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This setup is a small transmitter, working in the FM band, which will allow you to hear what is happening in another room, for example to monitor children, to hear a crying baby. It will be associated with a frequency modulation receiver which will preferably have a squelch system, that is to say an automatic cut-off of the sound in the absence of a signal.

Left: this is the acoustic treatment part. A 5 V regulator circuit powers the microphone, polarizes the first amplifier and provides a fixed voltage for setting the trip threshold. The audio signal is transmitted to CI2 amplified, it is then detected by D1 and D2; R5 delays the load in order to avoid triggering by too short a noise. In the presence of an AF signal, C5 will be charged and, as soon as the voltage reaches the threshold set by P1, the output of Chb goes to zero. The transistor T1 conducts, the oscillator T2 oscillates, it receives on its base the output voltage of the amp, this audio voltage then frequency modulates the RF carrier. The frequency modulated signal is transmitted to the antenna. When the sound has disappeared, the capacitor C5 discharges into R7, the output of Cl2b returns to 1, the transistor T1 blocks and the oscillation ceases. We have installed a switch in parallel on transistor T1, it allows adjustments or a permanent listening. If the FM receiver has a mute, the sound will be muted when the oscillator is in standby.

The assembly will be realized on printed circuit. There is no particular difficulty of construction for this apparatus, the components are conventional. Of course, there will be the orientation of the transistors and of the Cli which all have the same housing. Cl i is distinguished by three aligned lugs, that of the transistors are in a triangle. The coil L1 is made of enamelled copper wire, or possibly of the wire such as a telephone pair wire. The antenna will have a length of 50 cm; It will be made from a soft wire or, if you install the assembly in a box, by the same length of piano cord.


Résistances 1/4W 5%
R1 : 6,8 kR - R2 : 470 kR - R3 : 2,2 kR - R4 :1 MR - R5, R6 : 47 kR - R7,R15: 100 kR - R8:15 kR
R9 : 4,7 MR - R10 : 10 kR R11: 4,7 kR - R12 : 22kR - R13 : 4,7 kR -  R14 : 330 R

C1, C4, C7 : 22 nF céramique
C2, C3 : 1 MF chimique radial
C5 : 4,7 MF chimique radial
C6 : 0,1 MF, MKT 5 mm ou céramique
C8 : 2,2 nF céramique
C9 : ajustable 2/10 pF
C10 : 27 pF céramique
C11 : 82 pF céramique
C12 : 4,7 pF céramique

T1 : transistor PNP BC 308 T2 : transistor NPNBF 199

Circuits imprimés
CI1 : circuit intégré 78L05 - CI2 : circuit intégré LM 358 - D1, D2 : diodes silicium 1N 4148

L1 : 4 tours fil 0,5 mm sur 5,5 mm de diamètre
L2 : 3,3 mH
M1 : micro à électret

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