Delayed single pulse shot

Discussion in 'The Projects Forum' started by Shurik407, Jan 21, 2015.

  1. Shurik407

    Thread Starter New Member

    Jan 21, 2015
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    Hi,

    I am new to this forum is fairly new to electronics. College 15 years ago was about the last time I delved into it!

    Basically, I need a circuit to generate a delayed button press upon its power source being applied. Once the said circuit is powered on (power between 13V and 20V), there should be a delay of around 1 second, followed by a pulse lasting around 1 second, after which the circuit should remain idle until its next power on.

    I've found something like this:

    [​IMG]

    This seems to do what I want, however, there's no start-up delay here. Also, the pulse generated will have a slow falling edge (depending on capacitor used) - I don't know how it will affect the "button press" but I would prefer it to be digital type pulse, i.e. fast rising and fast falling edges.

    This could probably be achieved with some transistors or perhaps a 555 timer... but no really sure where to start :(
     
  2. djsfantasi

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    Your image doesn't appear. If you're having difficulty, this blog may help.
     
  3. Shurik407

    Thread Starter New Member

    Jan 21, 2015
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    I apologise, I am not sure what happened. Please see it attached (hopefully).

    CB_PwrUpPulseCircuit.gif
     
  4. Alec_t

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    Sep 17, 2013
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    What do you intend to control with the circuit output?
     
  5. Shurik407

    Thread Starter New Member

    Jan 21, 2015
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    An on/off button :D

    Basically.. I have a satellite receiver, which is connected, via an antenna cable, to the satellite dish positioner. This positioner is automatically switched on when the satellite receiver is on. It knows when the receiver is on, because the antenna cable will have 13 or 18v on it - it senses this and switches on.

    For weird reasons, I need the positioner device to remain off and this is the idea I came up with. The circuit that I am asking help with will simulate a button press to turn off the positioner.
     
  6. Alec_t

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    Sep 17, 2013
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    So, just to confirm, you want a circuit which, on receipt of 13 or 18V, provides a 1 sec duration output pulse of the same voltage but delayed by 1 sec? I don't see how that will achieve "I need the positioner device to remain off". Surely it will just turn the positioner on 1 sec later than normal? Can't you simply disconnect the positioner from the receiver?
     
  7. Shurik407

    Thread Starter New Member

    Jan 21, 2015
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    I can't disconnect the positioner from the satellite receiver because it needs to receive commands from it - via the same cable. It accepts commands and drives the dish even in so-called standby mode. The reason I need it off is because I have two of the same positioners and both respond to the same remote control. If the positioner is in standby then it ignores the remote control commands...

    So, to explain - currently the chain of events is thus, with the receiver and positioner both in standby.

    1) Receiver switches on and sends 13/18v down the antenna cable
    2) Positioner senses 13/18v and comes out of standby

    I want to add step 3) with my circuit:

    3) Wait 1 second and send a pulse to the power button to send the positioner back into standby mode.

    Anyway... Maybe the pulse is the wrong thing I need. With the pulse, I could trigger a relay/transistor of some sort to "short" the power button for half a second-ish. Maybe there's a better way to "push" the button?
     
  8. Alec_t

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    Sep 17, 2013
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    Ok. The circuit will need to provide two 1-sec delays and could drive a reed relay to short the power button. Is the 13/18V signal available continuously while the receiver is on, or do the commands interrupt it? If continuously, or any interruptions were sufficiently brief, it could be used to power the circuit. If not, what alternative power source do you envisage using?
     
  9. Shurik407

    Thread Starter New Member

    Jan 21, 2015
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    It is generally continuous - the commands sent do not interrupt the power supply. I would actually prefer the circuit to have its own source of power (I have a steady 5V source available from the positioner itself, through the 7805 regulator) with the 13/18v line from the receiver being the trigger for the circuit. For the sake of simplicity and my general newbie-ness, I wasn't thinking of the external power source - but if you have advice/ideas on this, then I am all ears :)
     
  10. Alec_t

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    Sep 17, 2013
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    I think this should do what you want :-
    DelayedRelayPulser.gif
     
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  11. Shurik407

    Thread Starter New Member

    Jan 21, 2015
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    This is brilliant, Alec! A quick one: will the relay be retriggered when Vreceiver voltage is reapplied?
     
  12. Alec_t

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    Yes, provided the 5V supply is present when Vreceiver is re-applied.
    A light duty relay (reed type or otherwise) shouldn't need much coil current, so the FET doesn't need to be a beefy one (but it has to be a logic-level type) and R5 may not be necessary.
    Bear in mind the CD4093 is static-sensitive.
     
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  13. Shurik407

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    Jan 21, 2015
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    Excellent! Many, many thanks for this... Need to try and figure out how it actually works - the logic of it.
     
  14. Alec_t

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    How it works :-
    When Vreceiver is applied, C2 takes ~ 1 sec to charge via R2 to the switching threshold of U1a. U1a output goes low, triggering a 1 sec pulse from a monostable (one-shot) consisting of U1d and U1b. The monostable period is set by C3 and R3. U1c inverts and buffers the U1b output to drive the FET gate. R4 limits the current to which the built-in input diodes of U1b are subject when C3 is driven high or low by U1d output.
     
  15. Shurik407

    Thread Starter New Member

    Jan 21, 2015
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    Thank you Alec for the great explanation. It makes sense.

    Now let's hope I don't blow the house up when this is built!
     
  16. Alec_t

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    Let's hope so! In particular, let's hope the positioner doesn't mind having its power switch shorted for 1 sec, and its 5V output is enough to provide the relay coil current.
     
  17. Shurik407

    Thread Starter New Member

    Jan 21, 2015
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    I am hopeful. The power supply is fairly beefy at 7.5A, so it should handle it... I can always play around the C3 and R3 values to adjust timings.

    Just out of interest - what program did you use to sketch and simulate this circuit?
     
  18. Alec_t

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    Sep 17, 2013
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    LTspice (as used by many members here). A free download from Linear Technology. Steep learning curve, but well worth the effort. (A study of the 'Help' is recommended). Yahoo has an LTS User Group for advice and free downloads of libraries of many additional third-party Spice models.
     
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