Help! My Flashing LED Circuit Isn't Working

In summary, the person is trying to create a flashing LED circuit, but the LED only lights up for a few milliseconds and then goes out. They replaced the 470 ohm resistor with a 100k ohm resistor and the 10 uF capacitor with a .1 uF capacitor, but the LED still does not work.
  • #1
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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.)
 
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  • #2
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.
 
  • #3
Just a note here. The 556 is a dual timer (two 555s in one chip). There's also a quad timer; the 558.
 
  • #4
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?
 
  • #5
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.
 
  • #6
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.
 
  • #7
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.
 
  • #8
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)
 
  • #9
It Works! thanks a ton! you made me figure out my calculations and now I understand it better! You rock.
 
  • #10
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.
 

Related to Help! My Flashing LED Circuit Isn't Working

1. Why is my LED not lighting up?

There could be several reasons why your LED is not lighting up. First, check to make sure that the LED is properly connected to the circuit and that the polarity is correct. Next, check the power source to ensure that it is supplying enough voltage to turn on the LED. If both of these are correct, then the LED itself may be faulty and you may need to replace it.

2. Why does my LED only light up dimly?

If your LED is only lighting up dimly, it could be due to a few reasons. First, check to make sure that the power source is supplying enough voltage to fully turn on the LED. If the voltage is too low, the LED may only light up dimly. Additionally, check the resistor in your circuit. If the resistance is too high, it can limit the current to the LED, causing it to light up dimly.

3. What do I do if my LED is flickering or flashing instead of staying on?

If your LED is flickering or flashing instead of staying on, it could be due to a few reasons. First, check to make sure that the connections in your circuit are secure and that there are no loose wires. If everything seems to be connected properly, then the issue may be with the power source. If the power source is not supplying a consistent voltage, it can cause the LED to flicker or flash. Try using a different power source to see if that resolves the issue.

4. Can I use any type of LED in my circuit?

No, not all LEDs are created equal. Different types of LEDs have different voltage and current requirements, so it is important to choose an LED that is compatible with your circuit. Make sure to check the datasheet for the LED to ensure that it can handle the voltage and current of your circuit.

5. Why is my LED not turning on even though the circuit is working?

If your circuit is working but the LED is still not turning on, it could be due to a few reasons. First, check the connections to make sure they are secure. If the connections are loose, the LED may not be receiving enough power to turn on. Additionally, check the resistor in your circuit. If the resistance is too high, it can limit the current to the LED, preventing it from turning on. Lastly, make sure that the LED is properly connected with the correct polarity.

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