470 ohms, the LED turns off for only a few milliseconds before turning back on.

From http://en.wikipedia.org/wiki/555_timer_IC#Usage :

The timer pulse is high for a time equal to
0.693 (R1 + R2) C

And the timer pulse is low for a time equal to
0.693 R2 C

For R2 = 470 Ω and C = 10uF, the low time (off time?) is
0.693 * 470 * 10 * 10^-6 sec
= 0.003 sec

With a 100kΩ, it will be 0.7 seconds which is noticeable. With 220 kΩ, it's 1.5 seconds.

Good luck! If it still doesn't work post back and we'll brainstorm some more.

 

[Atriusbread]

I pretty much put the whole idea of my calculations being wrong (with capacitor and resistors) by using a 470uF capacitor. The Led still light up like a Christmas tree. So, it has to be something wrong with how it's wired. I have gone over the circuit diagram a ton and I can't find what I did wrong.

 

[Atriusbread]

Ok, I remade the whole circuit based on this.. not quite the same function but it still should go on and off. I wired it exactly like in the picture and now when I give the board power the LED lights up for a second then goes out and won't come back on no matter how long I leave it on. But when I tap the batter on and off the led blinks once in a while every time a tap it against the leads.

 

[Redbelly98]

Are you sure it's wired for astable (2 resistors) mode and not monostable (just 1 resistor) mode?

If it is in astable mode, can you show how you calculate the on and off times? (Using your resistor and capacitor values)

 

[Atriusbread]

It Works! thanks a ton! you made me figure out my calculations and now I understand it better! You rock.

 

[Redbelly98]

Cool! There's nothing like building something and then figuring out how to make it work. That's one reason why this site is here--though it doesn't always work out with everybody all the time.