Using this simple circuit, you can talk on the phone without being interrupted. As soon as you pick up the phone to talk, the TV, music system, or any other device that might be making noise turns off. When you put the phone back on the cradle, it turns on. This is done with the help of a polarity-guard circuit with a bridge rectifier BR1, an optocoupler MCT2E (IC1), and a timer 555 (IC2), as shown in Fig. 1. Usually, there is about 48V across telephone lines. But it drops to about 9V when you take the phone receiver out of its cradle.

Fig. 1: Auto-muting circuit When the phone receiver is in the cradle, current flows through the zener diode and the LED inside optocoupler IC1. So, its internal transistor conducts and transistor T1 stops conducting. This makes IC2's pins 2 and 6 high, so its pin 3 output becomes low. Because IC2's output is low, relay-driver transistor T2 doesn't work, and relay RL1 stays off. Because of this, the normally-closed (N/C) contacts of the relay keep the TV or music system on. When the phone is taken out of the cradle, however, the Zener diode stops working and no current flows through the LED inside optocoupler IC1. So, its internal transistor stops working, and transistor T1 starts working. This makes IC2's pins 2 and 6 low, so its output at pin 3 goes high. Because IC2 is putting out a lot of power, the relay-driver transistor T2 starts to work. So, relay RL1 gets turned on to turn off the power to the TV or music system. At the same time, LED1 lights up to show that someone is talking.

Fig. 2: Power supply for auto-muting circuit The circuit runs on a 6V power supply that has been regulated. Transformer X1 takes 230V AC mains power and turns it into 9V at 200 mA power. Full-wave bridge rectifier BR2, capacitor C3, and regulator IC3 are used to fix the output of the transformer (7806).