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Amateur (hack) needs a little guidance...

Discussion in 'Electronic Basics' started by C Logan, Jul 28, 2004.

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  1. C Logan

    C Logan Guest

    I am simply a hack at electronics but I need a little help...

    I have two cats. One is fat because it eats the other one's food. I have
    create a special sensor using simple reed sensor switches, a 9 volt and a
    piezo buzzer. There is a magnet attached to the cats collar. It works
    quite well, when the cat approaches, the magnet triggers one of the reed
    switches to close setting off the buzzer.

    The problem is that the buzzer only buzzes for a micro - second as the
    magnet passes over the reed switch. I want a simple circuit or relay that
    stays on for say 5 seconds. When the reed switch closes, it sends 9 volts
    to the piezo...isn't there a simple relay or simple basic circuit that can
    hold a relay or switch closed for 5 seconds and then open again?
  2. P.M. Groen

    P.M. Groen Guest

    Try a Schmitt-trigger circuit. Google for it.
  3. John Larkin

    John Larkin Guest

    Just hang an electrolytic capacitor across the piezo. The sound will
    fade out over a period of time; the bigger the cap, the more time.
    Piezo oscillators don't use much current.

    That's the simplest way to do it.

  4. Bob Masta

    Bob Masta Guest

    The name of the type of circuit you are looking for is a "monostable",
    more commonly known as a "one shot". A pulse of any duration
    sends the output high, and then it stays high for a time determined
    by a resistor-capacitor time constant. You can buy ICs that are
    dedicated monostables, but I usually prefer to wire up something
    simpler from a CMOS gate. Don Lancaster's "CMOS Cookbook"
    has a number of circuits, but you can probably find plenty on the
    Web via Google.

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
  5. Ian Stirling

    Ian Stirling Guest

    And leads to the reed switch welding after a few dozen applications.
  6. C Logan

    C Logan Guest

    Dear Bob or others who might help:

    First of all, the information provided by Bob was excellent, and I was
    able to find an excellent circuit, which I promptly built, and it
    works excellent. The circuit I built on the 555 is here.

    Anyhow, I replaced the switch with magnetic reed switches, and placed
    a piezo buzzer in the LED slot. Works excellent except for one thing.
    The output voltage is too low...power is supplied by a 9 volt
    battery, but it is too quiet because the circuit apparently reduces
    output voltage significantly. Any suggestions out there?

  7. First, on s.e.b, please reply at the bottom of a post, never at the
    top. This is simply a convention.

    Secondly, you can use a pass transistor to get around the
    current/voltage limiting using an external transistor, like this:

    1k |
    Output ___ |/
    From -|___|- -| NPN like 2N2222 from Radio Shack or Maplin
    555 |>

    created by Andy´s ASCII-Circuit v1.24.140803 Beta

    When the output of the 555 goes high, it'll turn on the NPN
    transistor, allowing current to flow.
  8. Is your 555 a CMOS version (LMC555, TLC555) or a bipolar version (NE555)?
    Try the latter; it can conduct quite a bit more current.
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