Sound Level Attenuator

030 AUDIO & HI-FI SOUND LEVEL ATTENUATOR When the radio or record player is on at a fairly high volume, it is often impossible to hear the telephone or doorbell. A solution to this frequent difficulty is offered by this automatic attenuator. As soon as the doorbell or the telephone rings, it turns down the volume of the audio equipment. The circuit consists of an optically controlled attenuator and the requisite electronics to connect it to, say, the telephone. The attenuator is of fairly simple design and is based on a TL074. Its control part, consisting of a current-driven attenuator based on an LT2001 (a combination of an LED and two light- dependent resistors - LDRS - in a common enclosure), is incorpo- rated in the audio equipment. After the mains has been switched on, C9 causes the resetting of bistable ICI. The high voltage level at pin 6 of the 741 causes T1 and T2 to switch off: this, in turn, results in D2 not conducting and the voltage-controlled current source, T...