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555 monostable - no delay

Discussion in 'General Electronics Discussion' started by mike3317, Apr 15, 2014.

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  1. mike3317


    Apr 15, 2014
    I've been trying to get the 555 chip to work in monostable mode and can't figure out what the problem is. As far as I'm aware it's configured correctly, and the 3x 1M resistors and 1uF capacitor should turn the led on for ~3.3s after the blue wire is grounded. What actually happens is that grounding the pin 2 turns on the LED but is turned off the instant the blue wire is removed from ground. Thanks in advance for any help.

    Attached Files:

  2. neon


    Oct 21, 2006
    pin 2 is the trigger ok 50 micro amps will do going negative the 555 has 2 states output can source or sink 200 ma if you connect a led at pin 3 it might survive once depends on the Vcc voltage put 1k for 12v Vcc to limit the current then find out where the 1k +led is going to be tied to to lite it for the time
  3. mike3317


    Apr 15, 2014
    The supply is just under 5V, and the LED has an internal limiter and doesn't require a resistor under 12V.
  4. Chris Dancer

    Chris Dancer

    Nov 6, 2013
    Are you sure that is a 1uF capacitor? I've never seen a 1uF ceramic cap in that case style.

    I can't see anything obviously wrong in the layout. The problem could be the 555's infamous power supply glitch causing the circuit to misbehave. Try putting a 10uF electrolytic cap from pin 1 to pin 8 as close as you can to the IC, see if that makes any difference.
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Looks like a 0.1uF or even 0.01uF capacitor.

    Also, it is useful to place a small capacitor (0.01uF to 0.1uF) between pin 5 and ground (this will help even more than the suggested cap between pin 1 and 8).
  6. mike3317


    Apr 15, 2014 I am not. Just checked and it's 1000pF, I got nano and pico mixed up. Replaced it with a new 10uF and it works. I cant believe after all that I missed something so simple. Thanks for the help.
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