Micro Spy 88-108 MHz

LIST OF COMPONENTS
R1 = 15 kOhm
R2 = 100 Kohm
R3 = 100 ohms
C1 = 100 nF milfeuil
C2 = 220 pF ceramic
C3 = 220 pF ceramic
C4 = 2-22 pF adjustable
C5 = 4.7 pF ceramic
T1 = BC 550 NPN
L1 = Self (see text)
M1 = Micro electret
S1 = SPDT

A simple little montage you will listen on the FM band (88-108 MHz) everything happens in its environment in a clear and precise.

OPERATION
The assembly is constructed around a type oscillator that modulates COLPITTS electret microphone.
The transistor T1 (BC550) serves as both amplifier and oscillator frequency signal HF.
The transmission frequency is adjustable with C4 (2-22 pF) which allows to calibrate the transmitter at the desired location on the FM band.


Fig.1 Wiring diagram


Fig.2 Reproduction of the printed circuit having a wide side copper

MOUNTING
Wire resistors, capacitors (excluding the adjustment will be mounted with the latest electret microphone), the inductor and the transistor.

* L1 consists of six coils of copper wire enamelled 7/10ème mm of wound on the tail of a forest of 4mm.

The inductor L1 must be conducted with particular care on pain of a malfunction of the assembly. (Inability to stall in the FM band).


Fig.3 Plot plan

The size allows mounting of your choice to be installed in a housing containing the battery or be concealed in a cigarette pack. The switch is mounted either on the front of the case or inside a packet of cigarettes for example.

COMMISSIONING
1) Connect an FM receiver on an unoccupied frequency.
2) Set the transmitter.
3) Adjust C4 so as to capture the signal on the receiver from the transmitter.
4) Once the coarse adjustment, fine tune by adjusting the settings C4 and FM receiver to have the best possible sound quality.

(The setting of the receiver can be pushed to increase the sensitivity to listen to soft sounds).

The antenna was carried out on the circuit board, thereby to avoid disturbing the oscillator.

We noted a consumption of 6.5 mA and our test allowed us to determine a scope for about a hundred meters.
Important : The adjustment will be made using a screwdriver Amagnetic (ie plastic or with the help of a toothpick size).