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Using multiple 555 timers in astable

  1. May 26, 2012 #1
    Hello, I am relatively new to this forum as a registered member but have been reading as an unregistered guest for quite some time. I have searched and searched through search engines and forums and am really not having too much luck finding any information on using several 555 timers in an astable configuration together in the same circuit.

    Albeit there might be much simpler ways to accomplish the end result, I figured that this little project would be a great way to learn how to manipulate each and every tweak on a 555 astable circuit. I would like to use 5 555timers to light up 5 different LEDs at different intervals (when 1 goes off, 2 comes on, etc.). The end goal would be to have a set of three LEDs/555s that would operate independently from the last two, as they are different colors.

    Could someone kindly assist in this endeavour, or at least point me in the right direction to find some help with this? I am a novice electronics hobbyist who is pretty much self taught, and any advice would be great. I can wire the 555 in a simple astable and accomplish the goal of lighting 5 LEDs, however I cannot control the interval at which the 2nd and 3rd 555s begin their frequency cycling, etc.

    In addition, I would also be interested to hear what other solutions you might provide to accomplish the same goal.

    Thanks in advance for any help.

    CP
     
  2. jcsd
  3. May 26, 2012 #2

    vk6kro

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    Science Advisor

    I'm not sure what you really want, but you may be able to work it out if you know that the reset pin of a 555 stops all activity and forces the output to go LOW. (Low is 0 volts. High is the same voltage as the supply)
    Pin 4 of a 555 normally has to be high for the chip to work, but if it is low, the chip stops working.

    So, you could control one 555 with another which was oscillating and producing alternative high and low outputs.

    555s can also be used as a Schmitt trigger, which from your point of view might be a handy inverter.
    If the input is high, the output will be low and vice versa. So, you could have lights turning on and off in opposite phase.


    http://www.kpsec.freeuk.com/images/555buff.gif [Broken]
     
    Last edited by a moderator: May 6, 2017
  4. May 26, 2012 #3
    Use monostable timers in ring

    This is also my first post on this particular forum. But I first used a 555 timer about 37 years ago a couple of years after it was introduced, and I have used it a few times over the years. What you probably need is a monostable configuration of a 555 timer. Astable oscillators free run, so can not be easily triggered to run sequentially as you desire.
    • You can arrange the five 555 timers in a ring. Monostable mode allows each 555 to be triggered by the previous 555 in the ring. In this mode, each 555 generates one pulse then stops.
    • The first monostable flashes the first LED with pulse width T1.
    • When the first monostable pulse ends, the second monostable is triggered and flashes the second LED with pulse width T2.
    • When the second monostable pulse ends, the third monostable is triggered and flashes the third LED with pulse width T3.
    • When the third monostable pulse ends, the fourth monostable is triggered and flashes the fourth LED with pulse width T4.
    • When the fourth monostable pulse ends, the fifth monostable is triggered and flashes the fifth LED with pulse width T5.
    • When the fifth monostable pulse ends, the first monostable is triggered and the cycle repeats over and over again until power is removed.
    The problem with this arrangement is how to get the ring oscillator to start in the correct configuration when you apply power. You probably want the first timer to start on, then the ring of LED pulses would start when the first timer ended. If the 555 timers start up on power up with all outputs off, then the ring will never start. So it needs a special initial condition at power up.

    One solution would be to force the first 555 on at power up for an interval longer than all pulse times. This can be done with D1, D2, and C4 on this schematic. These components should only be on the first timer.
     

    Attached Files:

  5. May 27, 2012 #4
    Re: Use monostable timers in ring

    n5bb - This is EXACTLY what I am looking for. The attachment that you uploaded appears to be for the second monostable circuit. Is this correct?

    If you wouldn't mind, could you provide a schematic that would show both the first and second 555 timer? As I said before I am a novice and I understand the schematic you sent, I just need one more small push to be able to breadboard this circuit and determine if it will do what I want to do.

    Thanks so much!
     
  6. May 27, 2012 #5
    To answer your questions:
    • The circuit I posted was for the first stage, since it includes the stage 1 startup circuit.
    • The second and other later stages should not include D1, D2, and C4. Those components are a startup circuit to force stage 1 to start first. Without those three components on stage 1, nothing happens since all stages start in the off condition when power is applied.
    • I hope that is clear. I don't have time to create another schematic right now - maybe later. Remember: stage 1 is shown, the later stages don't include the startup circuit, and the stages are arranged in a ring, with the output of a stage driving the input of the next. The output of the last stage drives the input of the first.
    • Imagine 5 people in a ring, holding hands with those on either side. Initially, nothing happens. Then a dog bites person 1. He says "ouch" and squeezes his right hand (output) hard for 1 second. Person 2 then says "ouch" since her left hand (input) was squeezed, and she squeezes her right hand for two seconds. This continues around the ring, with people squeezing for longer or shorter times depending on their personality. The last person in the ring squeezes his right hand (output) so the left hand of person one (input) is squeezed and the cycle starts over again. The starting circuit acts like the dog, starting the pulse propagating around the ring.
    I built and tested a two-stage version of my circuit. The video is at youtu.be/ShNXs_w2NrU (sorry for the background sound -- the radio was on).
    • The output of stage 1 is connected to the input of stage 2.
    • The output of stage 2 is connected to the input of stage 1.
    • This is a video of a short ring, with only two stages. But you could add as many stages as you wish. The idea is that each stage generates a single pulse (which activates the LED for that stage). On the falling edge of that pulse, the next stage is triggered and generates a pulse.
     
  7. May 30, 2012 #6
    Last edited by a moderator: May 6, 2017
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