The circuit shown can detect when a shared telephone line is in use (see the figure). This is particularly useful when an automated device wishes to place an outgoing call without disturbing an established telephone line connection.
As the circuit demonstrates, when a telephone line isn't in use, the dc voltage across the line is between 42 and 52 V, with 48 V being the nominal. When any shared devices on the phone line go "off-hook," the voltage across the telephone line typically drops to somewhere between 4 and 18 V.
Clare's TS117 integrated relay and an optocoupler provide the circuit with 3750-VRMS galvanic isolation between the microcontroller side of the circuit and the telephone line. In this circuit, the device wishing to make an outgoing call asserts the ——Check_Line signal (i.e., low), which turns on LED D2. Next, the light from D2 activates the high-voltage solid-state relay K2 that provides a current path to LEDs D3 and D4. D3 and D4 are anti-parallel to allow the circuit to operate regardless of the telephone line polarity.
If the line isn't in use, the voltage across TIP and RING will be greater than 42 V, and Zener diodes D1 and D5 will conduct. Note that one of the Zener diodes will be conducting in the Zener region, and the other will be forward-biased, depending on the telephone line polarity. Such conduction causes a current to flow through the forward-biased LED D3 or D4. The light from this LED is coupled to phototransistor Q1, which is driven into saturation. This pulls the Line_In_Use signal low, indicating that the line is available.
If the device wishing to make the call asserts the ——Check_Line signal and the telephone line voltage is less than approximately 20 V, then Zener diodes D1 and D5 will not conduct. When this is the case, no current will flow through LEDs D3 or D4. Also, the Line_In_Use signal will be high, indicating that the line is in use. The device wishing to place the call should wait and retry at a later time.
R3 is selected so that the current drawn by the detector circuit does not assert an off-hook condition when ——Check_Line is asserted. V1 is a 300-V surge suppressor that can be a MOV or thyristor-type device. Relay K2 is used to comply with the FCC's on-hook impedance requirements.