Free-line indicator stops interruptions

JM Terrade, Clermont-Ferrand, France -- EDN, 12/7/2000

When only one phone line is available for two phones, each time you want to make a call, someone may be using the second phone. A simple circuit lights an LED, which indicates whether the line is free (Figure 1a). Batteries are unnecessary; the phone line powers the circuit, and an accumulator saves energy for an "in use" indication. A rectifier bridge ensures that the voltage is positive for the circuit. You can safely use this circuit on a private phone line, but you may need authorization before connecting it to your operator line.

A phone line has different voltages between terminals depending on the line's availability. Three possible states exist (Figure 1b). Figure 1b shows the absolute value of the line voltage, because you can switch the line terminals.

In phase 1, the line is free, and the voltage is a continuous 50V dc voltage. The series zener diode, D₁, decreases the voltage by 12V, and R₁ and D₂ further limit the voltage to 8V. The current now flows through the NiCd accumulator, R₂, and D₄. The green LED, D₄, turns on, and the voltage across D₄ turns on Q₁. Q₂ is off, and there is no current for D₃. R₂ limits the current to 3 mA, which is enough to charge the accumulator. The green LED is a low-current model. D₅ protects the accumulator against overvoltage. Total line consumption is 5 mA because an extra 2 mA of current flows through D₂.

In phase 2, when the line is ringing, an extra ac voltage with an amplitude of 50V adds to the 50V dc voltage. In this case, the value of C₁ is critical. If C₁ is 1 µF, both LEDs will turn on because the 15V voltage value will vary. If C₁ is 47 µF, the voltage remains greater than 15V, and D₄ turns on.

In phase 3, when answering the call, the voltage falls to a value of about 10V. Voice modulation adds to this continuous voltage. The operator considers a phone line as "in use" if a current in the phone draws close to 30 mA through its 300? equivalent impedance. These current and impedance values are not critical. The line voltage, which is less than 15V, blocks D₁, and the voltage across D₂ is almost zero. Current discontinues its flow through R₂, and D₄ turns off. Q₁ is also off, and Q₂ conducts. Current travels from the accumulator through R₃, and D₃ turns on. R₃ limits the current to 3 mA, which is enough for a low-current LED. The 300 mAh, 3.6V accumulator is a phone type. If you unplug the circuit, D₃ remains on until the accumulator discharges.