Amplifier for radon monitor (CB21202E)

This circuit was found in an Electronics Now from the 90's. See the previous article for the assembly of the detector head.

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Tunable notch filter (CB7404E)
We found this circuit in an English publication from the 1980s. The circuit uses current...
Telephone Relay (CB15524E)
The circuit shown in the figure can be useful in cases where a common telephone extension is used....
Proximity Switch (CB13489E)
We found this circuit in an American documentation from 1977. The circuit must not be powered by a...