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555 timer self reset

Discussion in 'General Electronics Discussion' started by sahil-siwatch, Jan 12, 2021.

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  1. sahil-siwatch

    sahil-siwatch

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    Jan 12, 2021
    I am new to working on 555 timer. I am running it in monostable mode. I am triggering it using a button on trigger pin. It works just fine. What I want is the LED connected to the out pin 3 to switch off when I press the trigger button again. If I don't press the button, LED will switch off after the specified time duration by the RC circuit. Is it possible? Thanks in advance. :)
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Trigger pulse to the 555 is low active.
    Use a NOR gate and an Inverter.
    Connect the output of the 555 to the input of the inverter. Connect one input of the NOR gate to the output of the inverter.
    Connect the second input of the NOR gate to the trigger signal.
    Connect the output of the NOR gate to the RESET input of the 555.

    You don't have to use integrated logic gates, you can construct the same function with discrete elements (examples).

    To avoid oscillations you may have to add an RC delay between the inverter and the NOR gate.

    BTW: the title of your thread is misleading: Your question is not about self reset but about active external reset. Unless I have misunderstood the question.
     
  3. sahil-siwatch

    sahil-siwatch

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    Jan 12, 2021
    Hello. Sorry if the title was misleading. I tried what you said on multisim simulator. It's not working. The timer doesn't get triggered. I am attaching an image. Can you see if I am doing anything wrong?
     

    Attached Files:

  4. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    This is where the issue with oscillations I mentioned may come into play: When the trigger switch ios closed, the lower input of the NOR gate is low. The output of the 555 becomes high. Consequently the inverter output goes low and both NOR inputs are low. This generates an active high RESET to the 555 and the cycle restarts.
    Try an RC filter (low pass) with a time constant longer than the duration of a pushed button (red).
    Also try to make the trigger a single pulse by adding another RC filter (high-pass) to the input (blue).
    upload_2021-1-12_8-37-29.png
     
  5. sahil-siwatch

    sahil-siwatch

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    Jan 12, 2021
    I don't think this is working sir. upload_2021-1-12_13-33-2.png
     
  6. Harald Kapp

    Harald Kapp Moderator Moderator

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    Your low pass is set to a too low corner frequency. Make C4 much smaller.
    Also try this change:
    upload_2021-1-12_11-4-7.png
    Now the NOR gate will see not the pulse but the steady signal from S1.
     
  7. sahil-siwatch

    sahil-siwatch

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    Jan 12, 2021
    I tried with the changes on the above diagram. I also tried with smaller values i.e 1,0.1, 0.01, 0.001uF as well. Nothing seems to be working sadly.
     
  8. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Can you be more specific? What does not work? The monostable, the reset, ...?
     
  9. sahil-siwatch

    sahil-siwatch

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    Jan 12, 2021
    monostable not even triggering....button doesnot have any effect here. after i make the connections. In normal connections, it works.
    upload_2021-1-13_11-16-34.png
     
  10. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Can you show the timing and amplitude of the signal at the 555's inputs TRI and RST? An at the output OUT?
     
  11. sahil-siwatch

    sahil-siwatch

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    Jan 12, 2021
    Pink one is trigger. blue(green underneath blue) are Reset and output.

    upload_2021-1-13_13-44-47.png

    when i press switch...voltage goes momentarily high.
     
  12. bertus

    bertus Moderator

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    Nov 8, 2019
  13. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    My fault, I beg to excuse. RESET is, in spite of its name, low active. Therefore you need to change the NOR gate to an or gate.
     
  14. sahil-siwatch

    sahil-siwatch

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    Jan 12, 2021
    after using the OR gate, I get 3.3V on RESET but output is still LOW. Could it be the values of RC ?
    upload_2021-1-13_14-2-59.png
     
  15. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    What is the power supply to the OR gate? It should be powered by the 7 V supply, too.
     
  16. sahil-siwatch

    sahil-siwatch

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    Jan 12, 2021
    I can't see any setting to do that.
    upload_2021-1-13_15-40-42.png
     
  17. Alec_t

    Alec_t

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    Jul 7, 2015
    Voltage at which point? Please be more specific with your responses.
    You need to set your logic parameters to suit the type of OR gate you are using. For a CMOS gate the switching threshold will be about half the supply voltage, i.e ~3.5V with a 7V supply. Logic output high will be 7V.
     
    Last edited: Jan 13, 2021
  18. sahil-siwatch

    sahil-siwatch

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    Jan 12, 2021
    trigger
     
  19. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Look under "logic levels". Currently it shows "3.3 V mode" with an output high logic level of 3.3 V. Set this value manually to 7 V.
     
  20. sahil-siwatch

    sahil-siwatch

    25
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    Jan 12, 2021
    hello. I have changed that. The timer is entering the monostable mode but on second press it doesn't switch off which is what I want. My circuit is configured for 3.3v logic level and Or is configured at custom(7V output high, threshold 3.5). The behaviour is same if I keep both levels custom(i.e 7v high, threshold 3.5v). This is my final circuit.
    upload_2021-1-14_0-35-29.png
    upload_2021-1-14_0-36-43.png have you tried this circuit personally?? Could my RC values be wrong ?? Please correct me if I misunderstood what you were telling about logic level.
     
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