Automatic house telephone set - uses solid state components and has Darlington pair becoming conductive when receiver is lifted and negative DC potential is applied to it

摘要

The house telephone set has the following solid state components. Light emitting diodes (OCl to OC7) opto electronically coupled to photo transistors (OC1' to OC7') diodes (D1 to D6), NPN transistor (T1) and Darlington PNP pair (T2, T3). The contactors (a1, a2) are in the position after the distant receiver has been lifted. Before that, the contactor (a2) connects to the 25Hz call signal terminal. When call signal is operative, 25Hz current passes through line (b) the distant bell system, the upper transformer secondary of a transformer (T2), the optoelectronic coupler (OC1), a resistor (R1) and finally to ground (mr). This alternating current does not pull the base of Darlington pair (T2, T3) sufficiently negative to make it conductive. Therefore, 48 volts passes through resistor (R4) and optoelectronic coupler (OC6). Both OC6' and OC7' photo transistors are conductive causing a relay (A) to draw and the call signal to be sent out via its contactor (a2). When the distant telephone answers and its receiver is lifted, a negative d.c. potential is applied to resistance (R1) and transistor base of the Darlington pair (T2, T3) causing it to become conductive so that coupler OC6/OC6' looses current. Delay (A) falls off and conversation can begin via the transformer (Tr).