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 1.8VCC.
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.