Isolated indicator signals telephone line's status

Part 68 of the FCC's (Federal Communications Commission) telecommunications regulations requires that certain signaling equipment connecting directly to the public-telephone network must present a line-to-line resistance of at least 5 MΩ. In addition, status signals that equipment derives from the phone lines must include electrical isolation to prevent interaction between earth grounds from the telephone network and attached control or communications equipment. Although a transformer can provide isolation for voice-frequency signals, the telephone-line-status-indicator circuit in Figure 1 meets FCC isolation requirements without incorporating a transformer (Reference 1). A diode bridge, D₁ through D₄, and R₁, a 5.6-MΩ resistor, supply a small amount of dc power from the phone line to a nanopowered combination comparator and a 1.2V voltage reference, IC₁. The Maxim MAX917 IC draws only 0.75 µA at 1.8V_CC.

Resistors R₂ and R₃ form the detection-voltage divider, and R₄ provides hysteresis. When IC₁'s output goes low, R₄ and R₃ form a parallel combination of 3.26-MΩresistance. To reach the comparator's reference voltage of 1.245V, the voltage across C₁ must reach at least 5.06V. Once IC₁'s output goes high, R₄ and R₂ form a parallel resistance of 6.67 MΩ, and the voltage across C₁ must reach 3.37V to deliver a 1.245V input to the comparator. IC₁'s output drives a photocoupler, IC₂, a Toshiba TLP190B. Unlike other photocouplers, IC₂ includes an array of photodiodes that, when illuminated, delivers a voltage output. Although weak by power-conversion standards, the photocoupler's output can deliver several microamperes at an open-circuit voltage that exceeds 7V, or enough to drive a MOSFET's gate or a microprocessor's input pin. In addition, the TLP190B carries a 2500V-rms emitter-to-detector isolation-voltage rating.

When a telephone is not in use, the on-hook voltage across its line of approximately –48V produces a current of 7 to 8 µA through R₁, which imposes a low-leakage requirement on C₁. The prototype version of the circuit uses an X5R-characteristic ceramic capacitor. When the voltage across C₁ exceeds 5.06V, IC₁'s output goes high and drives IC₂ through R₅, discharging C₁. When the voltage across C₁ decreases to 3.37V, IC₁'s output goes low, and C₁ recharges. The output from IC₂ comprises a 1.4-msec-wide voltage pulse with a repetition period of approximately 240 msec. When the phone is off the hook, the voltage across its lines drops to a few volts, which don't sustain pulse generation.