This circuit is from a 1969 Electronics World. The transistors may be modern general-purpose equivalents. The circuit is for frequency ranges between 250 Hz and 250 kHz.
Found in American documentation from the 90s, this configuration detects the direction of rotation...
Circuits similar to this can be found in this section. This is from 1967 General Electric...
This circuit uses two transistors and can be powered by just two small batteries. The frequency is given...