555 delay switching on

Discussion in 'General Electronics Chat' started by rjjenkins, Oct 29, 2011.

  1. rjjenkins

    Thread Starter Member

    Apr 16, 2011
    102
    2
    Hello
    sorry for another 555 question. Monostable this time.

    I want to use it to delay the switching on of a relay.

    I've found circuits where the timing will start as soon as power is applied to whole circuit (i.e. switch on, output is low for x seconds, output goes high)

    What I want is: switch on; output is low; push a button; output remains low for x seconds; then output goes high indefinitely. Can this be done with a single 555?
     
  2. jimkeith

    Active Member

    Oct 26, 2011
    539
    99
    Good challenge--Takes only the addition of a simple optical coupler (4N37) that serves as a latch. There are numerous opto-couplers that will work, but this one is cheap and readily available.

    When power is applied, both 555 inputs are high, so the output is low and remains low indefinitely because the capacitor cannot charge. The 2.2M resistor keeps it from charging via possible leakage current.

    When the switch is closed momentarily (or permanently), the LED in the opto-coupler turns on which turns on the photo transistor which completes the circuit for the 1M charging resistor and latches on the opto-coupler permanently. The capacitor now starts charging in the negative direction from the +12V bus until it reaches the trigger point (4V) of the 555 at which time the 555 output goes high and remains high because there is no reset path via pin 7. Timing roughly = RC or about 1sec.

    Very, very tricky as many 555 circuits are.

    Now, I did not test this circuit--hopefully it will work as planned--let me know how it turns out--happy tinkering!
     
  3. jimkeith

    Active Member

    Oct 26, 2011
    539
    99
    Woops, the 2.2M resistor is unnecessary as the 10K resistor performs the same function through the opto-coupler.

    Also, the opto-coupler must have a transfer ratio that exceeds 100% or else it cannot latch. The min for the 4N37 is 100%. If it does not latch, try reducing the 10K resistor to 2.7K--that will increase the LED current to about 3.5MA which falls on the maxima of the transfer ratio curve.

    Drat--wasted more cyberspace again!
     
    Last edited: Oct 29, 2011
  4. rjjenkins

    Thread Starter Member

    Apr 16, 2011
    102
    2
    Thanks - I can't get my hands on a 4N37 right now but would a 4N25 do it?
    Also, what are the determinants of the delay? I probably want about 60 seconds
     
  5. jimkeith

    Active Member

    Oct 26, 2011
    539
    99
    The 4N25 is not likely to work as its minimum CTR is 20%. Now it could be much higher than 20% as the upper limit is open ended.

    If it does not work, add a transistor as shown to turn it into a darlington.
    I indicated a 2N3904, but almost any NPN will do.

    To get 60sec, increase the cap to 47 or 63uf.
     
    Last edited: Oct 29, 2011
  6. rjjenkins

    Thread Starter Member

    Apr 16, 2011
    102
    2
  7. jimkeith

    Active Member

    Oct 26, 2011
    539
    99
    Try tying pins 2 & 6 together--I had 2nd thoughts about this detail after I did the thumbnail. Also, in case the opto is not latching, try holding the switch closed for the timeout period to see what happens.

    Boo--BC327 /BC557 transistors are PNP--will not work--we need NPN

    We will get it working--if you get stonewalled, I will breadboard it myself too to demonstrate (or not) that it functions.
     
    Last edited: Oct 29, 2011
  8. rjjenkins

    Thread Starter Member

    Apr 16, 2011
    102
    2
    Tying pins 2-6 together (on the other circuit) causes it to oscillate. Since I can't get hold of an acceptable opto-isolater this weekend I am going to try your idea but with a latching DPST relay instead.
     
  9. jimkeith

    Active Member

    Oct 26, 2011
    539
    99
    Last edited: Oct 30, 2011
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