The RV, journey trailer, and even automotive industries make heavy use of 12V fridges. Not like their bigger AC-powered brethren, these run on the 12V DC energy that’s typical in automobiles. Some also can swap to AC energy or run on propane. However a 12V-only automobile fridge is about so simple as they arrive, with that energy driving a compressor and possibly some management circuitry. Toxicantidote has an Evakool 55L automobile fridge of their automobile and it was failing on account of inaccurate temperature readings, so they refurbished the unit with some trendy {hardware}.

Many fridges comply with an working precept similar to your house’s central air-con. A thermostat merely activates the compressor if the inside of the fridge is just too heat, after which turns it off as soon as it’s cool sufficient. That’s strong and dependable, however it isn’t probably the most environment friendly technique to do issues. That isn’t a giant deal when power is reasonable and plentiful. However in a automobile, energy draw is a priority. For that cause, the Evakool 55L has a variable-speed compressor.

When the temperature is a lot larger than it needs to be — resembling when the fridge is first turned on earlier than a visit — the compressor can run at a excessive velocity. It might then gradual to develop into extra environment friendly because it will get nearer to the upkeep part. That performance necessitates management circuitry and so there’s a devoted board for the compressor. There may be additionally a board for the management panel, which lets the consumer set and skim the temperature. The defective temperature sensor (an NTC thermistor) connects the management panel board, which then tells the compressor board what to do (and in addition reads any fault codes despatched again).

Toxicantidote changed the management panel board along with his personal elements: a Raspberry Pi Pico W improvement board, an OLED show, a thermistor, a few buttons, and a buzzer. Collectively, these do virtually precisely the identical factor as the unique management panel. However Toxicantidote had hassle getting the Pico to learn fault codes from the compressor board, as a result of it doesn’t have {hardware} interrupts. In order that they added a Keyestudio Leonardo Tiny dev board that acts as an middleman between the 2.

The ultimate problem was controlling the compressor velocity. The compressor’s management board determines that velocity based mostly on a set resistance. However Toxicantidote couldn’t get a digital potentiometer working, so as a substitute they hardwired resistors for 3 speeds: off, low, and excessive. That isn’t as granular as the unique velocity management, however it’s sufficient for effectivity in most sensible conditions.

And as a bonus, this will join wirelessly to a central management panel as a result of the Pico W has a built-in Wi-Fi adapter.