Problem with 555 LED flasher (not flashing)

Discussion in 'General Electronics Chat' started by lefam, Dec 7, 2010.

  1. lefam

    Thread Starter Member

    Nov 30, 2010
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    Hi guys!

    I am having problems with my first 555 LED flasher. The LED is not blinking. It just stays ON when I feed power to the circuit.

    Details of the circuit:

    PIN8 connected to +9V
    PIN1 connected to ground
    PIN2 connected by a wire to PIN 6.
    Resistor R1 connected between PIN8 and PIN7
    Resistor R2 connected between PIN7 and PIN6
    Capacitor C connected between PIN2 and (ground)

    LED protected by a 330 Ohm resistor connected between PIN3 and ground.

    I tried the circuit with these configurations:
    C=1000uF, R1 = 1k and R2 = 5k
    C=1000uF, R1 = 5k and R2 = 1k
    C=1000uF, R1 = 100Ohm and R2 = 1k

    C=10uF, R1 = 1k and R2 = 5k
    C=10uF, R1 = 5k and R2 = 1k

    I don't think I am not wiring well the circuit. If the configuration of C,R1 and R2 should lead to a flashing LED maybe my 555 chip is dead.

    Please help me to understand why my LED is not flashing. What should be the values for R1 and R2 for a 1Hz flashing LED? Does it matter for R1>R2 and vice-versa?

    Thanks in advance
     
  2. retched

    AAC Fanatic!

    Dec 5, 2009
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  3. macgyver

    New Member

    Dec 7, 2010
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    what's the voltage reading in PIN3 of the 555?
     
  4. bertus

    Administrator

    Apr 5, 2008
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    Last edited: Dec 7, 2010
  5. retched

    AAC Fanatic!

    Dec 5, 2009
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    good call mac and bertus.

    This is a good reason why a drawn schematic is better than a spoken (typed) one.

    Also lefam, are you building this on a solderless breadboard?
     
  6. Wendy

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    Mar 24, 2008
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  7. bertus

    Administrator

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    Hello Bill,

    In his opening post he has a list with the values of R1,R2 and C.

    Bertus
     
  8. Wendy

    Moderator

    Mar 24, 2008
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    My bad, so he did. Tired computing will do that to you.

    Assuming he is using the same schematic (a large assumption), the first set of values will be around 0.13Hz, or every 7.6 seconds. 555 circuits also take a lot longer on the first flash, so it will be over 10 seconds.

    I'm betting on a simple wiring error, though there is another possibility.

    Really large caps, when they are old, can be finicky. The resistors values are low enough that it shouldn't matter, but there is always the possibility of a bad cap.
     
  9. lefam

    Thread Starter Member

    Nov 30, 2010
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    I realized that I was not connecting correctly the timing capacitor.
    So I bought a new chip and tried again attaching correctly the timing capacitor and adding the PIN4 wire connection to +V. But it is still not flashing.

    I used C=220uF, R1=2k, R2=10k

    I am pretty sure all the PIN connections are ok.
    I suspect the capacitor is dead. When I measure the capacitance using my multimeter it gives a wrong value. Does it mean the capacitor is dead?

    When I use the same capacitor to make a LED fade out circuit it works. The interesting thing is that I used to make the circuit with the capacitor attached backwards and it worked but took a very long time (>1min) to completely turn off the LED.
    When I attach the capacitor correctly, the LED fades out and turns off in 3 seconds.

    Today I will buy new capacitors (I suspect I killed all of them).

    The lack of the 10nF bypass capacitor may be the problem? Or just the timing capacitor?
     
  10. Wendy

    Moderator

    Mar 24, 2008
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    The chips are probably fine. We must have the schematic you are using before we can help, it is an absolute. After that I can tell you the voltages you should see at the various pins.

    I write a lot about this little chippie, it seems complicated at first, but it really isn't.

    Are you using a protoboard? This (as defined by me) looks a lot like this...

    [​IMG]
     
  11. lefam

    Thread Starter Member

    Nov 30, 2010
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    Hi guys. The LED is flashing now.
    I realized a few days ago that I was not connecting correctly the battery. That fact killed my first 555 ICs. I bought new ones and my LED is flashing now.

    Thank you. I learned a lot with you guys.
     
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