Modifying 555 timer alternating LED flasher to alternating triggers

Discussion in 'The Projects Forum' started by drlids, Feb 4, 2015.

  1. drlids

    Thread Starter New Member

    Feb 4, 2015
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    Hi, I'm tinkering with 555 timers to devise a clocker for analog synthesizers, using sequential triggers from a monostable 555 timer chain.

    Looking at the alternating LED flasher circuit (e.g., as found here http://forum.allaboutcircuits.com/blog/cookbook-entries.381/), it occurred to me that maybe I could derive triggers from both the sinking and the sourcing side on pin 3 instead, thereby using chained 555 timers to sequentially double the frequency of the triggers.

    I breadboarded the alternating LED flasher and a second 555 timer in monostable mode, but I can only get the second timer to trigger from the sourcing side of the flasher. Should the sinking side be able to provide a trigger, or am I misunderstanding how the outputs of both sides work?

    I'm new to all of this, so any help would be greatly appreciated.

    Thanks.

    dl

    P.S. I realize that what I'm trying to do could be more easily accomplished using a decade counter driven by a 555, but I like the challenge, and figure it's a good way to learn about the ins and outs of 555 timers.
     
  2. ScottWang

    Moderator

    Aug 23, 2012
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    The LED flasher is called astable multivibrator, what are the R,C values of astable multivibrator and monostable circuit?
     
  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    @drlids
    Yes, you can have two monostable 555 circuits trigger each other. I recently set one up to allow two completely independent controls of the alternating LED on times.

    The only things you need to worry about is how to trigger the first one (possible) and make sure to capacitively connect them in a way that your trigger pulse goes high again faster than the monostable that it was triggering.
     
  4. drlids

    Thread Starter New Member

    Feb 4, 2015
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    Thanks for your replies. Yes, I have the first 555 wired as an astable to trigger the second 555 wired as a monostable. I've tried the circuit with a variety of RC values, and it seems to now be working. What I'd like to do next is figure out how to edge-trigger the monostable, so that I can trigger it with both sides of the astable. Right now I have the monostable set up in restartable mode, with pin 2 tied to pin 4.

    GopherT, you mentioned capacitively connecting the two 555s. I've tried putting the astable output through a 10 uF capacitor into pin 2 (tied to Vcc through a 10K resistor), but it didn't trigger the second 555.

    Any advice for the next step?
     
  5. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    For falling edge triggering, use a 10nF capacitor to connect output of one to input of other. Tie pin 2 of the monostable to the Vcc with a 33k resistor as I did in the image of post 2.

    For rising edge triggering, connect the output of the a-stable to the base of an npn transistor (emitter to ground and collector to pin 5 of the mono-stable. Not a normal trigger but it works (it works better on a breadboard than it does on some simulators (not all simulators handle pin 5 very well).
     
  6. drlids

    Thread Starter New Member

    Feb 4, 2015
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    Oops, that should have been 100 nF, not 10 uF, for the capacitor I've been using for falling edge triggering. I'll try it again with a 10 nF capacitor and a 33k resistor to tie pin 2 to Vcc, and see what happens. (However, I didn't see any image in post 2.)

    I guess my question is, can I combine the capacitive coupling with restarting the monostable by tying pin 4 to pin 2, or are the two going to interfere in some way?

    Thanks for the help so far.

    dl
     
  7. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Sorry, I thought I had posted it. Here is an image of two monostable 555 triggering each other. Reset (pin4) should be tied to Vcc unless you need to cancel a trigger. Otherwise, output of your astable (pin 3) should trigger the monostable (pin 2).

    Cheers

    image.jpg image.jpg
     
  8. ScottWang

    Moderator

    Aug 23, 2012
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    If you need both of rising and falling edge to trigger the monostable, the circuit as below.

    DoubleFrequencyInverter_ScottWang.gif
     
  9. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    ....via a current-limiting resistor.
     
    GopherT likes this.
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