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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Active inductor (CB20748E)
This circuit simulates an inductor. Or operational amplifier of old type, but a configuration is...
Voltage reference with FET (CB21534E)
We found this circuit in a 1967 Electronic Design. The circuit uses contemporary transistors that...
Relay Control shield or interface (CB15483E)
To control relays safely with double insulation, we suggest the circuit in the figure. The optical...