Cheap frequency counter kit "1Hz‑50MHz Digital LED DIY Kits Crystal Oscillator Frequency Counter Tester"
original design
https://www.qsl.net/dl4yhf/freq_counter/freq_counter.html 
modifications
• A PIC based frequency counter kit ‑ part 1:building and modifying https://www.youtube.com/watch?v=0BxpMm6SLoE 
• A PIC based frequency counter kit ‑ part 2:changing the firmware https://www.youtube.com/watch?v=8rOCmhPWLWw 
• Frequency Counter Kit revisited: higher resolution and RPM mode https://www.youtube.com/watch?v=0D6EcgTQtNw&t=313s