This proximity sensor was found in a 90s Popular Electronics. The base is a PLL567 that operates at around 100 kHz. The circuit operates by capacitance sensing.
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...