Found on 80's Popular Electronics this circuit operates at 455 kHz. The explorer coil is formed by 10 turns of yarn 26 in a 4 inch shape. The indicator is a microammeter.
This circuit can function as a light alarm by changing the sound when light falls on the sensor....
Based on a unijunction transistor, this circuit was found in the January 1994 issue of Electronics Now...
Although it is a common configuration today, these circuits were taken from an old publication. The...