The transistors of this circuit found in an English 1980s documentation may be the modern BC548 equivalents. The circuit is battery powered and the earphone must be crystal.
This circuit was obtained from an English magazine from 1981. The magazine no longer exists, but...
Radio Electronics published this circuit in a 90's edition. The circuit makes use of an uncommon...
This TTL module can be used in counter and many of them can be cascaded to counts up to 99, 999,...