This is the schematic from the original article. In the upper left of the schematic is the frequency generator section. The bottom left is the power supply and the right half of the schematic is the counter / display section of the unit. The JU1 jumper in the schematic is the access point for external use of the internal counter.
To externally use the internal counter the jumper needs to be removed and the external signal (maximum level 5 volts peak to peak) is feed into the jumper side of capacitor C3. The power transformer requirements are 120 ac volt primary and 12 or 12.6 volts ac secondary and about a .5 to 1 amp output capacity.