The PIC Frequency Counter

Here is a nice project  that uses a minimal parts count.
For more information and the programming hex files for the PIC 16F628A,  see:

Wolfgang, DL4YHF has designed a very nice project.

counter rear

counter front


Component values for variant 2 (as shown above): R1...R8 = 1k, R9=R10=10k, D1..D4=1N4148, T1=BC547 or similar, C1=C2 about 22pF (select them to tune the crystal to exactly 20 MHz), C3=100nF, Q1=20 MHz, PIC: 16F628 programmed with firmware "counter2.hex".