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simple one-shot pulse detector circuit needed

Discussion in 'Electronic Basics' started by [email protected], Nov 20, 2012.

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  1. Guest

    Hi all,

    I seem to recall from distant memory that it's possible to make a pulse detector out of two of transistors and a few resistors, but I can't find the circuit anywhere. Searches keep throwing up stuff about astable multivibrators and nand gates and that's not what I'm looking for.

    Just need to be able to detect a 1 second pulse of 5v on an otherwise 0v line. The pulse comes only after several days of inactivity and should turn an LED on permanently. No reset required and any subsequent pulses must *not* turn the LED off again, so it's operating like a bistable latch (if that's the right term).

    Can anyone assist?

  2. I assume there is another power line to run it from (+5 V). I don't
    know the transistor circuit, but how about a S/R flip flop?

    George H.
  3. An SCR would do it, once triggered it only turns off if it loses power on
    the anode.

    You can get the effect of an SCR with two transistors, at least for this
    purpose. I can't remember, but doing a search for something like that
    should do it.

  4. Nice... but I had to reduce C1 from 47 Farads to 47nF :^)

    George H.
  5. mike

    mike Guest

    The 1 second requirement significantly complicates the solution.
    If any old pulse will do, use a latching relay...either magnetic latch
    or feedback from the contacts.

    As always, the devil is in the details.
    How much current can you get out of the 5V pulse?
    What voltages are available when the pulse is not present?
    How do you expect to reset the thing?
    What are the specific details of the 1 second requirement?
    Timing accuracy and boundary conditions?
    Assume you wouldn't have mentioned it if it weren't important.
    magnet/reed switch from a burglar alarm door/window ajar sensor.
  6. ... or just a resistor (eg. 4K7) to drive the gate of a 2N5064, in turn
    drive an LED with series resistor (4 parts). It will detect a positive
    pulse of any length greater than a couple microseconds by latching on,
    and thence cease to care about the state of the input, so the LED will
    stay on until the power is cycled.
    Should be battery powered sensors with RF interface available for
  7. mike

    mike Guest

    I missed that part of the specification.
    Probably want three or more magnetic sensors physically placed
    and logically connected to insure that it can't be spoofed by
    a magnet without intimate knowledge of the design.
    Encase the bullet-resistant box in concrete.
    Probably want to put a signal into the razor wire so you can use
    it as a proximity sensor. Maybe a satellite phone to call the
    cops when the phone lines get cut. And a bluetooth
    connection to the alarm system with an encrypted heartbeat
    so it can't be jammed.
    Probably want at least one guard dog with a wireless heart rate
    sensor just in case they get put to sleep.
    Of course, destroy all documentation and eliminate everybody
    you ever discussed it with. Crush the hard drive.
    Grind it up into dust and scatter it at least 10 miles off shore.

    If security isn't the primary requirement, there are lots of devices.
    The Powerhouse MS13A is a X-10 wireless PIR motion detector.
    You could hack a relay into that and get motion plus gate detection.
    They have other alarm sensors, but I don't know the numbers.
    There are devices designed to alert you when your mailbox door is opened.
    Driveway occupancy sensors.
    Wireless doorbells.
    Garage door openers.
    RF TV remotes.

    And my personal favorite...close the damn gate. ;-)
  8. There are waterproof inductive proximity sensors with all the
    circuitry in a little tube.
  9. phaedrus

    phaedrus Guest

    Can't get this to run for some reason, John. This is a LT spice
    netlist? Just get the following error message: "multiple instances of
    SYMATTR" Since someone else on this thread seems to have got it going
    (albeit with a slight mod to a cap value) I assume I'm doing something
    wrong (not an unknown experience).
  10. phaedrus

    phaedrus Guest

    Yes, I'm aware a SCR would be ideal, but I only have great fat
    thumping big ones lying around which would be a waste for such a small
    signal application.

    Malvino's book Electronic Principles shows the kind of thing I'm
    talking about, I've just discovered, at page 501. Vanishingly terse
    explanation with no reference to it in the index or contents for some
    reason. Uses a BJT PNP/NPN pair with bases and collectors shorted
    together between them and one resistor in the Vcc line. I reckon this
    should do the trick with any luck.
  11. phaedrus

    phaedrus Guest

    Here's the netlist of the Malvino design. Seems to run alright...

    "ExpressPCB Netlist"
    "LTspice IV Version 4.17"
    "Part IDs Table"
    "Q1" "2N3904" ""
    "Q2" "2N3906" ""
    "V1" "12" ""
    "V2" "PULSE(0 5 1 1n 1n 1 1 1)" ""
    "R_LOAD" "1200" ""

    "Net Names Table"
    "N003" 1
    "N004" 3
    "0" 6
    "N002" 9
    "N001" 11

    "Net Connections Table"
    1 1 1 2
    1 2 2 0
    2 1 2 4
    2 2 1 5
    2 4 1 0
    3 1 3 7
    3 3 2 8
    3 4 2 0
    4 2 3 10
    4 5 2 0
    5 3 1 12
    5 5 1 0
  12. Hmm, I just cut and pasted the whole thing into notepad... or pick
    some other text editor.
    Then save it with a .asc ending, into my ltspice/bin and run it.

    George H.
  13. Jasen Betts

    Jasen Betts Guest

    a reed switch like used for alarms...
  14. Jasen Betts

    Jasen Betts Guest

    just make an SCR out of your two transistors.

    one resistor on the gate input and a series resistor to limit the LED
  15. phaedrus

    phaedrus Guest

    I gathered that, Jim. Good suggestion on your part; I just can't get
    that link to bring up any results for some reason. :(
  16. phaedrus

    phaedrus Guest

    I must be having a really bad day, George. Ain't got no such folder on
    my system!
  17. mike

    mike Guest

    You keep adding requirements to the spec.
    Often, it's useful to monitor the thing you want monitored.
    Monitoring attendance is no guarantee that you actually get work.
    Stated another way:
    If you can't tell if the work was done, you're paying for work
    you don't need.
  18. phaedrus

    phaedrus Guest

    Finally! Thank you, John. Don't know what the problem is this end, but
    your list combine with my version of LTS generated a corrupted
    schematic with several components rotated and out of position!
    Anyway, I was able to reconstruct the corrupted diagram and it now
    works fine. Thanks again!
  19. phaedrus

    phaedrus Guest

    Many thanks.
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