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.
This experimental amplifier was found in the 1960 Mullard transistor reference manual. The...
This circuit from Motorola is from 1968, appearing elsewhere on this site. The transistors admit...
This circuit triggers the LED when the monitored battery voltage drops below 9 V for 12 V...