Our first attempts at controlling temperature began with frozen plastic bottles being dropped into the fermenting liquid (called must). This worked to a small extent but required a lot of effort and lots of bottles being re-frozen. It was useful to prevent the temperature becoming excessive but not effective to maintain low temperatures of 15-20 C or even maintain a consistent temperature. And there was the associated risk of contamination by leaking bottles or frozen debris from the freezer.
We then slowly acquired a number of previously discarded refrigerators and freezers. The internal shelving is removed and most of it recycled except for doubling up on the bottom shelf for reinforcement. Sometimes the containers may weigh in excess of 50 KG.
We purchased some inexpensive timers like the one above. The must is allowed to begin fermentation and then the fridge/freezer activated. Once the internal temperature of the cooling unit is close to the desired level the timer is activated. By trial and error the desired run time is ascertained. initially it may be 15 minutes every hour and as the activity of the yeast diminishes it may run for only 15 minutes every 3 hours. If the temperature gets too low the rest time is extended and if too hot the run time is extended. A bit fiddly but after a while a rhythm develops. The attraction of the timers at the time was their cost - 2 for $10.
The downside of the timers was that we also needed to buy these min/max thermometers with a long lead on the probe. Sitting the digital readout on top of the fridge/freezer and the probe inside the unit or even the must meant the situation can be assessed at a glance and it was also easy to look at the temperature fluctuations during the night. Accuracy was much better than frozen bottles.
But seeking perfection we came across a cheap ($20) digital temperature controller. The downside was that it runs more frequently attempting to maintain the temperature setting. It also has an annoying manufacturing fault. When plugged into a power point the readout is upside down. By using an extension cord this is alleviated.
Now we finally found a better controller ($20 also). The readout is larger (great) and there is setting for temperature variation e.g. The ideal temperature for Cider fermentation is 13-17 C. The controller is set for 14 C with a upper variation of 3 allowing the must to rise to 17 C before power is restored to the fridge/freezer. Bliss.
The downsides of this unit is it comes with only the probe and the power the connectors must be purchased and wired in. Not a complex task but a bit of care is required to make sure no bare wiring is exposed and the leads are secured well to prevent pull out. Fortunately we had some male plugs from old recycled appliances and only the female plug needed purchasing. It also has a short probe lead.