Montajul realizeaza schimbarea a doua tonuri de joasa frecventa produse de doua oscilatoare (1.400—2.500 Hz), cu o perioada de circa 1 secunda determinata de un multivibrator simetric.
Schema de principiu este realizata cu 5 tranzistoare folosite in mod curent in montajele de joasa frecventa (EFT 322, EFT 323, EFT 353, OC 70, OC 71, P13, P14, P15).
Tranzistoarele T1 si T2, aflate intr-o schema de multivibrator simetric, au o perioada de comutare determinata de constantele de timp ale circuitelor CI R2 si C2 R3.
Montajul functioneaza astfel: la cuplarea tensiunii de alimentare, intr-o perioada foarte scurta de timp, multivibratorul intra in regim, astfel incit unul din tranzistoare este in stare de conductie, iar celalalt blocat.
Presupunind tranzistorul T1 in stare de conductie si T2 blocat, intreaga tensiune a bateriei se va aplica pc rezistenta de sarcina R1 (rezistenta interna a tranzistorului este mult mai mica decit rezistenta de sarcina), putind culege din colector tensiunea pozitiva pentru alimentarea oscilatorului de joasa frecventa cu celula de defazare (R3) pe care il pune in functiune.
Dupa circa o secunda, tranzistorul T1 se blocheaza, ajungind in stare de conductie tranzistorul T2 care ne furnizeaza tensiunea pozitiva de alimentare a celui de-al doilea oscilator de joasa Ircrvcnta T4.
Semnalul de joasa frecventa, extras din colectoarele celor doua tranzistoare T3 si T4 prin intermediul rezistentelor R15 si R16 si al condensatoarelor electrolitice de 10 MF (C11, C12) se aplica pe baza tranzistorului final de joasa frecventa T5.
Pentru blocarea tensiunii negative de la multivibrator spre cele doua oscilatoare de joasa frecventa au fost prevazute doua diode D1 si D2.
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