This circuit was found in an American documentation from the 80's. The sensor is a special integrated circuit, uncommon in our times. There are modern equivalents.
This circuit from an old English publication from 1978 generates pulses of very short duration at a rate...
This high-impedance headphone circuit was found in a 1990s William Sheets. The power is low, but...
This circuit simulates a low-level siren and can be supplied with voltages from 5 to 15 V....