Simple circuit detects dc ringing

Jerzy Chrzaszcz, Warsaw Institute of Technology, Poland -- EDN, 8/17/2000

Ring detection is a common task in telephony-related digital design. The goal is to sense an ac ringing voltage applied to the telephone line by a central office or PBX (private branch exchange) and then pass an appropriate signal to the µC or µP. Figure 1a shows a typical ac-coupled ring-detection circuit. The serially connected capacitor closes the ac path and the optoisolator provides galvanic isolation between the line and the logic. The output capacitor integrates the pulse train to form a ring envelope signal. More sophisticated designs may use the unfiltered output for ring-frequency discrimination. Unfortunately, some PBX systems signal ringing to extension phones by raising the dc line voltage. A traditional ac-coupled detector is thus useless. The circuit in Figure 1b consists of a full-wave rectifier, a current-limiting resistor, a zener diode, and a high-gain optoisolator with output pullup. By changing component values, you can easily meet circuit requirements for circuit isolation, input-voltage range, threshold voltage, and line load. (DI #2567)