This light meter circuit was found in 80's Radio Electronics. The circuit uses a milliammeter as an indicator and the sensor is a common LDR. The power supply must be symmetrical.
Found in old documentation, this circuit makes use of amplifiers of the time, uncommon today. Next...
Rumble is a low-frequency oscillation that occurs in turntables due to variations in motor speed....
This circuit was found in a Popular Electronics from the 90s. The circuit uses common components...