LEAP#207 Frequency Counter
I found this circuit published in Electronics magazine (Sep 16 1976). It's a classic demonstration of the CD4026 "bucket-brigade" and CD4047 astable oscillator.
The frequency counter is governed by a CD4047 oscillator. Since this offers a clean 50% duty cycle, it is ideal for flipping the circuit between two modes: sampling period; display period.
As always, all notes, schematics and code are in the Little Electronics & Arduino Projects repo on GitHub.