This circuit was found in a 1971s English magazine. We can still assemble it using BC548 transistor. The other components are common. The supply voltage of 6 or 12 V depends only on the relay.
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...