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 source for devices for automotive use was found in a 1985 documentation. The power...
This circuit trips when the voltage set in the pot is exceeded. The tone depends on C3 and...
The circuit shown is from the 1977 GE semiconductor manual. The circuit uses a controlled silicon...