PH meter (CB20731E)

This circuit was obtained in an old American documentation. The integrated circuit used is uncommon today. Modern equivalents can be searched.

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Scratch filter (CB12054E)
The scratching noise is quite unpleasant for those who pass the sound of vinyl records to other...
Telephone Monitor in Use LM3909 (CB24688E)
This circuit for common analog telephones was found in March 1982 Radio Electronics magazine. It...
Amplifier for tunnel diode (CB23848E)
In the figure we have an RF amplifier for tunnel diode. The frequency range will depend on the...