Musical doorbell (CB9210E)

This musical doorbell was found in a Popular Electronics from the 90s. The U4 melody generator integrated circuit is uncommon today.

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HFE meter (CB16983E)
This circuit is from 1990 documentation. It uses a bridge with LEDs and the potentiometer must be...
Spark Generator (CB26136E)
This circuit has the spark duration controlled by a 555 timer. The circuit was found in the 1990...
Using Two Feedback Networks (CB19708E)
Taking Raytheon's 3900 integrated circuits as an example, uncommon today, this circuit can be used...