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DIY Bug - FM Audio Transmitter [Micro spy]

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Microspy FM audio transmitter for Hobby use

Micro spy [FM audio transmitter for Hobby use]

This Microspy transmits on the 160MHz frequency (if we use a 40MHz quartz) and therefore can be listened through a receiver tuned on this frequency. This circuit can be used to on various frequencies, for example on the FM band 88-108 just modifying some components, among which the quartz (25MHz)

Voice is detected by an electret microphone, then it is amplified and filtered by U1 pass-band in order then to be modulated from the carrier section, where through the varicap diode it "mixes" with the frequency generated by the quartz, that guarantees an adapted stability. Practically the modulating voltage is obtained applying the audio signal to the resonating circuit varicap diode that determines the carrier oscillation. The carrier frequency (160 MHz) must be greater than the modulating frequency (300-3300 Hz) audio band.
The transmission is on the fourth harmonic, therefore 160MHz, the oscillation frequency of the driver RF transistor Q1.
A small calibration is allowed acting on the L1 inductance and the C1 Trimmer Capacitor.
Whoever wants to elaborate on the overtone modality can leave a comment...
microspia.pngBug Components list

Item Quantity Reference Part
1 1 ANT1 ANTENNA 44.5 cm
2 1 C1 (2-7) pF
3 1 C2 10pF
4 2 C7,C3 100nF
5 2 C4,C8 1000nF
6 1 C5 1uF
7 1 C6 100pF
8 1 C9 1pF
9 1 D1 BB121A- BB405
10 1 JP1 HEADER 2
11 1 L1 five 0.1 uH, 4mm enamel wire windings
12 1 Q1 BFR96
13 1 R1 10M
14 5 R2,R3,R5,R7,R8 100K
15 2 R4,R6 10K
16 1 R9 150
17 1 R10 100
18 1 U1 TL061
19 1 X1 WM034DM
20 1 Y1 38.860 - 40MHz

Printed circuit PCB

The circuit can be mounted with traditional components that can be easily found. To obtain the best performances from this circuits find by your self the right numbers of windings for the output coil, matching it with adjustments on C1.

The antenna can be realized with a fragment of thread (of 0,22mm) of a 1/4 length of wave therefore:

Antenna length = (light speed (m/s)/ frequency(hz) ) /4 = (300000000 / 160000000) /4= 0,468 meters= 46,8cm

(to consider also a propagation factor on the antenna of 0,95)

The antenna will be therefore long 44,5 cm

Component Serigraphy (SST)


Solder Side (BOT)

microspia_bot.pngPrinted circuit miniaturized version (for SMD components)

Component Side (TOP)


Solder Side (BOT)


Component Serigraphy (SST)

bug_sst.pngDownload the complete PDF documentation --->> download

Buy the file GERBER Buy the file GERBER for the realization of the Printed Circuit (PCB) --->> microspia_gerber

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