Noob 555 Timer Question

Discussion in 'General Electronics Chat' started by blah2222, May 3, 2010.

  1. blah2222

    Thread Starter Well-Known Member

    May 3, 2010
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    Hi, I just have a question regarding the use of a 555 timer. My current knowledge of them suggests that using flip-flops they are able to turn a DC voltage into a square wave voltage, which could be considered like an AC source? I have been playing around with it but I must have misinterpreted something. Basically all I want to do is get a yellow diode to blink at a frequency of around 1 Hz, so I can actually see it blink.

    I thought this task would be pretty simple...

    I picked up two resistors (1K and 2K), and a 330uF capacitor.
    The timer gives the equation for frequency: 1.44/((R1 + 2*R2)*C), where R1 = 2K and R2 = 1K. Works out to around 1.09 Hz...

    So I set up the circuit using a DC wall adapter (120VAC -> 6VDC; max 330mA) and the diode lights up upon connection but then fails to light up again. The chip got very hot and I started smelling fumes, so I immediately disconnected the circuit.

    I am a complete electronics noob and need help with this "seemingly" easy circuit.

    Thanks alot,

    JP
     
  2. Bychon

    Member

    Mar 12, 2010
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    You've made a mistake (duh). Start with a drawing and we'll see if your design is wrong or you've connected something wrongly.
     
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  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Did you put a current limting resistor in series with the LED? Failure to do so can cause the LED to fail, the 555 to get very hot, and fumes to emanate from the circuit.:eek:
     
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  4. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
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    Always keep the LED current below 20mA by placing a resistor in series.

    Just divide the output voltage by 0.02 will give the resistor value, e.g. 6/0.02=300Ω in your case. Any resistor values from 270Ω or higher will work. For some high intensity LEDs, they will be too bright to look at even with a value as high as 1KΩ.

    Forget about diode voltage drop for the moment in calculation as LED works with current below 20mA too and it doesn't usually make a difference to the eye unless you are working with low voltage of 3V or less.
     
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  5. Wendy

    Moderator

    Mar 24, 2008
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  6. SgtWookie

    Expert

    Jul 17, 2007
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    You will also be much better off to use larger values for R2 and smaller values for C1.

    The smallest size you should use for R1 is 100 Ohms per volt of Vcc. Since you are using 6v, that's 6x100 = 600 Ohms. 1K is fine for up to around 10v.

    For R2 and C1, instead of 2k and 330uF, use 200k and 3.3uF. You will have very nearly a 50% duty cycle output.

    You could also use 560k and 1uF to get very nearly 1 second on/off intervals.

    A 270 Ohm resistor from +6v to the anode of a new LED (as yours is fried) and connecting the cathode of the LED to pin 3 should give you decent brightness without letting the magic smoke out.
     
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  7. blah2222

    Thread Starter Well-Known Member

    May 3, 2010
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    Thank you all for the replies.

    I managed to fix the circuit and got it to work. The problem was for some reason I had a short going from 5 to ground instead of a cap or open circuit.

    Sorry about that... I should have debugged my circuit more thoroughly. The links are helpful as well, I'm still new to this stuff and appreciate your help.

    JP
     
  8. blah2222

    Thread Starter Well-Known Member

    May 3, 2010
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    Would a variable resistor be a good idea? To be able to manage the frequency?
     
  9. Wendy

    Moderator

    Mar 24, 2008
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    Depends on the application. It is more fun to be able to adjust it, but many times space is a problem. A fixed resistor uses much less space.
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    Yes, you could replace R2 with a potentiometer wired as a rheostat.

    As Bill noted, fixed resistors are a lot smaller and a lot cheaper. They are much more reliable, too.

    A 1 MEG pot would give you from what seems like constantly ON (flashing very, very fast) to perhaps one flash every two seconds.
     
  11. blah2222

    Thread Starter Well-Known Member

    May 3, 2010
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    Cool stuff, I think I got the hang of the circuit and using it now, but I'm still not sure how it all works yet. Thanks again!
     
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