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Wednesday, 15 April 2015

Dehydrator MK 2

After success with Dehydrator MK 1, I decided that the cardboard box would probably give out after a while, but before that happened the supply of 100W light bulbs would run out. Several bulbs failed and they are now difficult to buy, so I decided to build a more solid solution.

I'd need a heat source, a fan, and something to control it all. In the end I added a temperature sensor and a humidity sensor, and attached an LCD panel that I had hanging around. An Arduino Uno controls everything.

The power for all the circuitry comes from a slightly modified power supply PCB from an old video recorder. I modified a couple of supplies to get 12V and 5V and added voltage control from one of the Arduino PWM ports so that I could control the fan speed using firmware.

Video recorder PSU PCB

The electronics is in a separate area to the drying chamber, with a hinged front panel to allow access.

The arduino and the display are on one panel:

The fan and power supply and the power resistor controller are on a second panel:

The heat source I settled on was a 200W wire wound resistor that was connected across the mains supply using a triac with an opto isolated trigger from one of the Arduino GPIO lines. The algorithm works on a 30s cycle, altering the duty cycle of the power drive between 0% and 100%, in order to get different power outputs from the power resistor.

I also added an I2C temperature and relative humidity sensor so that I can attempt some form of automation in the future. All the power settings and readouts from sensors fits on the LCD display:

The trays fit in from the top (something I may change in a dehydrator MK 3, as it's a bit fiddly but it does make the air flow easier) and are wooden frames with plastic mesh on them.

I've dried apple, tomatoes and mango in the drier so far and the results are pretty good, I can't tell dried mango from the dehydrator from shop bought dried mango.

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