We found this circuit in the 1964 RCA transistors manual. The sensor may be a common LDR and the transistor BC558. The relay has an operating voltage according to the power supply.
This circuit was found in National Semiconductor documentation from the 1990s. It applies to...
Found in Popular Electronics, this '90s circuit generates harmonic-rich signals for testing audio...
The circuit shown in the figure is suggested by ON Semiconductor (www.onsemi.com), based on its...