This signal injector and tracer (follower) was found in an old American documentation. The transistors can be general use and power is made with one or two common cells.
August 2016 - It can be obtained from Mouser Electronics, Modules Intel Curie low power containing...
Old phones with dials can be used in a simple home intercom system, as this circuit from Popular...
Molex Sealed Industrial USB Solutions achieve superior signal integrity in harsh factory and...