This is very simple circuit that shows how to measure a frequancy.
There are only two digits. It's easy to extend. TTL ICs allow switch frequency up to 10 MHz. This circuit may be extended up to 7 digits.
The logic train above is the measuring frequency.
The logic train below is 1Hz synchronized oscillator connected to D-trigger.
The red LED shows seconds.
Every odd second the Frequency meter is showing a measured frequency.
Every even second the Frequency meter resets and then it is counting.
