5 to 200 seconds timer (CB21401E)

This circuit was found in a 1967 RCA documentation. The semiconductor components are from the time, and modern equivalents can be tried.

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Astable with the LM3900 (CB4037E)
The frequency of this astable depends on the capacitor that can be changed over a wide range of...
Micro spy (CB24456E)
This circuit is from a documentation from the 90s. The transistor can be any general-purpose FET...
Intercom with common phones (CB9292E)
In the figure we have one of the many options found in this section for an intercom using ordinary...