Alarm system designs often require circuitry that can detect whether a phone line is active or broken. With this type of design, the primary difficulty is drawing less than 5 µA from the phone line over a line-voltage range of 24 to 58 V as the standard dictates.
When the phone is in its "on-hook" state, the central office exchange (CO) acts as a current source. The circuitry on the line is restricted to impedances that create less than a 5-µA current from this source. When the phone goes off-hook, the phone's impedance lowers significantly. Consequently, a CO-detected voltage drop is produced. The CO creates the ringing signal by adding a low-frequency signal to this dc bias signal.
Shown here is a circuit based on a micropower oscillator biased by the telephone line (see the figure). This oscillator generates a differential signal, which is coupled to a detector circuit via high-voltage capacitors. These capacitors supply the required isolation. The detection circuit merely recognizes the presence of the oscillation while presenting negligible output loading to the oscillator itself.
The oscillator uses an astable multivibrator. Instead of relying on the traditional collector resistors, which would otherwise become very large, it employs 1-µA current sources (Q4 and Q6). These determine the supply current. Also, capacitor-charging resistors (which establish the oscillation frequency) are based upon current sources of 0.1 µA nominal (Q3 and Q5).
A full-wave rectifier (D1, D2, D3, and D4) and a smoothing capacitor (C6) bias the oscillator from the phone line. C6 has strong impact on the time needed to detect a line break due to the high impedances involved in this circuit.
To meet the galvanic isolation requirements for telephones, the oscillator section must be isolated from the detection circuitry. Traditionally, this has been achieved by means of a transformer. In addition to its cost and size penalties, the transformer is unable to couple dc current, which is needed for detection when the phone remains "on-hook."
At this point, capacitor coupling comes to the rescue. Low-capacitance, high-voltage devices couple the oscillator to the externally supplied detector circuitry. Normally, a capacitor such as C5 would be discharged by the Q7 current source (0.1 µA nominal). But the detector in this application contains a half-wave rectifier that charges C5. This is a better solution than using a simple resistor, which would require a very high value (i.e., greater than 100 MΩ).
Finally, a low-impedance output is provided by means of a MOSFET device. Since the MOSFET includes a gate-source zener diode, it's necessary to employ a large gate resistor. Doing so will prevent excessive loading when it enters the conduction region.
The possibility of high voltage levels in the phone lines determines the ratings of the devices to use. Otherwise, the component values are noncritical. To reduce leakage effects, very high valued resistors (over 10 MΩ) should be implemented by means of multiple 10-MΩ resistors in series. Solder flux must be carefully cleaned. This will prevent leakage around the resistors between the component pads on the printed-circuit (pc) board. Also, pc-board layout must be well planned to minimize the effect of external noise sources caused by the high impedances involved.