Batteries are required to power your fan in the field which can be cumbersome and expensive. An on/off switch of some type will help conserve batteries. A simple momentary push button switch may be the most elegant solution as a strong fan only needs to be turned on for a few seconds at a time. Rechargeable NiMh batteries can be used to soften the financial blow caused when the fan is accidentally left on. Multiple cells can be wired in series to achieve the required voltage. A typical high quality setup would be 8 AA cells with a momentary switch. Other more expensive battery options are 12v NiMh clusters used in RC cars/planes and cordless drills. The fan controller shown here uses (2) 9v cells to keep the controller small and lightweight and a voltage regulator to control the current. The controller also features a timer circuit which automatically shuts the fan off.
This is the revised schematic for the fan controller circuit wich eliminates the problem of unintentional battery drain. It uses two reed relays to isolate the timer circuit from the power source. The relays are small enough to fit inside the pictured project box alongside the batteries and the additional resistor will fit on the main board. It is also possible to use smaller switching components like transistors in place of the relays. This circuit uses a double pole (DP) switch which triggers the two separate circuits at the same time providing 12v at the fan. The toggle style power shutoff switch has been eliminated from the design but is easy to add next to the battery if desired. Large fans running at 12v can easily vent or mix in 2-3 seconds, 26 seconds is a little overkill. This circuit features a bit of circuit protection so crossing your output wires on accident shouldn't cause any harm.