This circuit of the 70's uses transistors of the time. The circuit can amplify signals from 10 Hz to 100 kHz and modern equivalent components can be used. The circuit is from Ferranti.
This interesting capacitive water level sensor was found in Popular Electronics. The circuit can be...
In the original 2000, this circuit was designed to interface the parallel port with external...
This 10 kHz filter was obtained from an article in Electronics Now magazine in February 1995. The...