Electronic solution to mimick this switching??

Discussion in 'The Projects Forum' started by triggernum5, Feb 3, 2009.

  1. triggernum5

    Thread Starter Active Member

    May 4, 2008
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    How can I mimick a momentary pressing of a normally closed dead-man's switch on a permanent change of signal from low to high without triggering a 555 oneshot or something that complex??
    A transistor won't work, because I need full collector-emmitter current both before base voltage change (when low), and after the base voltage change (permanently high after this transition)..
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    I assume when you activate the switch it is open? You said normally closed.

    How about a FET circuit?
     
  3. triggernum5

    Thread Starter Active Member

    May 4, 2008
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    I gotta read up on FETs I guess.. This is related to that timer.. I posted the original plans that use physical switches to manually reset the counters at desired time on first cycle..
    I'm backtracking away from that logic butchering where I was tieing the outputs of those additional counters into the reset pins of the daylength counters..
    I know I can do it if I put a monostable 555 in between the output of each of the additional counters, and put pnp transistors in place of the pushswitches from the original desgin.. The 555 could sink the pnp base current both before the additional counters trigger its pulse, and after the pulse expires.. Just seems like too much for the task doesn't it??
     
  4. triggernum5

    Thread Starter Active Member

    May 4, 2008
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    Here is what I mean.. Use a capacitor in series with the outputs of ICs 6&7 to trigger the 555 to momentarily cut the base current..
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    The reason I sugested the FET is it is a high impedance device, and should respond easily to a 1MΩ resistance.

    Looking at it, I'm not clear where you're going with the 555. I'm not looking too deeply at this schematic, just for things that might let smoke out.
     
  6. triggernum5

    Thread Starter Active Member

    May 4, 2008
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    Basically the 555 just kills the base current from the pnp's briefly while they are outputting high, 555 output pin can sink the base current allowing the pnp to conduct at all other times.. So when IC 6 or 7 finish their set count, they no longer sink the 9V coming through the 100k resistor, that 9V can then charge a series cap which will cause a low on the 555 trigger, and it will output high for just long enough to stop the base current sinking, mimicking opening the pushbutton until the 555 goes low and sinks the base current again....
     
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