Telephone Spy & FM transmitter

Discussion in 'The Projects Forum' started by baba16, Jun 17, 2012.

  1. baba16

    Thread Starter New Member

    May 17, 2012
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    I made this circuit with the following configuration but it is not working.This is the way I implemented the circuit..Can some on please tell me what mistake I have done in my circuit..

    Bridge is in parallel with 200 Ohm, A933 and Coil are in series with 200 ohm and Coil is in series with 12 k ohm. 12 pF and 25 pF are in series. 330 pF and 470pF are in series LED and 1 k Ohm are connected in series with each other and are in parallel to 200 Ohm. IN2 & OUT2 are connected in the same vertical column of breadboard which are all connected vertically. IN1 & OUT2 are connected as shown in figure.[​IMG]
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    The stray capacitance of a breadboard is much too high for this circuit that operates at the veryu high frequency of 100MHz. Use a compact pcb layout instead.
     
  3. baba16

    Thread Starter New Member

    May 17, 2012
    6
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    What do you mean by compact PCB? How can be the capacitance too high the website from which I took this circuit says that these specs are all OK and people are saying that the circuit is working for them.

    I just made following 2 changes in the circuit, added C557B TRANSISTOR instead of A933 and used 4007 diode instead of 4384 diode. as they are short in the market
     
  4. wayneh

    Expert

    Sep 9, 2010
    12,089
    3,027
    Capacitance (and inductance) does not come from just the components, but from the wire leads, the breadboard, everything. You cannot operate at that frequency without designing for those parasitic effects.
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    A breadboard works for DC and low frequencies. Its capacitance between its rows of contacts is too high for high radio frequencies and each connecting wire has too much series inductance and picks up interference from strong radio stations. A pcb should be used with its parts close together (compact).

    I made my FM transmitter on stripboard with all the strips of copper cut as short as is possible and the parts close together and it works perfectly.
     
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Your written description does not match the schematic.
     
  7. absf

    Senior Member

    Dec 29, 2010
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    Is this how you wire up your circuit?:D
     
  8. Audioguru

    New Member

    Dec 20, 2007
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    100MHz does not work when the wiring is messy.
    This FM transmitter also did not work because its parts are too far apart:
     
  9. #12

    Expert

    Nov 30, 2010
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    Post #7 is a perfect example of what won't work at 100MHz.
     
  10. luckydat

    New Member

    Apr 13, 2012
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    Thanks !It is a great sample for the spy tools
     
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