Parallel Connection of Modern Telephones
Modern telephone can be conveniently linked in parallel, allowing households to place them in multiple locations like bedrooms, kitchens, halls, or studies. Nevertheless, determining whether a phone at a different location is in use or has answered the call is not always straightforward from a single position due to visibility and audibility limitations.
Voltage Levels and Polarity in Telephone Lines
The voltage on most telephone lines typically diminishes from 48 V to roughly 8 V when one device is in use, and it further reduces to approximately 5 V when two telephones are active. Additionally, the polarity on the line changes when a telephone is engaged. Given this dynamic polarity change, it is necessary for circuit L to adopt a bridge-type configuration, as illustrated in Fig. 1. To maintain a consistent and low current drain at varying voltage levels, an efficient indicator LED requires a current source. When the voltage between points a and b remains at 50 V, the circuit remains inactive. However, as this potential decreases to around 8 V, the current source activates. When the voltage level drops to about 5 V, the’switch’ connecting the upper LED opens, causing both diodes to illuminate.
Practical Circuit Diagram (Fig. 2)
The circuit, as depicted in Fig. 2, incorporates a bridge rectifier using diodes D1–D4, and T3 functions as the LED switch. When the applied voltage exceeds 30 V, T1 conducts, turning off the current source T2. In this state, the circuit’s current draw does not exceed 0.5 mA. However, when the applied voltage decreases to 8 V, T1 turns off, allowing T2 to conduct. The current source circuit is then completed by D5, D6, R3, and R5, resulting in a current supply of approximately 1.5 mA.
Switching Mechanism for Multiple Telephones
In this configuration, T3 is active due to the values of R1 and R2, causing D7 to be short-circuited. When an additional receiver is picked up, the line voltage falls to around 5 V, leading to the deactivation of T3. Consequently, D7 illuminates, indicating the use of the second telephone.
Maintaining Low LED Current and Appropriate Resistance Values
To ensure the LEDs operate at a minimal current, it is imperative to keep the current below 5 mA. For two telephones, R5 should be in the range of 270–330 Ω, and for three telephones, it should be between 390-470 Ω. The use of high-efficiency LEDs is crucial under these conditions.