A Replacement 12V Fridge Control
This is the design of circuit of the fridge controller made on Fritzing for a more friendly format. The actual design file is available for download at the end of the page, along with a parts list.
Anyone is free to copy and build a similar control but any constructor will need to be competent at adapting it to their own needs and ensure its reliability. Just because it worked for me doesn't mean it works for anyone else.
It's based round an ESP8266 microcontroller module which is extensively used for control of home automation systems and incudes a wifi interface. An Arduino Nano and Uno have been used successfully to run the system with the omission of the wifi.
An overview of the system is as follows:
- Power is supplied to the system as 12V, the raw batter power is supplied to the relay contacts where needed, a buck convertor is used to reduce this to 5V for the microprocessor and relay electronics. The 3.3V regulator on the microprocessor board is used to supply power to the I2C circuits.
- The temperature sensors are mounted inside the fridge compartment and were connected via the existing wiring for the thermostat which ran at 12V. A 3.3V regulator was fitted alongside to power them correctly.
- I2C is used to communicate between the microprocessor, the OLED screen and a parallel digital output chip, a PCF8574.
- A rotary switch is used to provide input to the microcontroller for menu settings displayed on the OLED display.
- The relay board is used to switch the main power outputs of the controller at 12V, and also emulate the thermostat operation, switch the speed control resistor.
For the motor speed control, a fixed resistor is shown but in practice this has been replaced by a variable resistor to allow different speed settings. Automatic switching of the speed was considered for time of day operation and power reduction but not implemented.
The code for the microcontroller is held on github and this incudes more description of the operation, however this is a short description.
The controller is expected to be powered at all times and provide a local webpage to indicate it's status with the appropriate microcontroller.
Power to the fridge is supplied via the fridge via the relay board under the control of the microcontroller and an external 12V signal can be used to trigger this. In practice, a switch was used to isolate both the controller and fridge but the option is there if required.
Settings for fridge operation are displayed on the OLED screen, as is the current operating status. The screen blanks after a couple of minutes of non-use, illuminating again when the rotary switch is operated.
As is typical with fridges. the thermometers measure the temperature inside the fridge and the logic activates the fridge compressor when the measured temperature is too high, and cuts off the compressor when the temperature is judged to be OK again. There is some hysteresis in this operation.
In addition, the microcontroller runs two sets of fans, one internal which it activates to even out the temperature inside the fridge to prevent freezing, and an external one to remove heat from the system, which is run for a while after the compressor stops to remove residual heat from the system.
If the temperature is measured to be too low and in danger of freezing stored goods, the compressor is cut off until the temperature is safe again.
At it's core though, the fridge controller is a thermostat which activates the compressor circuitry when the fridge internal temperature is to high, and turns it off when correct, the rest of the circuit and software is to make that more efficient and easier to use.
