The final design settled on utilised the 'Piezo' and 'Bell' digital signals that feed the system LEDS. These two signals are translated into more useful controls though the application of some combinational logic.
The function of the logic can be represented in the form of logic tables as below.
Here the 'Bell' and 'Piezo' signals are effectivly ORed togterth thus enebeling the 555 whenever any of them are true.
The second logic group ensures that the 555s output is modulated whenever the'Bell' and 'Piezo'
signals occur at the same time and effectively produce an AND function of the two signals.
While the 'Bell' output is false, a pulsed tone that is generated in sympathy with the true/false state change of the'Piezo' output and a changing Hi/Lo tone is generated while the 'Bell' output is true.
Once breadboard tests where complete this was translated into a strip board design as below:
The final design was then constructed so that further tests could be carried out, note that
the components that determine the final output frequency of the tones have been mounted on
solder pins as to allow experimentation of the final values.