NE 555, LED Lamp Sleep mode. ( Monostable Configuration)

Discussion in 'The Projects Forum' started by wolf441, May 23, 2016.

  1. wolf441

    Thread Starter New Member

    May 23, 2016
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    Hi guys, So I'm working on a project where I have to make an LED lamp that can go into sleep mode after a 5-10 second delay when a button is pressed.
    We're supposed to use a NE555 timer. I figured out how to use it and I made the proper circuit. The delay function works fine but the only problem is that the LED lamp is not on when I push the sleep button. It is only after I press the button that it turns on and turns off after an 8 second delay. I need the LED lamp to be on before I press the sleep button and then turn off after an 8 second delay only when the button is pushed.

    I'll attach a drawing of my circuit. Pin 3 outputs a voltage of 4v when the tap switch (sleep button) is pushed.
     
  2. wolf441

    Thread Starter New Member

    May 23, 2016
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    A NAND circuit seemed like the perfect solution until I realized there was one big flaw in it.
     
  3. #12

    Expert

    Nov 30, 2010
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    Give the LED about 220 ohms in series and connect to the 5V line, not the ground.
     
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  4. wolf441

    Thread Starter New Member

    May 23, 2016
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    I tried doing that. But the LED is always on then. When the sleep button is pushed its brightness increases for 8 seconds and then it stays on at its normal brightness.
     
  5. #12

    Expert

    Nov 30, 2010
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    You have something else wrong if the NE555 sinks current into pin 3 all the time.
    Maybe you burned up the chip by not using a resistor to limit the LED current.
     
  6. wolf441

    Thread Starter New Member

    May 23, 2016
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    No, The 555 chip is working fine.

    Wait, so did you ask me to connect the terminal of the LED thats connected to the ground back into the 5V supply line instead of ground?
     
  7. #12

    Expert

    Nov 30, 2010
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    Yes. I said to let pin 3 drive the output just as it has been and connect the LED and its series resistor to the 5 volt supply. That will reverse the on and off of the circuit.
     
  8. wolf441

    Thread Starter New Member

    May 23, 2016
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    The LED doesnt turn on. :/
    I don't understand how the circuit is completed if you do that.
     
  9. wolf441

    Thread Starter New Member

    May 23, 2016
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    Well I reversed the polarity of the LED and did what you said. Now the LED is on, and when i press the sleep button it turns off, but turns on again after 8 seconds.
    Thats not what I want,I want it to be on, and then when I press the button, to turn off after 8 seconds.
     
  10. #12

    Expert

    Nov 30, 2010
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    Well, I assumed the circuit was right and you just had the output reversed. It seems worse than that and I'm feeling rushed right now with 3 other customers running at the same time. If I get time, I will examine this properly.
     
  11. wolf441

    Thread Starter New Member

    May 23, 2016
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    That's perfectly fine. Take your time. :)
     
  12. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    So, to clarify what you want...

    Lamp is ON

    User presses a button

    8 to 10 second delay

    Light turns off

    =====

    Is that all? Any requirements of how the light turns on?

    Maybe you can tell us the reason for the project? Did you consider other options?
     
  13. wolf441

    Thread Starter New Member

    May 23, 2016
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    Yes!!
     
  14. #12

    Expert

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    The 555 starts with its output low. When you hit the trigger (low) it changes to output to high immediately, waits for the capacitor to fill to 2/3 Vcc and then switches back to low. If you want the LED to be on, stay on when you hit trigger, and stop when the output switches back to low, you must have something attached to the out put which only responds to a falling edge. This can not be done in one stage.

    Meanwhile, the moment of startup should have a delayed reset to avoid glitches. This means you add a resistor from Vcc to the reset pin and add a capacitor to ground from the reset pin to hold reset low for a few milliseconds. When reset comes up, the timer is in the output low condition and stays there until trigger goes low.

    Boy am I glad to see Gopher. I always have to re-teach myself the 555 timer every time I need a circuit.:mad:
     
    Last edited: May 23, 2016
  15. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Then you need a 555 timer (1-shot). To delay a rise by 8 seconds

    Then a 555 wired as a flip flop that will toggle the lamp
     
  16. wolf441

    Thread Starter New Member

    May 23, 2016
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    What do I do with the flip flop?
     
  17. #12

    Expert

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    Watch out. I wrote the logic wrong and just corrected it. You want to respond to a falling edge.
     
  18. wolf441

    Thread Starter New Member

    May 23, 2016
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    I think I understand what you mean. But the problem is we're only given 2, ne555 timers and we're supposed to use one to make a dimmer + flasher circuit.
    I can probably use the output from the flasher circuit as a flip flop but the lights need to be flashing at 5hz. So I cant change the duty cycle.
     
  19. GopherT

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    I gotta go. Work calls. Sorry
     
  20. wolf441

    Thread Starter New Member

    May 23, 2016
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    Thats fine. :)
     
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