Circuit eliminates PC echoes
Hans Krobath, EEC, Nesconset, NY -- EDN, March 30, 2000
Long-distance-telephone services available via the Internet often require the PC user to wear headphones of a headset to prevent echo caused by the microphone's picking up the loudspeaker outputs. The circuit in Figure 1 eliminates the echo while using the existing PC microphone and speakers for a comfortable conversation. The interface is between a standard electret condenser microphone and the microphone input of the PC. The loudspeaker output of the PC serves to mute the microphone input. R1 and R2 provide biasing for both the electret microphone and the Q1 emitter follower. Q2, a p-channel FET, acts as a switch that opens with the application of a gate voltage greater than 6V and closes with a gate voltage of 0V. Q3 compensates for the switching-circuit losses and buffers the output. R9 and R10 provide an appropriate input impedance to Q3 and limit the output to 5V p-p, thus preventing any possible damage to the PC's microphone input.
IC1, which acts as low-level retriggerable monostable multivibrator, controls the Q2 FET switch. Loudspeaker voltage levels as low as 15 mV from the PC cause comparator IC1A's open-collector output to discharge C6 via R18. The falling voltage of C6, passing the threshold of comparator IC1B, produces a high output that turns off Q2. Any input from the PC's loudspeaker output discharges C6. The absence of an input allows C6 to charge within approximately 40 msec to the IC1B threshold, producing a low-level output and turning on Q2. LED D2 lights whenever no loudspeaker output is present, and the microphone input to the PC becomes enabled. D1 reduces the Q2 gate voltage to 0V when the IC1B output saturates. You should set R1 such that approximately a 100-mV p-p microphone input just triggers IC1, as indicated by the LED's extinguishing. This level prevents any noise from the PC's loudspeaker output from falsely triggering the monostable multivibrator. (DI #2508).
