This circuit is from a 1964 Electronic Design. The sensor is an LDR and the transistors are general-purpose types. The circuit triggers when the light that falls on the sensor is cut off.
Found in an Elektor from the 90s, this circuit is suitable for viewing audio signals on an...
This project was found in a 1982 publication, using only two transistors. Signal injectors are...
Found in documentation from the 1980s, this circuit uses components from that time. The circuit is...