This is what you'll need:
- Styrofoam box big enough to fit your yogurt jar
- 8 Ohm 25W power resistor (USD 3.80 at aliexpress)
- 12V 2A power supply (got mine from an old laptop)
- Arduino Nano board (USD 11 at dealextreme)
- 12V relay module for Arduino (USD 3.40 at dealextreme)
- DS18B20 thermometer (USD 2.50 at dealextreme)
- 4.7k 1/4W resistor
- Heat sink or piece of scrap metal
- Some wiring
Since I got the Styrofoam box and power supply for free, the total amount I spent on this was less than 21 bucks!
Step 1: Wiring the box
In order to better dissipate this heat I screwed the resistor to a piece of scrap metal sheet I had lying around. A better option would be a heat sink+fan from an old computer, but I don't have one right now...
One wire from the resistor connects to the negative of the power supply, the other one connects to the C (center) connector of the relay. The NO (normally open) connects to the 12V of the power supply.
The 12V relay module has 3 pins: Vin goes to the 12V, GND to the negative and data goes to Arduino pin 3. This module has an optocoupler, which protects the Arduino from any interference from the relay.
The Dallas DS18B20 thermometer also has 3 pins (look for the data sheet). All 3 connect to the Arduino: 1 goes to GND pin, 2 to pin 2 and 3 to +5V pin. Use some thin wire, soldering and heat shrinking tube to connect the thermometer to the Arduino. Note that you need to solder a pull up resistor (4.7K) between pin 2 and 3. Position the thermometer in the box with some tape.