I am considering building a frequency counter.  In part of that, I’ve been learning and playing around with logic chips.  I started by playing with a few decade counters and setting those to count based on pulses from an Arduino.  As I continued, I felt the Arduino was too slow and replaced it with a 555 timer used as an astable multivibrator and only used the Arduino as a power supply.  Then, just to mess around, I put a photoresistor on it and used it to adjust the frequency.  Below are the Fritzing image and a video showing how it works.