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 was found in a 2000 Poptronics. The magazine no longer exists, but the circuit can be...
This 80's circuit has an input resistance of the order of 880 M and gain 10, using operational...
This circuit can be used in a toy organ. The frequencies are determined by the capacitors selected by...