Remote listening amplifier (CB21957E)

Today we can call this circuit a baby monitor. It is a microphone amplifier found in a publication by Hugo Gernsback from 1960.

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Complex Sound Generator (CB16437E)
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Fire siren (CB21232E)
This circuit that emits siren sound can be used in a fire alarm. We found it in an old American...
Simple Alarm with SCR (CB24114E)
Found elsewhere in this section, this circuit is from the April 1971 Radio Electronics magazine. Note...