Many of my little 'summer' projects have been completed and seem to work. First - my bathroom extract system has been installed without me once falling off the ladder or putting my foot through the bathroom ceiling, second - I'm now producing tasty pizza and bread from my DIY wood-fired bread oven without upsetting my neighbours and third I've constructed four battery powered 'dew zappers' for camera lenses and scope objectives.
The following is proof: - 'Dew zapper' on the Canon EFS 18-55mm lens worked for just over 2 hours, allowing star trails and a time-lapse movie to be made, and thus exceeding the design parameters for the heater and battery.
Many thanks to Alan Sheehan B.E on IceInSpace for his excellent article and Excel Spreadsheet relating to building 'dew zappers'. Thanks to Alan I could concentrate on the difficult bits including burning my fingers with a soldering iron and 'wrinkling up' the duct tape.