Help with "non retriggerable monostable" trigger time issues

Discussion in 'The Projects Forum' started by Squidtone, Jan 30, 2009.

  1. Squidtone

    Thread Starter New Member

    Jan 29, 2009
    3
    0
    Hello, I am looking for how best to construct what I think may be called a "non retriggerable monostable".

    The situation:
    I want to remotely trip shutters on a set of 80's era SLR film cameras. These feature electromagnetic shutters, which need only be shorted to ground. I've already made custom electronic switch cable releases that work nicely.
    For remotely switching, I believe a small relay will do the trick as far as completing a circuit to ground on the shutter releases, but the trick now is how to remotely actuate the relay.
    So, for the "remote" actuation, I purchased an RF doorbell set and took apart the receiver. (Coincidentally, after I bought the doorbell kit, I found a nice write up on using this very RF doorbell rig from John at www.hackersbench.com.) The receiver outputs a signal of about 3 seconds duration to a transistor to which a speaker is attached.

    Okay, connecting the transistor output of the doorbell receiver to a relay might work, but I want more control over the time duration. So I figure a 555 timer circuit in a "oneshot" mode would work. I would be able to change the R/C network to be able to have an output time duration of anywhere from 0.1s to 30s.

    The doorbell receiver is powered by two C cells (3V), and I would like to run the 555 timer circuit and relay off the same battery source. So, I purchased some low power (CMOS I guess?) TS555IN timer chips. I made a test circuit as seen by Tony van Roon here: http://www.uoguelph.ca/~antoon/circ/pulsetimer.html

    It works fine, and I can tailor my output time duration by changing the resistor/capacitor network.

    I then substituted transistor Q1 of Tony van Roon's circuit with the output transistor of the doorbell circuit in my test 555 timer breadboard. The RF doorbell receiver output indeed triggers my timer circuit, however, the duration of the R/C network in the timer circuit is apparently being overridden by the long duration trigger from the doorbell circuit. How do I make the output "unresponsive to long duration or repeated input triggers".

    The allaboutcircuits chapter about monostable multivibrators says it's possible, but I don't know how.

    One more issue: how do I get the timer circuit to NOT do an output when I apply power to the circuit?

    Hopefully I've made myself clear. Any help is appreciated! Thanks!
    Dave
     
  2. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    This one is easy. Do not make direct connection to the 555 but use a capacitor of 0.1uF for isolation instead. Just use a 0.1uF capacitor and connects pin#2 of 555 to the collector of the doorbell transistor via the capacitor. Doing so the capacitor then only couples the voltage changes to the 555 and the 555 will time out regardless of the status of the door bell.

    This one is more difficult. When you power up, the circuit need time to stabilize and the time needed would be anything from about half a second or more. During this time, unexpected output will result.

    An easy way out is to use a power switch with several positions and only connect the "Load" to it after the circuit stabilized. Something like "Off-Start-ON" and relies on moving through the positions manually during switching on sequence.

    A more elegance solution is to electronically block/remove the load connection for a few seconds upon power ON but it is a bit more involve.
     
  3. Squidtone

    Thread Starter New Member

    Jan 29, 2009
    3
    0
    eblc1388,
    Thanks for the reply. I have tried what you suggested by using a 0.1uf cap between the collector of the doorbell circuit transistor and pin 2 of my 555 timer. It did not even trigger the timer.
    Then I started to wonder if I should keep the 10K resistor (R2 in the circuit I referenced http://www.uoguelph.ca/~antoon/circ/pulsetimer.html). So I tried putting that on my breadboard and it still did not work.
    So I'm still stuck. Any other ideas?
    Thanks for your help.
    Dave
     
  4. triggernum5

    Active Member

    May 4, 2008
    216
    0
    A diode might help if whats happening is the output pinof the 555 is sinking the transistor pulse while in its low state..
     
  5. Squidtone

    Thread Starter New Member

    Jan 29, 2009
    3
    0
    In the interest of closure, I'm posting a solution to my problem. I tried the suggestions offered here, but I found a working solution elsewhere. I'll bet there are other ways also, but this solution worked for me because I already had some low power LM555 IC's.

    To recap my goal: I wanted to use a cheap wireless doorbell to trigger a relay. The doorbell transistor signal (trigger) was about 4 seconds, I wanted to set my own shorter timer duration for my relay (test LED in the picture).

    As you can see, the solution is a two stage timer circuit. The designer said the first timer stage should exceed the doorbell output. The output of the first stage triggers the second stage which is not influenced by the duration of the first stage.

    Not sure why, but it works! (remember, I don't know this stuff)

    Anyway, thanks for the help.
    [​IMG]
     
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