Swimming pools are dangerous places, especially for toddlers – as the table above right chillingly shows. And the pool in your own back yard is certainly not exempt; in fact, statistics show that’s where more than half of all toddler drownings occur.

Even while taking the photographs for this article, with mother millimetres out of shot and grandfather (Ross) in front taking the picture, 14-month-old Keira (who cannot swim) needed no prompting to attempt to get in the pool – not once but again and again.

While swimming pools these days must be fenced off, there is always the possibility that a toddler will find a way in. That can be as simple as a gate not latching properly or a determined youngster climbing the fence.

So while fences may appear to make a pool secure, they are never foolproof. A secondary defence, one that warns if someone falls into the pool, can literally be the difference between life and death.

A way to add secondary safety is with a pool alarm. The type of pool alarm described here monitors the amount of pool water movement and sounds an alarm when this exceeds a preset level.

Of course, wind can also create movement in the pool water – after all, that’s what makes waves in the ocean. The last thing you want is false alarms – remember the boy who cried "Wolf!"?

The SILICON CHIP Pool Alarm can be set to a level which ignores typical wind movement but screams its head off when that level is exceeded – ie, someone falls in.

Fig.1: the Pool Alarm in block diagram form. Pressure variations due to changes in the water level are detected by Sensor 1. Its weak output is amplified and then processed by the PIC microcontroller which controls the alarms and drives the status LEDs.

Fig.2: this cross-section diagram shows the internal structure of the MPX-2010DP pressure sensor. The strain gauge varies its resistance according to the applied load. P1 & P2 are the two port openings.

OK, let’s see how it works. Fig.1 shows the block diagram of the Pool Alarm. It uses a pressure sensor to detect sudden increases in water depth, as happens when an object falls into the pool creating waves.

The unit is built in two sections, each in a weatherproof box. One houses the sensor while a second, which we have dubbed the Pool Alarm box, houses the PIC-controlled alarm circuit. The two are connected via a 4-way cable.