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.
This circuit, shown in the figure, can operate from 4 to 100 MHz, based on a 2N2222 transistor or...
We found this circuit in a 1988 publication. A touch of the sensor causes the relay to trip and...
This dual flasher for high brightness LEDs was found in an American documentation from the 90's....