The modern telephone can easily be connected in parallel, enabling a household to have one in the bedroom, kitchen and hall or study. It is, however, not always easy to see (or hear) at one position whether the phone at one of the other positions is in use or has taken up the call.
The d.c. level on most telephone lines drops from 48 V to about 8 V when one appliance is in use and to around 5 V when two telephones are in use. Also, its polarity changes over when a telephone is in use. Since the polarity changes over, the circuit L is of necessity a bridge type as shown in Fig. 1. To keep the current drain small and relatively constant at varying voltage levels. a current source is needed for the high-efficiency indicator LEDs. At 50 V across a and b, the circuit is inactive: when that potential drops to about 8 V, the current source is on; when the level has dropped to about 5 V, the ‘switch’ across the upper LED open so that both diodes light.
The diagram of the practical circuit is shown in Fig. 2. The bridge rectifier is formed by diodes D1-D4, while T3 serves as the LED switch.
When the applied voltage is greater than 30 V, T1 is on and switches off current source T2. In this situation, the circuit draws not more than 0.5 mA. When the applied voltage drops to 8 V, T1 is off and T2 is on. The current source is completed by D5, D6, R3, and R5: with values as shown, the current supply is about 1.5 mA.
In this situation, because of the values of R1 and R2, T3 is on, and D7 is short-circuited. As soon as a second receiver is lifted. the line voltage drops to some 5 V, resulting ire T3 being switched off so that D7 lights.
The current through the LEDs IS necessarily small and should in any circumstances not rise above 5 mA. With two telephones that means that R5 must be 270-330 Ω, and for three telephones, 390-470 Ω.As already stated, this means that high-efficiency LEDs are essential.