Using a sensitive crystal or ceramic microphone, this circuit can pick up faint sounds through a wall. The circuit has a good range given the use of an RF amplifier transistor.
We found this circuit in Radio Electronics Special Projects from 1992. The circuit turns on the...
Using neon lamps, this regulator was found in a1969’s documentation. The lamps are types available...
This interesting automotive alarm was found in a 90's Popular Electronics. The circuit uses...