During the various seasons, fruit and vegetables can at times be purchased cheaply and or relatively easily grown, but how to store this fresh produce for later consumption. Canning and freezing are possible solutions, however, this can be relatively time consuming, and with freezing continues to use electricity over time. Dehydrating food is an excellent alternative, particularly if the excess fruit or vegetables coincides with copious sunny weather. Unfortunately, available sunny weather does not particularly coincide when our area has surplus fruit. Fruit season in our area corresponds with the "wet season", that is, tropical thunder storms and high humidity. This lead to the idea of making a food dehydrator/dryer that could be used when rainy/humid weather made the "natural" sun-dried method impractical.
The teardown of old electronic equipment (see the Teardowns Section for examples) often provides some cooling/circulation fans, and particularly with older computer equipment, sometimes components such as peltier devices. This provided the salvaged equipment to form the basis of a DIY food dehydrator. A "quick and dirty" survey of commerical food dehyrators via the web, shows these consumer machines to vary in price from about $100 (basically for an enclosure that has fan forced heated air flowing past the food to be dried) up to $400-$500 for basically the same thing (although with a few more heating options/better control, larger size, better circulation of heated air to given uniform drying etc - "supposedly"). A number of reviews of various models on different sites also complained about power usage, noise, and slow drying times, which seem to be the other major difference between a "cheap" dehydrator and a more expensive model.
The electricity usage was a particular concern, as it seemed counter productive 'environmentally' trying to produce and store your own fruit/vegetables for later use by consuming large amounts of electricity, and the concurrent carbon dioxide/pollution. It appeared the major 'problem' with the commerical dehydrators is that the heated air is not recirculated to any large extent, but passed through the dehydrator in effect wasting electricity heating the room within which the dehydrator is located. The difficulty being if the heated air is recirculated within the dehydrator, it eventually becomes saturated with moisture and drying of the food does not eventuate. This lead to the idea of using a peltier device to provide a cold surface onto which moisture could condense and be removed, allowing the same heated volume of air to be recycled as much as possible. Utilizing a PIC microcontroller with temperature and humidity sensors, the additional energy usage by the peltier device is offset by energy savings due to decreased heating requirements.
The section in Learning and Component Testing gives details on my various experiments/trials and tribulations with using power supplies and other components/IC's for this project. In particular the use of the peltier device for cooling/condensing of moisture, thermistors for monitoring temperature and the DHT-11 humidity sensor.