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 was found in the August 1970 issue of Radio Electronics magazine. It employs RCA's...
This circuit is from 1968 General Electric documentation. It can certainly be adapted for any...
This tachometer from a 1994 publication has a bargraph scale with the well-known LM3914 integrated...