Michael Lake added a high pass filter
(for power line hum) and a low pass filter to reject AM Broadcast
stations, and produced and sold some very nicely constructed
surface-mount SWT's. See http://www.speleonics.com.au/business/index.html
for details and schematic.
A SWT was published in the British CREG Journal. A slightly
modified version is at http://www.hamil.org/swt/index.htm
. This more complex design includes a standby mode and a ringer.
Desirable features in a rescue SWT include an LED "ON" indicator (only
bright enough to see on a cave, could just blink); provision for a
ground lead; use of AA cells for a power source (which most cavers
carry) or built-in lithium cells (with a foolproof OFF switch) and