We found this touch control on a Popular Electronics from 1970. The circuit makes use of an integrated of the time. It can be adapted to use modern equivalent components.
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...