External hardware is not strictly within the scope of Beltower software, but a few hints are offered here.

Proximity detector sensors, which respond to changes in the resonance of a tuned circuit, are amongst the most reliable for tower bells.  

A new design developed by Aidan Hedley, using a TDA 0161 chip, is illustrated below.  

The IC is a TDA0161 available from Farnell (see http://uk.farnell.com: search for part number 146-7823).   It is easy to use and has a 1/10mA current loop output.

Interfacing is two wire into a simple resistor transistor current to voltage conversion and a series LED will give supply/activation indication.

Performance is excellent - the activator is detected from 60mm, a sheet of aluminium will not be detected until within 15mm.

C1:   680 pF
C2, C3: 1nF
R1:   10 K
L1:   Coil 20 turns 0.25mm wire on supported on T0220 mounting washers and nylon screws.

Full details can be found at http://www.st.com/stonline/products/literature/ds/1441/tda0161.pdf   from   STMicroelectronics.

An older design using the obsolete CS209A chip can be found at Old Design.