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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Window detector (CB9706E)
This configuration that appears several times with variations on this site is from old American...
Parallel Transformers Power Source (CB20205E)
This configuration (somewhat critical, as the imbalance of characteristics between transformers can cause...
Siren for motorcycle (CB6491E)
This circuit is from a Popular Electronics of the 90's. Any power MOSFET can be used. The circuit...