This circuit was found in RCA documentation from the 1980s. The circuit uses LDRs of the time and the 50 uA instrument has zero in the center. R1 and R2 may have 2k2 and R3 10k typically.
We found this transmitter circuit in a 1990s spy book. The coil data is provided along with the...
This circuit, from an old publication, can also use modern transistors. Note that the unit is...
This analog tachometer was found in Radio Electronics from the 90s. The components used can still...