Before my modification, I used two timers to distill with the stock unit. The amount of water to fill the collection vessel is added to the boiling chamber as well as a small amount of extra water that will remain in the unit after the vessel is full, ensuring the holding tank does not run dry. I would set a timer for 25-30 minutes after first turning on the unit. This allows the water to heat up enough to distil about 50 ml, called the foreshot in moonshining terminology. This shot is comprised of all the compounds that evaporate leading up to 100 deg C, such as many volatile organic compounds (ie Cl+). If I were distilling ethanol, this would be alcohols (such as methanol), ketones, aldehydes, and other byproducts of yeast fermentation. Since I am distilling only for the water, the foreshot is tossed down the drain. Now a clean vessel can be placed under the drip to collect the distilled water. This is where another timer is set to the amount of time it would take to fill the vessel but not run the boiling chamber dry. Fed up with this method for being prone to error (miss the last timer, ruin the water if not the heating element or circuit breaker), I set out to automate the process and make it user-friendly and safe for anyone in my household to operate.
This is where I salvaged a few pieces from the control box of a temperature/humidity controlled terrarium that I developed. This includes two Crydom D1210 120 VAC relays, each controlling a 2-output wall outlet, a wall outlet that's always on for the converter that powers an arduino, and an electronic rotary dial for input. The dial was removed from a clock tower toy that was left at my apartment's refuse area. Everything is connected to a home-built shield that plugs into the arduino.
The dial input varies the amount of time the heating element is to be turned on and when the fan starts, as the fan is unnecessary until the water begins boiling. This approach allows different sized vessels to be filled without having to read from a list of times for each vessel's volume and manually set two timers. User input is required (turning the knob any direction) to turn the unit on. This prevents a power outage from powering on the unit when the power returns. This still has a 250-watt heating element and can produce 1 liter per hour. I routinely distill over a gallon a day.
Stillduino-v1 code can be found on GitHub.