This circuit was found in a Popular Electronics from the 90's. Its purpose is to detect the ionization of the air. The meter can be replaced by the current scale of an ordinary multimeter.
We can implement this filter with more modern equivalent operational amplifiers, since the LM110 is...
This stethoscope or amplifier for hearing aid using a crystal headset and microphone was found in a...
The normal use of a 4520 as a divide-by-16 counter is shown in Figure bellow. In the normal...