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Transistor circuit what works when it shouldn't? Why?

Discussion in 'Electronic Basics' started by Tristar500, Jul 19, 2005.

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  1. Tristar500

    Tristar500 Guest

    I'm looking to build a circuit that will alert me to the location of a
    lost model airplane based on modifying a commercially available lost
    plane finder. I de-soldered the wimpy 5 VDC piezo alarm off the board
    of the commercial unit. I probed the two pads where the piezo alarm was
    powered and confirmed I get a steady on-off pulse of 5VDC with a
    one-second interval. (I did a test with a small reed relay and got
    enough current to throw the relay and close a circuit on a much more
    powerful 12VDC piezo siren (this unit has a piezo driver and just needs
    12VDC and it starts hollering!)

    I would have just stuck with this however my goal was to get all the
    goodies back into the black case the loud piezo siren. Space is tight
    so I looked to use a switching transistor instead. I hooked up the
    commercial lost plane finder so that it had power and as I was
    completing the wiring it just started working! My intention was to
    connect the emitter to the negative side of the Piezo siren/lost plane
    finder (labeled simply as "on/off 5 VDC) in the photo)

    Now I'm confused why this is working and am hesitant to start hooking
    up more wires when its working already.

    How can this circuit work with the emitter not connected to anything?

    Here's a link to the photo I took of a schematic I have drawn of the
    current working config.

    Any thoughts on this?

    Thanks, Lawrence
  2. kell

    kell Guest

    That is one cute circuit.
    The transistor is conducting through its base-collector junction, which
    is basically a diode.
    Your batteries are in series so that you actually have 17 volts
  3. ehsjr

    ehsjr Guest

    330 /
    +5------[R1]-------+------b| NPN
    | \
    -(5)-----+ 10K [R2] e|
    | | |
  4. Tristar500

    Tristar500 Guest

    Sorry, I don't understand. I checked the voltage with an analog meter
    at the 12 Volt piezo and it's 12 volts so I dont see that the piezo is
    "seeing" anything more then just the 12 volts powering it.

    Any other ideas anybody?

  5. Kitchen Man

    Kitchen Man Guest

    A previous poster is correct, in that you have simply forward-biased a
    PN junction (the B-C path), and thus have no switching control. Now,
    could you clarify that schematic? What is the 5V on-off thing you
    have drawn there? You say it's the piezo alarm, but you have a second
    box labeled "pieza siren." What is really going on there?
  6. Tristar500

    Tristar500 Guest

    The 5 volt on -off thing is a commercially available lost plane finder.
    It works well and also alerts you to low battery voltage etc. It just
    isn't all that loud. As I mentioned, I removed the piezo off the
    board of the unit and are now using the 5 volt signal that used to
    power the little piezo to switch on a much bigger noise maker.

    My intention was to use the switching transistor as a relay.

    Does this make sense?
  7. John Fields

    John Fields Guest

    Yes. Your circuit should be hooked up like this: (View in a
    non-proportional font like Courier

    | |
    [SIREN] |
    | |+
    C [12V]
    +----[1KR]---B [BAT]
    +| E |-
    [PLANE ] | |
    [FINDER] | |
    -| | |
  8. ehsjr

    ehsjr Guest

    A small current at the base of a transistor controls a larger
    current through the emitter-collector. In your diagram, a large
    current exists at the base, and there is no emitter current.
    Your transistor is acting as a diode. That is an undesireable
    situation for what you want.

    The base current of a transistor should be kept small - too much
    current through the base will burn the transistor out. There is
    nothing in your existing setup to protect the base from too
    much current.

    The diagram I drew shows the transistor properly configured
    as a switch, with a small base current (that current is limited
    by resistor R1) that controls a large emitter-collector current.

    I added some insurance in the circuit with R2. It is very
    likely not needed, but insures that the base of the transistor
    is connected to something (ground in this case) in the event
    that your switched 5V circuit leaves it "floating".


    I checked the voltage with an analog meter
  9. Tristar500

    Tristar500 Guest

    HI John, I tried again and imported your schematic into word and this
    time I could decipher. I got it wired up and it works! Brillant!

    Thanks John, and thanks everybody for helping me through this.

    Next project will be to build my own lost plane finder from scratch. I
    plan to have it start making noise once the transmitter is shut off an
    it looses signal.
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