Found in an old Radio Electronics, this circuit applies to frequency meters for frequencies up to 100 MHz. The input impedance is in the order of 1 M ohm.
This circuit was suggested by Texas Instruments in the January 1973 issue of Radio Electronics magazine....
This circuit was found in Motorola documentation from 1968. Transistors can be modern equivalents, just...
To invert the rotation of a motor by changing the levels of an Arduino output or another...