FM interphone circuit

The circuit is as shown in the figure. The triode V, inductance coil L1, capacitor C1, C2, etc. form a capacitive three-point oscillation circuit, which generates a carrier frequency signal with a frequency of about 100MHz. The integrated power amplifier LM386 and capacitors C8, C9, C10, Cll, etc. constitute a low-frequency amplifier circuit. The speaker BL is also used as a microphone. When the circuit is working in the receiving state, set the receiving/transmitting switch to the "receiving" position, and the signal received from the antenna ANT is detected by the super regenerative detection circuit composed of the triode V, inductance coil L1, capacitor C1, C2, and high-frequency choke coil L2. The detected audio signal is coupled to the input end of the low-frequency amplifier through the capacitor C8, and after amplification, the capacitor Cll coupling pushes the speaker BL to sound.

When the circuit works in the sending state, S2 is placed in the "sending" position, and the voice is converted into an electrical signal by the loudspeaker. After IC low-frequency amplification, the signal is added to the base of the oscillator V by the output coupling capacitors Cll, S2, R3, C4, etc., so that the bc junction capacitance of the tube changes with the change of the voice signal, and the bc junction capacitance of the tube is connected in parallel at both ends of L1, so the frequency of the oscillator circuit changes accordingly, realizing the function of frequency modulation, And transmit the modulated wave from the antenna through capacitor C3.

During debugging, S2 should be placed at the "receive" position first, and then the speaker should have a large noise. Touch the triode case and the noise disappears, indicating that the receiving circuit is basically normal. Then place S2 at the "sending" position, take an FM radio and place it nearby, and adjust the receiving frequency to about 100MHz. At this time, there should be a loud whistling sound in the radio. The whistling sound disappears when the radio is about 10 meters away, and then aim at the microphone to speak. Clear and bright sound should be heard in the radio. If there is no sound or the sound is low, adjust the frequency of the radio. After the above debugging of the two walkie talkies, the interworking test shall be carried out, and the spacing of L1 shall be properly adjusted so that the receiving and sending signals can be unified to the same frequency. When the frequency overlaps with that of the local radio station, the resonant capacitor C1 needs to be replaced to prevent mutual interference and affect normal use.