555 problem

Discussion in 'General Electronics Chat' started by hassan maysara, Nov 16, 2012.

  1. hassan maysara

    Thread Starter New Member

    Oct 23, 2012
    14
    0
  2. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Your link DOES NOT WORK.
    Please copy and post the schematic HERE to your reply.

    The datasheet for the LM555 says, "When the reset function is not in use, it is recommended that it be connected to VCC to avoid any possibility of false triggering."
     
  3. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,869
    1,393
    I think this is the circuit the OP is referencing.
    [​IMG]
     
  4. hassan maysara

    Thread Starter New Member

    Oct 23, 2012
    14
    0
    this is the circuit i think there is a mistake in the connection from T to 4 so i make changes but the problem i talk about is that if i don't attach any wire to 4 the voltage on it is 0.92v so the alarm works this means if i send 0 v to 4 the alarm work too what is the soultion
     
  5. ScottWang

    Moderator

    Aug 23, 2012
    4,855
    767
    You may try as below:

    1. Talking green LED away.
    2. Connecting E of T1 to GND.
    3. +9V → R4(430Ω) → (+)Green LED 2V/20mA(-) → C of T1
    4. R2 changing from 1K to 10K.

    * Green LED :
    2V/20mA 5mm, using 80% of 20mA is 16mA, R4 = 430Ω
    2V/10mA 3mm, using 80% of 10mA is 8mA, R4 = 875Ω,choose 820Ω, about 8.5mA
     
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  6. hassan maysara

    Thread Starter New Member

    Oct 23, 2012
    14
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    I speak about pin 4 in 555 if i don't attach any wire to it there is a voltage 0.92v on it so is the problem in the ic or what??????
     
  7. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    The alarm will run with pin 4 disconnected or floating.
    Do this:
    1. Connect pin 4 to ground, if the alarm still sounds then the IC is not working correctly, replace it.
    2. Rewire the circuit as suggested in post #5 This ensures that reset is pulled close to ground, the circuit in post #3 does not do that.
     
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  8. hassan maysara

    Thread Starter New Member

    Oct 23, 2012
    14
    0
    I'll try it
     
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