This circuit, found in an American publication from the 1970s, produces an audible tone in a speaker. It is a traditional configuration that can be mounted with BC548 and BC558 transistors and fed with voltages from 6 to 9 V. The adjustment pot can be 220k or even 470k. A 1k resistor in series with C1 helps to improve circuit performance. The speaker can be 4 or 8 ohms and for greater power supply the circuit with 12 V exchanging T2 for a BD136. Reduce C1 if the sound is too low.
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