Obtained from a 1969 documentation, this circuit can operate at frequencies up to 500 Hz, counting up to 999. The circuit makes use of neon lamps of the time.
This circuit was found in an Analog Devices manual and can supply a 3 mA current or drain a 10 mA...
This 1967 Raytheon circuit uses an optical coupler of the era and has a range of 800 to 200 Hz....
The integrated circuit used in this project is not common today. It was found in a 2000...