Astable 555 Circuit with Ton < Toff

Discussion in 'Digital Circuit Design' started by José Alfredo Sánchez, Jun 14, 2017.

  1. José Alfredo Sánchez

    Thread Starter New Member

    Feb 1, 2017
    20
    1
    Hello dear friends.

    I need your valuable help for a little circuit I'm designing.

    It is a small circuit whose output I want to keep ON for one second and then stay OFF for 8 seconds. I've tried with a 555 in astable mode, but in all settings I've read the Ton> Toff. I have been testing with two LEDs connected to the 555 output (one connected to VCC and one to GND).

    For simulation purposes, I am using two resistors of 47k and 10k with a capacitor of 220uf which turns on the red led for about 8 seconds and the green led for 1 second. But the behavior I want is the red led 1 second ON and then the green 8 seconds ON.

    green-red.png

    I know you will recommend me to use a PIC or AVR. I have similar circuits that work perfectly, programmed in PIC 12F675 or ATTiny85, but I prefer to leave the uC for more complex projects.

    Any suggestions on how I can use a 555 for this?

    Thank you very much in advance.
     
  2. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
    5,434
    2,056
    Swap the LEDs and recalculate the RC to provide a Ton of 8 seconds and Toff of 1 second.

    Then Ton > Toff
     
  3. bertus

    Administrator

    Apr 5, 2008
    19,903
    4,138
    Hello,

    Do you want the leds to alternate?
    Why is R6 connected to the TH and TR pins of the 555?
    I would expect that connection of R6 to be connected on ground.

    Bertus
     
  4. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
    8,134
    2,015
    You can make the on and off times independent by connecting a diode across R5, anode to the '555 pin 7 and cathode to pin 2 and 6.

    I certainly agree with that. It is not going to work properly as drawn.
     
  5. José Alfredo Sánchez

    Thread Starter New Member

    Feb 1, 2017
    20
    1
    Ooops.

    I will review that.

    Thanks a lot!
     
  6. José Alfredo Sánchez

    Thread Starter New Member

    Feb 1, 2017
    20
    1
    Excelent!

    I tryed that and my circuit works fine now!

    Thanks a lot!
     
  7. Wendy

    Moderator

    Mar 24, 2008
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  8. bertus

    Administrator

    Apr 5, 2008
    19,903
    4,138
    Hello,

    @Wendy , The OP wanted an asymetric on/off.
    You circuit will be about equal on/off times.
    When you double the timing post and use some diodes, the on and off time can be adjusted independently:

    555Circuit_wendy_indipendend on off ratio.PNG

    Bertus
     
  9. José Alfredo Sánchez

    Thread Starter New Member

    Feb 1, 2017
    20
    1
  10. José Alfredo Sánchez

    Thread Starter New Member

    Feb 1, 2017
    20
    1
    Hi!

    Thanks to the excellent advice from all of you, in particular from @Wendy and @bertus, I have achieved what I needed. I have a timer that activates a led for about 0.69 seconds and another led every 1.15 minutes. Actually, what I want to do is activate a device to chase animals, which I do not want to be retriggered for 1 minute.

    PIR.png

    The timer works fine, but I have not been able to activate it with the PIR motion sensor. The cycle is activated every time the PIR is triggered, but I can not get it to work properly.

    Any suggestions?

    Thanks in advance.
     
  11. Wendy

    Moderator

    Mar 24, 2008
    21,838
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    I think Bertus called it.
     
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