1. Jean Claude Abeille

    Thread Starter New Member

    Oct 26, 2015
    3
    0
    Hi



    I wanted a circuit with a timer controlling an output preamp relay. A LED should blinking meanwhile and stay ON when the relay

    get closed.

    I designed this one with RL3 reseting U4. Q41 also starts RL2 the output relay.

    It works but after a while, D7 restart blinking. Maybe there is not enough current for 2 relays.

    How can I fix this ?

    Mainly, is there a more clever way to achieve the goal ?
     
  2. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    2,503
    380
    hi J,
    The Reset pin 4 of the 555 should not be 'floating', connect to +V via a resistor, say 4k7.

    E

    EDIT:
    If the only purpose of the 2nd relay is to Reset the 555. this option should work, see image.
     
    Last edited: Jun 4, 2016
  3. AlbertHall

    Well-Known Member

    Jun 4, 2014
    1,941
    383
    A lot depends on the resistance of the relay coils. The current you might expect from Q41 is around 35mA which isn't all that much. Probably the easiest way of reducing the needed current is to eliminate RL3. Connect the 555 reset direct to Q41 collector.

    I agree with ericgibbs that the 555 reset shouldn't be left floating but that won't cause the symptoms you describe as, once the delay time has elapsed RL3 should hold that reset grounded.
     
  4. Jean Claude Abeille

    Thread Starter New Member

    Oct 26, 2015
    3
    0
    I use 90Ω coil relays. So about 120mA on each one.
    The only purpose of RL3 is to ground the reset pin at the end of countdown.
    So I will discard it and replace it by a connection to Q41 collector, directly or with a resistor.
     
  5. AlbertHall

    Well-Known Member

    Jun 4, 2014
    1,941
    383
    Blimey, that's really stretching Q41. You might need to add another transistor between the 1.7M resistor and Q41 base.
     
  6. Jean Claude Abeille

    Thread Starter New Member

    Oct 26, 2015
    3
    0
    some news.
    I tried your solution, Eric and Albert, but it does not work the way I expect.
    Effectively, U4 is reset.
    I didn't precise that I want the LED to stay ON after countdown.
    With reseting by connecting to Q41 collector, with or without resistor, LED stays OFF.
    Voltage at reset pin decreases progressively, too slowly.
    Damn !
     
  7. AlbertHall

    Well-Known Member

    Jun 4, 2014
    1,941
    383
    You can fix the slow switch of Q41 and the possible current availability of Q41 like this. R1 and R2 provide positive feedback to make the switch-over fast. T2 also amplifies the base current of Q41. T2 can be any small signal PNP transistor. I think this will also extend the time delay so you might need to reduce either R61 or C49.

    upload_2016-6-5_11-42-20.png
     
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