The sensor of this circuit found in Popular Electronics can be formed by a piece of fabric between two bare wire screens. When wet, the fabric will pass the current between the screens, triggering the alarm.
Waking up to a loud and clear sound from an audio oscillator, after up to 5 hours of sleep, or...
This circuit was found in a publication from the 90s. The circuit can be assembled with modern...
This conventional oscillator with the 741 can be precisely adjusted to produce a 400 Hz signal. The...