555 Monostable has some output issues.

Discussion in 'The Projects Forum' started by Vemuri Suresh Kumar, Aug 26, 2015.

  1. Vemuri Suresh Kumar

    Thread Starter New Member

    Aug 26, 2015
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    Hello all,
    Am new here and I was impressed the way people interact and help each other here.;):)
    I was making a project based on PIR motion sensor.
    The circuit works as the motion sensor senses the motion, it gives output (3.3V) to the 555 timer (Pin 2) which i have used in Monostable mode.
    Now the output of this 555 timer (Pin 3) is connected to the relay in such a way that as the o/p gets low, a lamp turn ON through relay.

    This circuit works fine <ON BREADBOARD> and as i have expected

    Now when i made this made the same circuit on a PCB it doesnt function as in breadboard. And frankly, i have checked it about 10 times and i found no mistake or shorting in the Souldering.

    ISSUE is:
    Whenever the Motion sensor triggers the timer (Pin 2), Pin 3 should go low. But it isnt going low. I need help....... :(
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    What is your supply voltage to the 555? A standard 555 is marginal in its operation even at a Vcc of 5V. How does a 3.3V pulse trigger a 555 that is running on 5V? Post a schematic...
     
  3. Vemuri Suresh Kumar

    Thread Starter New Member

    Aug 26, 2015
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    Actually My supply voltage to 555 timer is 5V DC. The triggering voltage needed for 555 timer is 1.67 V. Thats why i managed to give 1.7V to the i/p (Pin2) by adding resistors between o/p of the sensor and i/p of 555.
    The schematic is attached.
     
  4. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    745
    Your circuit is wrong, you need to feed a 0v trigger to pin 2, then the timer will go high for the set period, then it will go low, if pin 2 is kept low for longer than the time pulse, pin 3 will stay high permanent,

    if your pir gives out a positive pulse, you need a Npn transistor to invert the pulse to make it work.
     
    Vemuri Suresh Kumar likes this.
  5. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    If your PIR put out a logic signal that swings from 0V to 5V, it would be this easy:

    pir.gif

    Note that the AdaFruit PIR has an internal jumper to make the output signal either 3.3V or 5V.

    If yours is not strap-able, then you will need a level-shifter to convert the 3.3V signal to a 5V one. Like this:

    pirls.gif

    Are you sure that your PIR sensor doesn't have an open-collector output?
     
  6. Vemuri Suresh Kumar

    Thread Starter New Member

    Aug 26, 2015
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    Model of my PIR sensor is DYP-ME003.
    I am giving it a Vcc of 5V and it's default output is 3.3V (when motion is sensed).
    Now am giving this o/p voltage to pin 2 of 555 timer.
    This time instead of giving the o/p directly to the timer circuit, I have given it through a NPN transistor (BC547). As suggested by Dodgydave to give inverted i/p as to get fruitful o/p.
    The problem now arised was, whenever there's a trigger, the o/p gets low making my Lamp turn OFF. That's why i used another inverter at the i/p of the relay and it works fine. <again only on breadboard>
     
  7. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    If you invert the input, there is no need to invert the output:
    piri.gif
     
  8. Vemuri Suresh Kumar

    Thread Starter New Member

    Aug 26, 2015
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    Ohh yes @MikeML I recognized that yesterday and that was so helpful ... Impressed by the way you explain stuff. Thanks for that:)
    And Which circuit simulator do you use?
     
  9. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    LTSpice. free download at linear.com
     
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