That project is of a micro transmitter of FM using bf494 of easy assembly, so that any one can obtain success in your assembly.
Operation of the circuit of the transmitter of FM with transistor bf494
The Schematic of the circuit is basic of micro transmitters of FM of low potency. Where consists of an oscillator of of high frequency. That frequency is determined by the resonant circuit L / CV. Cv is one it can be adjusted for the oscillator to cover the fm strip.
The feedback to maintain the oscillation comes from the capacitor of 4,7 pF. The audio is captured by a sensitive microphone of electrets of two terminals and coupled to the the base of the transistor bf494 through capacitor C5. That sign makes the modulation of the circuit. The antenna for the circuit is a piece of rigid thread of + / – 20 cm. Para to test the transmitter it ties a radio FM and tune in in a free strip, then adjust the trimmer with a of plastic key to capture a stronger sign, when it tunes in a stronger sign it moves away of the radio if the sign disappears it re-does the.
The components for assembly of the transmitter of FM with bf494
The transistor bf494 can be substituted by the bf495 without problems, that transistor easy of being found in the trade.
Attention with the value of the capacitors, so that there is not a position change, C1, C2, C3, C5 should be ceramic. CV is a trimmer for 3 to 30 pF. it can be ceramic or of plastic.
L is the coil, it consists of 4 you exhale of enameled thread 20 AWG, nucleus of air, use a pencil to wind the coil.
(that thread type you find in the scrap, of an old transformer or of it winds found in an old plate).
If you have a Lcmeter or Inductimeter, that coil it should present inductance of + / – 0.3µH.
Suggestion of Printed circuit board for assembly of the circuit
Printed circuit board side of the components
It lists of components for assembly of the transmitter of FM
||Two to four piles (3 to 6 volts)
||47nF (47n, 473, 0.047)
||4,7pF (4.7, 4p7, 4r7)
||100nF (104, 100n, 0.1)
||3pF – 30pF
||bf494 or equivalent *
||2k7 (red, violet, red, gold)
||8k2 (gray, red, red, gold)
||5k6 (green, blue, red, gold)
||47 (yellow, violet, black, gold|
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