Found in a 90’s American documentation, this circuit indicates the frequency of input pulses up to 1 kHz. The circuit has a very explored configuration on this site.
We found this circuit in documentation from the 1970s; The circuit uses Japanese transistors of...
This circuit goes into operation producing a sound on a small speaker when the supply voltage...
Suggested by Harris in documentation from the 80s, this circuit uses components from that time. It...