Block Diagram

Analysis of the Block Diagram

The block diagram of Project SWIM-SA is shown on the previous page. The sensor itself consists of a piezo element which supports a weight. As the piezo element is attached to a floating box which floats on the swimming pool surface. Any upwards-movement of water will cause the floating box to rise, pushing against the piezo which doesn’t move as quickly due to the inertia of the attached weight. 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.

With research done on several textbooks it is fully confirmed that a downward movement of the box will not usually cause decompression of the element. This is because the floating box drops with the gravity at the same rate as the mass.

The signal from the piezo detector is amplified by IC1a and filtered so that only frequencies below about 2Hz pass through. The amplifier has a gain of 33 for frequencies below 2Hz.
As the mass on the piezo element also damps out any fast movement (again due to inertia), it reduces the high frequency response of the piezo element. Thus 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 must be exposed to the outside air so that it can be heard. It also needs to be made as loud as possible to attract attention.

By feeding the siren into a tuned port would help increase the amplitude of the siren generated. This port would be covered to prevent the device from getting wet.

After referring to some calculations actual dimensions of the port and 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 channeled through a small tunnel will maximize the sound output level.

Searching for parts being in the device for the tower for the siren is quite hard as parts are unique. But fortunately an alternative has been determined as several scrap parts of other devices such as the flange port of a light bulb and a ping-pong ball could be used. This should provide the same equivalent characteristics needed to improve the alarm.