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an old 27 Mhz transmitter - help!

Discussion in 'Electronic Basics' started by andrew_h, Feb 9, 2006.

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  1. andrew_h

    andrew_h Guest


    Back in the late 70's or so, my late father made a remote which, when
    the antenna was pulled out and button pressed, would switch on two
    mains lights in the house. The idea was to give it to mum, so when she
    came back in the night she could switch the lights on.

    I am new to electronics and am learning more each day (did Physics but
    didnt pay as much attention as I should have).

    Opening the remote, I see a 27.445 Mhz crystal - along with basically a
    heap of resistors, 4 transistors, an inductor and some ceramic caps.
    (powered from 9V battery).

    My question is - what would the most-likely role be of the 4
    transistors?? Would they amplify the signal that the crystal outputs???
    Also, I've noticed that with at least two of the transistors, one
    collector connects to another on the second transistor. Would that be
    to amplify the current greatly?? (sort of like a darlington pair ?)??.

    If more info is needed pleast let me know - would appreciate any help
    on the subject. Thanks! Andrew
  2. Crystals don't output a signal, they filter a signal. That is, they
    are very narrow band pass filters. When you connect one in a feedback
    loop around enough gain, the loop has enough gain to sustain an echo
    (oscillation) only in the narrow band of frequency that passes through
    the crystal. One or more transistors may provide the gain for this
    loop. The others may be additional power gain between the oscillator
    and the antenna.

    The two connected collectors may be part of a push pull complementary
    output stage. Are you sure it is two collectors that are connected?
  3. Ron Hubbard

    Ron Hubbard Guest

    I took Electronics way back in high school although I was interested in
    it ever since I read Heinlein's book, Have Space Suit Will Travel, way
    back in the 6th grade. :)

  4. Crystals do "output a signal". The crystal, in conjunction with an active
    component such as a transistor in this remote is used in an oscillator
    circuit to create the 27.445 MHz signal used to communicate with the
    receiver. Crystals in receivers are typically used to create a reference
    frequency which is used to down-convert the received signal for
    amplification by an IF (intermediate frequency) amplifier stage or stages.

    Types of "crystals" known as resonators (typically ceramic devices) are used
    to filter signals.

    Transistors in transmitters are used in oscillators to create transmitted
    frequencies, boost this level of the transmitted frequency, modulate the
    transmitted frequency, create the modulating signal and other tasks possibly
    needed in such a device.

  5. That is what I said in different words. A crystal, by itself, has no
    output, but in conjunction with a gain element and feedback, can
    become an oscillator, with the crystal being the frequency determining
    element (filter).
    All crystals act as resonators, whether or not they are called
    Sounds like you are agreeing with me.
  6. Andrew Holme

    Andrew Holme Guest

    The purpose of the transistors could be as follows: one as a 27 MHz crystal
    oscillator; two could be an audio frequency multivibrator; and the fourth
    would then be a modulator - switching the oscillator on and off at audio
    frequency. If there is only one inductor in the whole circuit, this
    configuration seems likely. Designs like this were published in 1970s hobby
  7. Tom Biasi

    Tom Biasi Guest

    Hard to say what your father did.
    Back then we often took a CB unit and transmitted a tone to a PLL receiver
    to activate something. Sometimes to control lights and sometimes to control
    something that went BOOM.
    But I'll say no more, I don't know abot statue of limitations:)
  8. andrew_h

    andrew_h Guest

    I will take a picture of the circuit board of the remote and post it.

    I appreciate people's help and time on this - if someone could even go
    through it and attempt to work out exactly how it worked, I would be
    greatly in debt!

    For my own sake, I'd love to know exactly how it worked.
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