This circuit converts a high voltage into a frequency using an optical coupler. The transistor can be 2N2646 and the coupler admits equivalent. The circuit is from a 1974 publication.
This circuit operates with an AM radio tuned to or near 900 kHz if there is a station at this...
This circuit was found in a 1965 General Electric documentation. SCRs and unijunction may be...
Found in a 90s Popular Electronics, this circuit uses a simple 9 V source. The power can be used...