This simple Bargraph photometer with 4 LEDs was found in a 1997 documentation. The scale is adjusted in a trimpot and the sensor can be any common LDR. Power can be supplied by cells or battery.
Suggested by Maxim in documentation from the 90s, this circuit has its response curve given next to...
Found in a 1969 Ferranti documentation, this circuit uses only bipolar transistors from that time....
Found in a 1960 General Electric documentation, this crowbar circuit blows the fuse in case of...