Abstract: An integrated ringing signal detector sufficiently small for implementation on a single PCMCIA card. The detector has a DC-blocking capacitor 28 disposed between the telephone line and a bridge rectifier 27. The output 29 of the rectifier 27 connects to a voltage detector 23 which detects when the ringing signal exceeds a predetermined voltage. Output 30 of the voltage detector 23 triggers thyristor switch 24 into conduction. Switch 24 remains conductive until current flowing therein falls below the holding current of the thyristor switch 24. Current limiter 25 limits the current flowing therein to a substantially fixed predetermined amount of current, sufficient for the adequate operation of the LED in the opto-coupler transducer 22. The transducer 22 is shown here as an opto-coupler having an LED transmitter and a phototransistor receiver to provide galvanic (high-voltage) isolation between the telephone line and the utilization device, here a modem.