Tower bell optical sensors
This section describes a tower bell sensor design that uses infrared photocells to detect when the bell would normally ring and to signal this to Abel.
There are three components in the optical sensor: the sensor head itself, which is mounted on the bell frame next to the wheel; reflectors mounted on the wheel; and an interface box which contains the electronics necessary to connect the sensor head to the PC running Abel.
Tower bell sensor circuit
The circuit diagram for the sensor head is shown above. The sensor comprises an infrared light emitting diode (LED) with its associated series resistor, and an infrared phototransistor. These components are built into a small box as shown below, with the optical components arranged so that the light from the LED will shine out of one hole drilled into the case, and so that reflected light can shine on the phototransistor through another hole in the case. Within the case itself, the components should be shielded optically. You may like to put some red plastic over the holes, to help prevent spurious signals due to stray reflections.
Suggested types for the optical components are given in the diagram above. If these or similar components are used, then the 150 ohm series resistor will allow a supply voltage of up to 9 volts to be used; however, with the interface circuit described below, the sensor will work quite adequately on just 5 volts.
Arrangement of optical components within sensor head
The sensor head is connected to the PC running Abel via the interface circuit shown below. This circuit is required both to amplify the signal from the sensor, and to stretch the signal pulse so that it is long enough for Abel to see it. The variable resistor VR1 will allow the length of the signal pulse to be adjusted from approximately 20 mS to about 540 mS. Or, you can use a fixed resistor to give you a signal pulse of about 50ms.
Tower bell interface circuit
One sensor head and associated interface circuit will be needed for each bell that is to be connected to Abel. It is most convenient to build all the interface circuits into a single unit that can be sited in the ringing chamber near the computer. A single cable run with the appropriate number of cores can then be run up to the belfry.
The output of the interface circuit can be connected to any of the four input signals described in Connecting bells using the serial port, and the sensor earth should connect to the GND pin. Then configure Abel as described in How to configure Abel to use bell sensors and check that the program can detect the signals from your sensor.
The diagram below shows where to mount the sensor head with respect to the bell: it is usually positioned on the frame next to the wheel, on the pulley side of the bell. You can use either one or two reflectors. If you use one reflector, you mount it immediately in front of the sensor when the bell is down and hanging level; in this case, you will need to configure Abel to use software delays to ensure the bell strikes at the correct time. If you use two reflectors, the diagram shows the approximate positions to mount them. Aluminium foil, or the yellow highly-reflective material used by cyclists is suitable for reflectors. The position shown for these reflectors is only approximate: you may find that the positions need to be adjusted slightly to correct any odd-struckness.
Position of reflectors and sensor head