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.
The simple optical intercom we show is from a 1995 documentation. The LEDs can be infrared or...
With this circuit it is possible to monitor a signal that is being recorded on an old tape...
An oscillator that can be frequency controlled by a resistive sensor is shown in the figure. The...