Triggering a 555 from both edges.

Discussion in 'General Electronics Chat' started by m121212, Mar 12, 2014.

  1. m121212

    Thread Starter Active Member

    Jul 24, 2011
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    Moderators Note: Trying to take over someone elses thread is called hijacking, and is not allowed on AAC. I have therefore given you a thread of your very own. This from How would u trigger 555 monostable on both edges?.

    Sorry to revive an older thread.

    There's something I don't understand about Ron's circuit suggestion with the common base for the negative edges.

    I built the circuit as drawn, but when I connect the output to a 555 pin 2, the spike train vanishes. It should just be a high-impedance comparator input, no?

    I tried to buffer the output with a common collector buffer, but got the same result. I will try again with a pair of common emitters (double inversion), but in the meantime maybe someone knows what's happening?
     
    Last edited by a moderator: Mar 14, 2014
  2. tindel

    Active Member

    Sep 16, 2012
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    That's a great circuit from Ron... Good thinking!

    As far as your circuit... if you're driving a high input impedance, 100k output impedance is probably too high try lowering the pull up resistor value
     
  3. m121212

    Thread Starter Active Member

    Jul 24, 2011
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    Yes, I played around with decreasing that resistor from 100k to 8.2k and to 1k, but without success.

    Building the circuit, with the solitary exception that I am using a 5v supply instead of a 10v, I get the screenshot shown at the output of Ron's circuit.

    When I load it with the 555 trigger pin (should be a high-impedance comparator input) I get the second screenshot. The falling edge detection (common base bjt) seems to have some trouble.

    Simulation shows the same behavior (instead of 555 pin I used an LM111 comparator).
     
  4. tindel

    Active Member

    Sep 16, 2012
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    Can you provide us with a circuit diagram? It's hard to troubleshoot something that you don't know what it is!

    If it is driving a capacitive load... It may be taking too much time to charge discharge the capacitor
     
  5. m121212

    Thread Starter Active Member

    Jul 24, 2011
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    Sure, see attached.

    Vin is a square wave from 5v to 0v, 10khz.

    Circuit is the same as Ron's except for the 8.2k pull-down.
     
  6. Metalmann

    Active Member

    Dec 8, 2012
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    I had to Google "Spike Train", and all I got was info about brain neurons........:confused:
     
  7. m121212

    Thread Starter Active Member

    Jul 24, 2011
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    It's the spike equivalent of a pulse train :) Perhaps not exactly common parlance, but perhaps now it's clearer.

    As a sanity check, I re-ran using an actual 555 model instead of the comparator. It's slightly better, but something is still going on with that falling edge.

    The 555 is configured as a monostable pulse generator , to output ~25uS wide pulses when the trigger is fired.

    As you can see here, the falling edge can't quite pull the trigger. Emotionally weak I guess!
     
  8. crutschow

    Expert

    Mar 14, 2008
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    3,233
    Try increasing the value of C1 and reducing the value of R2. Note that all the current to pull the output low has to come from C1 and R2 through T1 since T1 is common-base.

    The simplest circuit is probably a CD4077 XNOR (not XOR) gate with a resistor and capacitor as shown in Post #11 by Ron H.
     
    Last edited: Mar 13, 2014
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  9. m121212

    Thread Starter Active Member

    Jul 24, 2011
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    You're absolutely correct. That made all the difference. Thank you! And Ron, thanks for teaching us the clever trick.
     
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