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

Aidan Hedley now recommends magnetoresistive in preference to proximity sensors because the proximity detector sensors seem to be unavailable.

---

Proximity detector sensors, which respond to changes in the resonance of a tuned circuit, are amongst the most reliable for tower bells.   An earlier design developed by Aidan Hedley, used an CS209A chip, but that became unavailable.   His subsequent design using a TDA 0161 chip, which sadly also seems to be unavailable, is illustrated below.

The receiving circuit could be the timer section of the Old Design - see below.

The IC is a TDA0161 and was 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 supported on T0220 mounting washers and nylon screws.

This used to be available from STMicroelectronics.

The receiving circuit could be the timer section of the Old Design, from and including R3 and the LED through to R6 at the ouput, and the whole thing could be built on a larger board inside the casing.