This 4-tube counter was found in Radio Electronics magazine from December 1959. The circuit uses a coil as a sensor to detect the passage of metallic parts or magnets.
This circuit found in a French documentation from 1982 can be applied to any operator. The...
In this circuit of the 1980s, the T1 transformer has 30 turns of 28 primary wire and the secondary...
Found in American documentation from the 90s, this circuit triggers a buzzer with a Sonalert-type...