This circuit uses a lamp driven by the Hertz loop and measures its brightness with ordinary photovoltaic cells. The circuit is from a Radio Electronics from February 1964.
This circuit was found in a 1999 English magazine, but can be easily assembled with BC548...
This sound relay or Vox was found in 70s RCA documentation. SCRs can be common types today and...
This telegraph microtransmitter for a frequency determined by the coil and the crystal was found...