This circuit is from a 1997 English magazine, but it is very current due to the components it uses. The transistors can be BC548 and the supply of 6 to 12 V depends only on the relay used. The circuit triggers a relay when the microphone picks up sound. This circuit is also known as Vox. The 7555 can be replaced by the non-CMOS version which is the common 555.
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