555 timer switch

Discussion in 'The Projects Forum' started by lotusmoon, Aug 1, 2013.

  1. lotusmoon

    Thread Starter Member

    Jun 14, 2013
    203
    4
    I am making an LED flasher with a 555 timer (timer 1) . I also want to add another 555 timer circuit (timer 2) to it, so that (timer 1) is on for a certain period of time and then turns it self off.
    So what I am unsure of how it do is - I want only way for (timer 1) to turn back on is from a manual reset button/switch, and not from when (timer 2) comes back in again. Could someone help me with this? Thank you
     
  2. WBahn

    Moderator

    Mar 31, 2012
    17,777
    4,804
    You need to be more explicit in your discription. It is very ambiguous to talk about a 555 timer being "on" since it can be set up to run in an astable or a monostable mode and "on" means very different things in those two cases.

    If am reading between the lines correctly, you want timer 1 to run as an astable multivibrator (i.e., an oscillator) in order to turn the LED on and off. But you want timer 2 to be configured as a monostable multivibrator (i.e., a one-shot) that gates timer 1.

    The easy way to do it is to just run timer 1 as a free running timer and AND the output of both timers so that the LED is on only when both timers are HI. Your reset button then simply retriggers timer 2, which goes HI for your "certain period of time" and then goes LO again until the next time the button is used to trigger it again.

    One (fixable) downside of this is that the first and last LED flash may not be full cycle. But you can use the reset button to reset both timers so as to synch them and adjust the second timer duration so that it shuts down midway between two pulses from timer 1.
     
  3. lotusmoon

    Thread Starter Member

    Jun 14, 2013
    203
    4
    I have 555 "timer 1" running in astable mode to run an LED flasher at different frequencies.
    I wanted to put in another 555 timer "timer 2" also in astable mode which will give some time periods (30, 60, 90, 120 seconds) that the LED's will run for and then switch off
    when that particular time period is over "timer 2" will turn off "timer 1" and the LED's will stop flashing.
    from what you say if i put "timer 2" in monostable mode it just works once "(i.e., a one-shot) that gates timer 1"
    and then it would have to be restarted by a manual reset button, which is what I wanted.
     
  4. #12

    Expert

    Nov 30, 2010
    16,343
    6,827
    You are correct. The first stage is a "one-shot" and the second stage is the flasher. Have you made an attempt at a schematic so we can see how far you were able to go with this?
     
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