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 uses in operational amplifier to produce square and triangular low frequency signals....
This shield can be used both to interface microcontroller circuits or TTTL. The resistor in series...
This circuit can gerenate square signals and be comanded by na external control. When the input...