Discussion

The main component of Project Swim-SA is the Piezoelectric Transducer which supports a weight. In the scenario of the box oscillating up and down , this motion will cause the weight attached to the transducer to push against the plate. When this occurs, the piezo element generates a small voltage output.

The piezo element is the main component of the swimming pool alarm as its function depends on its ability to detect the movement.

The signal from the piezo detector is amplified by IC1a and filtered so that only frequencies below about 2Hz pass through.

The output from the filter only changes for slower movements.

The signal is squared up by the following Schmitt trigger (IC1b) and has an adjustable threshold to allow setting the sensitivity to pool movement.

The Schmitt trigger output is a low frequency square wave which changes with the piezo detector output. The signal drives a charge pump which requires at least two pulses from the Schmitt trigger before the output from the charge pump is low enough to trigger the following timer. This requirement before triggering the timer reduces the likelihood of false alarms.
The timer produces a high signal for about 50 seconds which drives the siren driver (Q1) and siren. The siren should be sufficiently loud to attract attention.

The circuit is housed in a sealed box to prevent water getting in.

However, the siren is exposed to the outside air so that it can be heard. It has also been designed to be as loud as possible to attract attention.

After referring to some calculations, the cover has been designed to look like a light house as based on theory this design would help amplify the sound as it reaches the air as sound is channelled through a small tunnel will maximize the sound output level.

The user is only required to switch on the device and place it at the source which he/she would be monitoring. (Source = Swimming Pool)