Optocouplers have been commonly used for telecom ring detection for many years due to their simplicity and ease of use. Figure 1 shows a typical half wave optocoupler circuit. All the components except U1 and D1 are in series， therefore the order in which they are connected is not important. The ring detect circuit must be AC coupled and present a relatively high （60kΩ minimum） impedance to the line with low amplitude AC voltages. This is to prevent the circuit from interfering with the audio signals when the line is in use. In Figure 1， C1 provides AC coupling for the circuit. R1 limits the peak currents and D2 and D3 provide a method of preventinglow-level ac signals and noise from triggering the circuit. Back-to-back zener diodes are used so the load is symmetrical on both AC half cycles. U1 requires D1 in parallel with the input diode opto-emitter to prevent the input of the optocoupler from acting as a rectifier and pumping up a DC voltage on C1 during ringing. If this were to happen， the circuit would stop detecting the incoming ring as the DC voltage builds up on each ring half cycle.