This is what I came up with. Enjoy!
Step 1: The task
This simply screams for some micro-controller driven decision making.
Aside from a picnic cooler, the final bill od materials consisted of:
- an arduino board
(we used a pro mini 328/5V/16M, simply because they are the cheapest Arduino clone on eBay @ $2.83),
- 2 relays (5V/1ch relay modules from eBay at $0.99 each),
- 2 momentary push-buttons (stolen from colleague Klemen)
[Klemen comments: "Borrowed not stolen. One day I will have them back."],
- an electronics thermometer (we used a DS18B20, eBay @ $1.04),
- 5 resistors (see schematics, later on),
- an LCD 2x16 display module (HD44780 compatible module, eBay @ $2.26),
- a 7805 voltage regulator (eBay @ $0.90) and its two decoupling capacitors,
- an electrical enclosure box from the hardware store ($6.00 for the cheapest one!),
- many, many, many... wires (depths of supervisor's drawer).