Triggering a One-Shot 555 with a 4017

Discussion in 'General Electronics Chat' started by DanRilley, Apr 16, 2011.

  1. DanRilley

    Thread Starter Senior Member

    Jan 13, 2008
    107
    0
    Hi I'm building a basic 4-step sequencer that needs to generate a CV and Gate signal. The Gate should be a 50ms pulse at +5V and the CV should be between 0 and 5V. I've come up with the following circuit using various resources online, however, when building it I actually ran into a problem, the pulse shaper fails because the 4017 doesn't go low long enough to retrigger the one-shot.

    The circuit works like this: a 555 (IC1) clocks a 4017 divider (IC2) which divides the frequency. The 0 output of that divider is fed as the clock to another 4017 (IC5) which sequences the four channels. The channel outputs go through 4 switches and each switch is bussed into a 555 one-shot (IC3) to shape the pulse into a 50ms burst. But as I said the last part isn't working. Does anyone have any tips on how to make these short bursts from a 4017 in a better way? I thought this would work since its possible to clock a 4017 from another 4017 but it seems the 555 trigger is not picking up the low gap in between sequencer steps and just reads the gate as always high...
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    You have the outputs of IC3 and IC4 connected together. This is not correct because when one of them goes high, current will flow into the output of the other. This does not allow Q2 to turn on and the 555s might burn too. Use one resistor per 555 instead of a common one (R1). Also, 100K is quite high to drive a transistor, use 10K.
     
  3. Wendy

    Moderator

    Mar 24, 2008
    20,764
    2,534
    You may have destroyed IC3 and IC4. The outputs of the 555, while not rail to rail, are very close and have no current limiting. If they are in different states you have an effective short on both parts. A simple diode on each output would have prevented this.

    Which 555 is not responding?

    Second time someone has beat me to the enter key today.
     
  4. DanRilley

    Thread Starter Senior Member

    Jan 13, 2008
    107
    0
    OK, that's good to know, but at the moment I don't even have IC4 hooked in yet so that can't be the problem.

    Also I didn't mention in the original post, but the circuit does appear to work if I insert a gap into the sequence pattern by turning off one of the switches. When the sequencer pass from an off switch to an on switch, the one-shot does fire as intended. It is just when the sequencer goes between two steps that are 'on' that it fails to fire the gate pulse. Does that make sense?
     
  5. Wendy

    Moderator

    Mar 24, 2008
    20,764
    2,534
    You have a fairly long RC constant using the values for C4 and R7. How fast are these working? Try dropping R7 to 10KΩ. When the transistor Q1 stops conducting the capacitor C4 must discharge through R7 and R2.

    If R7 were 1KΩ there wouldn't be any increase in current, as pin 2 is a high impedance input for the comparator inside the 555.
     
  6. DanRilley

    Thread Starter Senior Member

    Jan 13, 2008
    107
    0
    I will try modifying this resistor tomorrow, I'm away from my studio now. But also I just noticed on my schematic here c4 is .01u, but I have since changed it to .1u when I was fiddling with it. I selected the values .1u and 100k from the attached schematic, but will try lowering the 100k and see if that helps anything. The only difference between my version and this version is that I've replaced the switch with a transistor controlled from the sequencer steps.

    Also to answer your question, the sequencer at its fastest will be about 6hz. However I don't know how fast the 4017 is internally in terms of going low/high between steps.
     
  7. DanRilley

    Thread Starter Senior Member

    Jan 13, 2008
    107
    0
    Maybe this is stupid but perhaps I also need to remove R2.
     
    Last edited: Apr 16, 2011
  8. Wendy

    Moderator

    Mar 24, 2008
    20,764
    2,534
    R2 needs to be there. Your Q1 is an inverter in front of the 555, that monostable triggers on the leading edge.
     
  9. DanRilley

    Thread Starter Senior Member

    Jan 13, 2008
    107
    0
    Unfortunately lowering the value of R7 didn't have any effect to the triggering of the 555. I also tried lowering c4. It appears that the 555 simply won't trigger from the small signal change that occurs between 4017 steps. The only solution I can think of now is to do an AND gate with the output of IC2, that way there is a longer off-time capable of triggering the 555, but that also means that my sequencer can't run as fast as it would have in this configuration. Hmm any other ideas?
     
  10. Wendy

    Moderator

    Mar 24, 2008
    20,764
    2,534
    Why do you have a diode AND gate on IC2 pin3 and IC1 pin 3? If Q0 is low (0) then the gate is permanently low, and there is no signal going to IC5. BTW, with most schematics the ICs are labeled U, as in U1, U2, etc. No big deal, it is convention.

    Try drawing up a block diagram of the logic. I can't tell what you are trying to do.
     
  11. DanRilley

    Thread Starter Senior Member

    Jan 13, 2008
    107
    0
    Sorry I've since removed that AND gate that you are referring to (it was originally to make the output of the clock divider a shorter pulse, but it didn't work so I removed it). I guess I've stated that the crux of the matter is I want to trigger a 555 one-shot from a 4017 divider on EVERY count of the divider. The problem seems to be that since I'm using every count of the divider the signal never goes low to reset the 555 so it interprets it as an always ON signal. But I will attach a block diagram nevertheless since you've been quite helpful and maybe it could open up some other ideas (maybe replacing the one shot with something else).
     
  12. Wendy

    Moderator

    Mar 24, 2008
    20,764
    2,534
    You can do this, but it will require a steering diode, and and input conditioner on every output you want to trigger from.

    The steering diode to to prevent the output going low from canceling the output going high. The input conditioner (which is a capacitor) will make every pulse narrow, so the 555 can have a signal to detect.

    I suspect the signal going low and the signal going high are nulling each other, canceling each other out.

    You show 4 outputs from the 4017. Is that all?
     
  13. DanRilley

    Thread Starter Senior Member

    Jan 13, 2008
    107
    0
    Yeah I'm only using 4 outputs. 4 channel sequencer. In what configuration would the steering diode/input conditioner capacitor be placed. I am currently directing each channel through a diode after it's switch so maybe I just need to add a cap?
     
  14. Wendy

    Moderator

    Mar 24, 2008
    20,764
    2,534
    Try it. A small value. Use a diode to supress the up swing or prevent the other cap from passing the positive edge. Be sure to add a resistor to allow the capacitor to discharge.
     
Loading...