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 att wireless communication

 Circuit Description : 

   The circuit is basically a radio frequency (RF) oscillator that operates around 100 MHz (100 million cycles per second). Audio picked up and amplified by the electret microphone is fed into the audio amplifier stage built around the first transistor. Output from the collector is fed into the base of the second transistor where it modulates  the resonant frequency of the tank circuit (the coil built into the circuit board and the trimcap) by varying the junction capacitance of the transistor.  Junction capacitance is a function of the potential difference applied to the base of the transistor. For a  FULL DETAILED technical description of the operation of two stage FM Tx like this one see:

                                                         COMPONENTS

Resistors (carbon, 0.25W, 5%):
100R (brown, black,brown)                                  R5                            1
1K   (brown, black, red)                                        R4                            1
12K   (brown, red, orange)                                   R1 R2                       2
2M2   (red, red, green)                                         R3                            1
BC338 transistor                                                  T2                             1
BC548 transistor                                                  T1                             1
Electret Microphone                                              MIC                          1
9V Battery snap                                                                                    1
10pF ceramic capacitor                                         C3 C4                       2
4n7 ceramic capacitor                                           C1 C2                       2
22n 223 ceramic                                                   C6                            1
trimmer capacitor 2-30pF                                     C5                            1
K18 or K128 PCB (both are the same)                                                 1
Hookup wire for aerial                                         160cm

L1 Inductor                                                         8 turns ; 6mm dia ; 0.5mm wire 

WHAT TO DO IF IT DOES NOT WORK:

Poor soldering is the most likely reason that the circuit
does not work. Check all solder joints carefully under a
good light. Next check that all components are in their
correct position on the PCB. Thirdly, follow the track with a
voltmeter to check the potential differences at various parts
of the circuit particularly across the base, collector and
emitter of the two transistors.
A check list of other items:

att wireless communication
              ·  are the transistors in the correct way and correct
places.
              ·  is the battery flat. did you add the LINK
              ·  Check that the following collector-emitter voltages are
present; 2V across the 548, 5V across the 338.
              ·  If you hear an oscillation or 'putt-putt' at all
frequencies then it is possible the unit is in oscillation
due to the load resistor on the microphone being too
low. Increase it to say 22K or 47K. This should
overcome the problem.

The simple halfwave antennae used in the kit is not the
most efficient. Greater efficiency may be gained by
connecting a dipole antennae using 50 ohm coaxial cable.
Connect one lead to the Antenna point and the other to the
earth line. In experiments using this type of aerial and 18V

supply we have got transmitting distances of 2 miles in the
open. You may experiment with higher voltages to see how
this increases the range. Adding a small capacitor in

parallel with the trim capacitor (eg, 10pF) will shift the

tuning range of the trim cap.

 If you face any problem than you can contact with me: This e-mail address is being protected from spambots. You need JavaScript enabled to view it