Simple mechanical timers, which you can buy for a couple of pounds in every home improvement centre, are suitable for switching something on and off one or more times per day. They can be used to control a wide variety of devices, such as lamps inside or outside the house,lighting for bird cages and aquariums, sump pumps, battery chargers, etc. If you need to control something over a longer period than the standard 24 hours, you can use two timers with the second one plugged into the first one (see photos). To determine what you can do with this arrangement, you first need to determine how often the load needs to be switched. For example, if the first timer has 48 tabs the shortest ‘on’ time is 30 minutes in 24 hours. This means that the second timer will run for 30 minutes every 24 hours, so the maximum duration of a full cycle is 48 days. A device such as a charger for diving torches can be connected to the second timer. To prevent the ‘on’ time of the second timer from e...