Ron Hardin wrote:

We used to build them with 1N34A diode, capacitor across the
headphones,
and a coil wound on a toilet paper tube, with taps on the coil
that you'd clip an allegator clip to, for tuning.

We still do! And they still work great!

They're not hard to construct with inexpensive readily-available items.
There are several good websites with tips and hints for simple
construction, and they're great fun. Try making one!

Bruce Jensen