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 is the circuit of a digital clock published in the January 1975 issue of Radio Electronics magazine....
This output stage can directly excite a small speaker. The circuit is from a 1960 Mullard...
In the figure we have one of the many options found in this section for an intercom using ordinary...