Wolfgang, DL4YHF has designed a very nice project.
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