Monostable 555

Discussion in 'The Projects Forum' started by tornado151, Apr 30, 2010.

  1. tornado151

    Thread Starter Member

    Apr 15, 2010
    34
    0
    Hi,
    I have built a 555 monostable circuit according to the attached pdf.

    However when I set the trigger off it immediately completes the circuit there is no time delay. I have adjusted the variable resistor but it makes no effect.

    Any idea whats wrong?

    Thanks
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    If, by "according to the attached PDF", do you mean you have put such a kit together?

    If so, have you carefully inspected your soldering to insure no pads got bridged?
     
  3. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    Is the 555 fitting well in the socket?
    If you are using the type of socket as shown on the pictures, they may make bad contact.

    Bertus
     
  4. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    The pin layout on the PDF is not the layout as the actual 555, so if you used this as a wiring diagram and not the kit:

    Be sure you have the package in the right direction, and have identified pins 1 through 8.

    Now be sure you attach the wires to the proper pins.
     
  5. tornado151

    Thread Starter Member

    Apr 15, 2010
    34
    0
    I bought the entire kit. All the soldering looks good, I cant understand what the problem is.
     
    Last edited: Apr 30, 2010
  6. retched

    AAC Fanatic!

    Dec 5, 2009
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    312
    Is the 555 in upside down?
     
  7. Wendy

    Moderator

    Mar 24, 2008
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    You should be able to see the capacitor charge up. It starts from ground, then slowly (slow enough to be seen by a meter) charges to 2/3 the power supply voltage. If this isn't happening then there is where the problem is.

    Pin 4 should be Vcc, and stay there.

    This circuit is very similar to the 555 Monostable in the AAC book.

    I would recommend a thorough read of the theory of operation.
     
  8. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    When you apply power, does the red LED light? When you press & hold the trigger switch closed does green LED light & stay on?
     
  9. tornado151

    Thread Starter Member

    Apr 15, 2010
    34
    0
    The 555 chip is up the correct way.
    The capacitors, diode and transistor are facing the correct way.

    When applying power LED2 light ups. When the trigger is set off, LED1 lights up and LED2 is switched off.

    I will try testing with the capacitors with a volt meter.
     
  10. Wendy

    Moderator

    Mar 24, 2008
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    I just took a closer look at your PCB vs the schematic, they don't match. You have Q1 on the PCB, which appears to be a driver transistor. Bad form that, a schematic should always have all of the parts, including part numbers (which yours doesn't).

    Don't forget to check the voltage on Pin 4, it is important, and this PCB swaps sides several times getting the voltage there. It should be at Vcc.
     
  11. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Every thing seem to be working right except for the charging ofC3 470 uF. On schematic 1k from VR1 is R5 which by parts list is 100k. When checking for V rise on C3, be sure to hold trigger sw closed to keep pin 7 from discharging C3.
     
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