Tower Bell Sensors Overview

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

Types of sensor

There are five main types of sensor including:

Push to make switches

These can be used with dummy hand-bells and call switches.

Magnetic reed switches

  and mechanical flip switches
Magnetic reed switches are Ok, provided the wheel runs true and the gap between the magnet and the switch is small, although magnets can decay in time.

Optical or photo-sensitive sensors

These respond to reflected light and are the recommended sensor design for tower bells.   The latest designs of infrared photo-electric sensors use infrared light to avoid the risk of picking up stray lights in the visible spectrum.

Proximity detectors

Proximity detector sensors, which respond to changes in an electro-magnetic field, act like metal detectors and are amongst the most reliable for tower bells.

Magnetic Hall effect sensors

Magnetic Hall Effect Sensors are like proximity detectors but relying on a magnet for the signal rather than a tuned circuit.

Some sensors produce high signals (+5v active, 0v inactive), and some produce low signals (0v active, +5v inactive).   Most sensors produce very short pulses which need a timer/delay circuit to make a reliable interface.

Different types of sensor can be used interchangeably, provided they all produce similar signals.

Sensor Configurations

There are three main configurations for bell sensors including:

Signal at Strike

This is most appropriate for use with hand bells.

Single Signal at Mid-Point

The recommend configuration for tower bells is using a single signal at the mid-point.   When the bell is rung, the sensor will send a signal, as the activator passes the sensor, at the mid-point of each stroke, and the strike is then delayed by hardware or software, and, either way, the software lets you set the delay for each bell.   This configuration has the added advantage that the signals are still active when lowering, and are valid almost to the point of chiming.   Beltower's early, late and odd striking features can be applied to sensors in this configuration with delays applied by Beltower.

Twin Signals

With twin signals, the alternate signals are accommodated by software but the drawback is that every signal is critical.   If one is missed, it confuses the software.   Re-positioning of the actuators might also be required to allow for any odd struckness.

This can be difficult to configure, because twice as many signals are being sent and received, and each of the following is therefore doubly critical:
1. the delay before your PC+Windows detects each signal,
2. the actual signal duration / pulse length / debounce time,
3. the Beltower debounce time setting, if required, and
4. the period between the first and second signals.
If any signal is missed it can be difficult to get back in step without restarting the ringing.

Problems can also arise with twin signals when the ring speed increases (eg. most noticable when lowering) because the separation between signals gets shorter.  

Types of Interface

There are three main types of interface between a sensor and a PC:   either serial or by individual pins.

Connection to Individual Pins of a COM (serial I/O) port

A simple sensor will only be reliable if the signal lasts long enough for the PC to respond to it, but this depends on both the length of signal and the speed of the PC.   Most sensor designs and interfaces also include a timer circuit.   These are more reliable and such designs often allow the user to vary the duration of the signal pulse.

Different types of sensor/interface can be used interchangeably, provided they all produce similar signals.

USB ports can be used with COM to USB adaptors.

Serial Stream of Bell Numbers

A typical serial interface will gather together the signals from a number of bells, and send bell numbers (maybe after pre-defined delays), as a data stream.   This is probably the most reliable interface for tower bells, but also the most expensive.   The delays can be selected in Beltower.  

It will not usually handle GO, BOB etc. so any call switches would need to be connected directly to a separate COM port.

Cabled or Wireless connection

A serial interface can be connected by cable either directly to a COM port, via a COM-USB adaptor, or wirelessly to USB port.

For information on wireless serial interface units see
http://www.simbell.co.uk/ for Simbell Wireless Simulator System (email: sales@elginhill.com) or
http://belfree.co.uk/ for Belfree interface system (email: info@belfree.co.uk).

For a DIY design, see
http://www.simulators.org.uk/ for Liverpool Ringing Simulator Project and click GitHub, simulator-type2 then docs and, from there, click Getting Started, Build and Config as required, but do check for subsequent updates.

Connection to the Computer

Where sensors send their signals to individual pins, on one or more COM (serial I/O) ports, Beltower detects the change of state, on either the rising edge or the falling edge of a positive pulse.   Normally, the signal, whether high or low, will last in the region of 100 milliseconds, but Beltower is also happy with extra long pulses provided they do not flutter.

A serial/USB adaptor will work fine for computers not equipped with a COM (serial I/O) port.

You can input up to 4 sensor signals through each COM port.   Connect your sensor(s) output to CD, DSR, CTS and/or RI of your COM port(s) and connect the return to GND.   You can take power from RTS, but be careful not to over-load it.

Looking at a COM port on the back of your computer you should find the pins arranged in a D shape   eg.

      1   2   3   4   5
        6   7   8   9

9 pin port

  as indicated above
        Pin 1 = CD, 6 = DSR, 8 = CTS, 9 = RI, 7 = RTS, 5 = GND

25 pin port

        Pin 8 = CD, 6 = DSR, 5 = CTS, 22 = RI, 4 = RTS, 7 = GND

Pins CD, DSR, CTS and RI are reliable, but their results can be influenced by the applied levels of voltage, current and line resistance.   The COM port will normally output about 9v from RTS, and it should accept 9v input signals.   The author recommends 5v, and a minimum of 3v, with less than a 3k resistor on RI.

Since Beltower v12.03, input on the RI pin is completely reliable.

Call Switches

Call switches should be 'push to make' switches which can be constructed from components with a resistor to limit the current, and mounted in a block, for foot or elbow operation.   They can be connected directly to unused pins of a COM port.

On the other hand, switches are not absolutely necessary, if someone else is available to use the keyboard.

Software Input Modes

Beltower can accept bell signals in any one of 5 modes:
1, Every signal    2, Twin signals    3, Single signal with delay    4, Serial Interface and    5, MIDI.

Input signals can be verified using Beltower's diagnostic facility.

Call switch signals should always be applied to individual pins of a COM port (or using the keyboard).


While every effort is made to ensure the accuracy and reliability of this information, it is given in good faith and the author cannot be held responsible for any loss or damage arising from the use of any information provided.
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