Those fortunate enough to swim in a 50-metre pool don’t have to count very many laps in order to cover a reasonable distance.

For example, just 20 laps means that you have swum a kilometre. But even then, as you plough up and down the pool, it is pretty easy to get distracted and lose count. Some people cope with the problem by swimming five laps freestyle, five breast-stroke, five back-stroke and so on.

Here's what it looks like close up. The box at left is merely the lap sensor - hit the switch and it sends a burst of air via the clear hose to the main box, right. This actuates a microswitch which in turn increments the count by two. You can set the count for odds or evens, depending on which end of the pool you mount the unit. The air hose can be quite long.

The problem is worse if you’re swimming in a 25-metre pool (as many top-level swimmers regularly train in) and much worse if you’re swimming in your home pool, which may be only 10 or 15 metres long. For a 10-metre pool, you need to do 100 laps to cover a kilometre.

Believe us, trying to keep track of that many laps in a home pool while you swim back and forth is practically impossible.

This is where our Pool Lap Counter comes to the rescue. It will display the number of laps you have completed on a 2-digit or 3-digit readout, so you can let your mind wander, do mental arithmetic or compose your new symphony while you swim up and down.

The Pool Lap Counter consists of two small plastic boxes. One, the "main" box, contains the Picaxe counter circuit and 2-digit readout, while the other contains a large air-switch pushbutton which connects to the main box via a thin air hose and actuates a microswitch when pressed. This is to avoid an electrical connection (even in a battery-powered, low voltage device) around the very damp(!) chemical-laden pool environment.

If you swim more than 99 laps you will have to add 100 to the count or add the third 7-segment LED display.