555 Timer Circuit Short pulse Help

Discussion in 'The Projects Forum' started by Michael Knight, Nov 5, 2015.

  1. Michael Knight

    Thread Starter Member

    Nov 5, 2015
    71
    3
    View attachment 94100

    I am working on a way to make latching DPDT rocker switches and other latching switches be able to generate a 1 second negative pulse needed for a DTMF tone request. So far I have this 555 timer circuit which works well for one rocker switch. What I'm wondering is if there is a way to make the circuit reset after the ground pulse from the DPDT rocker switch has been sent?
    Otherwise I would need a circuit like this for each DPDT rocker switch, I'm working with 5 rocker switches but would also like to implement SPST latching switches into the scenario as well so that they too could be made to generate a 1 second negative pulse for the DTMF request.
    If anyone knows a good way to do this that would be awesome :)

    DTMF_TimerCircuit_993x768.jpg
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,986
    745
    The 555 is wired as an auto triggered monostable, so it needs a negative pulse applied to pin 2, to make it work which it gets at power up,
     
  3. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,800
    1,103
    If you want any one rocker switch closure to be able to trigger the pulse even if another switch is already latched closed then you could capacitively couple the switches to pin 2 of the 555 and keep pin 1 permanently grounded.
     
    Michael Knight likes this.
  4. Michael Knight

    Thread Starter Member

    Nov 5, 2015
    71
    3
    Thanks Alec_t, that sounds like a possibility, is it possible that you could digram that out based on what I have here? That would be a great help.
     
  5. Michael Knight

    Thread Starter Member

    Nov 5, 2015
    71
    3
    Hi Alec_t,
    I'm not totally sure I follow, in my diagram I already have the DPDT rocker switch connected to the #2 pin via a capacitor. I did try connecting the # 1 pin directly to ground so it has a connection to that all the time and I did not get the results I was hoping for so I'm sure I don't follow what you mean by:
    "you could capacitively couple the switches to pin 2 of the 555 and keep pin 1 permanently grounded."
     
  6. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,986
    745
    Use a multple capacitor switch pulse on each switch like this,

    DSC_0042.JPG

    When each switch is pressed a pulse is sent to pin 2,try 100k resistor and 100nf capacitor, for starters..
     
  7. Michael Knight

    Thread Starter Member

    Nov 5, 2015
    71
    3
    Thanks Dodgydave. I'll see if this will work, this is running off a 12V btw, I guess I should have mentioned that ;)
     
  8. Michael Knight

    Thread Starter Member

    Nov 5, 2015
    71
    3
    Question? It looks like you make my rocker switches into Positive feed switches? They need to be Negative pulse as that's how I have them wired:
     
  9. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,986
    745
    Ok increase the capacitor to 1uF, how fast do you want your 555 pulse output.
     
  10. Michael Knight

    Thread Starter Member

    Nov 5, 2015
    71
    3
    I need a pulse to just simulate the pressing of a momentary button, a second or less should be more that sufficient to request the DTMF tone, but you didn't answer my question about my Rocker switches sending a negative pulse and NOT a positive pulse. You'll have to forgive me but I'm still pretty new to a lot of electronics ;)
     
  11. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,986
    745
    No the rocker switches are connected to negative supply, so they pull the capacitor down when pressed a give a negative pulse.
     
  12. Michael Knight

    Thread Starter Member

    Nov 5, 2015
    71
    3
    OK I have probably got this wrong but take a look and let me know if I am understanding your diagram correctly compared to mine.

    DTMF_TimerCircuitV2_993x768.jpg
     
  13. ScottWang

    Moderator

    Aug 23, 2012
    4,854
    767
    The files size that you attached was a little big, I was compressed them to the normal size, they are from about 760Kb compressed to about 69KB, next time before you upload the files, please compress the photos to 800x600 or 640x480, if necessary then you can compress to 1024x768, almost the photos using 800x600 image is enough.

    Too much file size will affecting the connecting speed of members get to the forums, thanks for your understanding and cooperation.
     
    Michael Knight likes this.
  14. Michael Knight

    Thread Starter Member

    Nov 5, 2015
    71
    3
    OK I thought it was getting a little confusing with the two different drawings so I tried it Exactly like you have here, I hooked the pin 3 output to an LED, and the out to device to another LED, the Device LED stays on as it should but so too does the LED from the number 3 output pin??
     
  15. Michael Knight

    Thread Starter Member

    Nov 5, 2015
    71
    3
    I also tried that other circuit you posted a link too. It works and gives a 1 second pulse but I am still unable to get it to work with multiple switches, I mean each switch will activate the pulse but if a switch is latched then the second switch when pressed is still not able to get a pulse from the timer IC.... I feel we are missing something here, perhaps the reset pin needs to be triggered somehow when each switch is turned off??
     
  16. marcf

    Member

    Dec 29, 2014
    137
    19
    Try momentary NO switches. Look like you have a hardwired /OR with toggle switches.
     
  17. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,986
    745
  18. Michael Knight

    Thread Starter Member

    Nov 5, 2015
    71
    3
  19. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,986
    745
    Ct and Rt are the timing cap,resistor for your output pulse, R1,C1 in your circuit.
     
  20. Michael Knight

    Thread Starter Member

    Nov 5, 2015
    71
    3
    "Well Fog a Monkey!"
    This one actually works!!
    I added and extra momentary button into the scenario, and I held the buttons down to simulate my latching switches and I'll be buggered it works.
    Now my guess is that for 10 switches I would need to change the value of that 100K resistor right?
    [​IMG]
     
    Dodgydave likes this.
Loading...