Looking for a count down delay circuit

Discussion in 'Analog & Mixed-Signal Design' started by Radiotechnician, Sep 1, 2016.

  1. Radiotechnician

    Thread Starter New Member

    Sep 1, 2016
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    Hello. I need a simple delay circuit. When depressing button, it needs few seconds untill the relay actives. A circuit diagram or perhaps even a cheap circuit sold on eBay. If somebody knows how this circuit is called, that would help. I've tried "delay circuit" "count down circuit" "555 timer" etc but every circuit needs only a short triggering pulse to activate. Now I need one where you must hold the switch down for certain time.
     
  2. crutschow

    Expert

    Mar 14, 2008
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    After the delay period does the relay stay activated indefinitely?
    If so, how will the relay be reset?
     
  3. AnalogKid

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    Aug 1, 2013
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    Your description of the requirements for the circuit you want are not complete. To start with, lets proclaim that by a "few seconds" you mean 4 seconds. Is this what you want:
    1. Press button - nothing happens.
    2. Release button after 3 seconds - nothing happens.
    3. Press button - nothing happens.
    4. Hold button down for over 4 seconds - at 4 seconds, the relay closes.
    5. The instant the button is released, the relay opens.

    Is this correct?
    What power is available for the circuit?
    How much power does the relay coil require?
    What is the button, a SPST switch, a SPDT switch, etc.?

    ak
     
  4. Radiotechnician

    Thread Starter New Member

    Sep 1, 2016
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    Hi. Yes, that's exactly what I need. The relay can be of very small current one because it just drives a small horn. And it isn't a problem to use a transistor to drive the relay. I can do any power needed 5, 12, 24 volts etc. Perhaps 12V is the easiest. The switch is just an ordinary momentary push button -type having probably some noise due to bad contact. We are using an old doorbell button. This is a project for my son's movie.
     
  5. AnalogKid

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    The most simple would be this:
    Use 24 Vdc source.
    Measure the DC resistance of a 12 V relay coil.
    Place a resistor of that value in series with it.
    Place a large capacitor across the relay coil.

    When you press the button, the cap will prevent the relay from closing until the cap is charged up to the relay's minimum operating voltage, typically 8-9 V for a 12 V relay, through the resistor. Very simple. but the the cap is very large. Things get better with the addition of one transistor, but now there has to be a way of discharging the cap when the button is released so the timer is ready to go without waiting for a long time.

    1. What is the minimum time available for the circuit to reset itself between presses?
    2. What is the coil resistance?

    ak
     
  6. Radiotechnician

    Thread Starter New Member

    Sep 1, 2016
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    Thanks. Good, old advice. This was my first idea too but it has not very accurate time constant. It depends on environmental temperature, component heating, capacitor behaviour, voltage stabilization etc. I need more accurate system using 555 or similar. I have been doing electronics for 50 yrs and am professional too. I have quite a huge amount of components, so there's no need to use certain ones. And I can buy new ones if needed.

    The idea: When button is depressed, movie camera starts to move and in exact time after that the horn gets off. The time must be adjustable but it is easy to make adding a pot or so.
     
  7. AnalogKid

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    Now that I know your background, about 80% of what I was going to say goes away. Sounds like you have more than enough horsepower to do this unaided.

    A simple R-C ramp followed by a comparator of some kind will work. The comparator can be 1 CMOS gate, 1/2 of an LM339, etc. However, accurate, repeatable, multi-second delays are difficult with a R-C, because of the C. And you still have to reset the cap somehow.

    Another approach is the CD4060 oscillator/divider. For any given delay, the capacitor is 8000 times smaller, and reset is instantaneous. The switch applies power to the entire circuit. After a 10 ms power on reset, the oscillator starts and the counter starts counting up to 16,384. Half way there, the high order bit goes high, and both turns on a 2N7000 MOSFET that drives the relay coil, and inhibits the counter. Release button, all power stops.

    Schematic to follow.

    ak
     
  8. Radiotechnician

    Thread Starter New Member

    Sep 1, 2016
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    Thanks. That sounds good. I was looking for much simpler but I believe it isn't possible if you want accuracy. Sorry I didn't tell my background earlier. I am Senior Electronic Maintenance Engineer and been servicing TV-studio cameras and Betacam VTRs 1975-2001 but I'm not good in designing these kind of circuits. I'm glad if you can draw the schematic for me.
     
  9. crutschow

    Expert

    Mar 14, 2008
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    Here's an LTspice simulation for a 555 circuit that, I believe, does what you want.
    The delay before the relay is energized can be adjusted by U1.
    The pulse is terminated when the PB is released.

    upload_2016-9-2_10-51-4.png
     
    Last edited: Sep 2, 2016
  10. AnalogKid

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    First pass, adjustable from approx. 1 s to 5 s. R6/D2 indicate that it is working. Add a 1N4004 diode across the relay coil.

    ak
    Button-Delay-1-c.gif
     
    absf likes this.
  11. Radiotechnician

    Thread Starter New Member

    Sep 1, 2016
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    That's quite nice idea to use Q1 there. This might work. Thanks.
     
  12. Radiotechnician

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    Sep 1, 2016
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    This looks nice too. And I already have CD4060B ICs too. Thanks.
     
  13. ian field

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    Oct 27, 2012
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    Your description pretty much suggests a monostable.
     
  14. crutschow

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    After seeing AK's circuit I realized I could simply the 555 circuit by eliminating the transistor and using the PB to control the circuit power, as shown below.
    This also has that advantage of requiring no power in the quiescent state.

    upload_2016-9-2_13-41-48.png
     
    Last edited: Sep 2, 2016
  15. AnalogKid

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    True, but your first circuit had better reset handling of the capacitor. In post #14, the cap discharges through the relay coil and D2 until the transistor drops out, based on the (unknown) power-down characteristics of the 555. An activation 1 second after the end of a previous one might have a shorter delay period caused by residual charge in C1. Gotta love counters for long delay/instant reset.

    ak
     
  16. crutschow

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    Good point.
    Below is the circuit with an added resistor to help with that discharge.
    The simulation shows the cap voltage [V(PB,trig1)] settling to 215mV in 1 second.
    That should be sufficient to discharge the cap between camera shots

    upload_2016-9-2_19-41-53.png
    upload_2016-9-2_19-42-34.png
     
    absf likes this.
  17. segtronics

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    Sep 3, 2016
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    i suggest a monstable multivibrator which will delay the triggering for some time as designed
     
  18. crutschow

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    I take it you haven't read the posts between the first one and your post. o_O
     
  19. AnalogKid

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    See posts 5, 7, 9, 10, 14, 15, and 16.

    ak
     
  20. dannyf

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    Sep 13, 2015
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    fairly easy with a 555 timer, or a logic gate.
     
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