Details
The relay coil must be rated for 12 VDC
and shouldn't draw more than 50 mA or so with the transistor shown.
The relay contacts must be rated for whatever voltage and current the load
to be controlled draws. I've wired mine to replace the thermostat
in my fridge (actually it's an up-right freezer). Physically, I didn't
run the wires to the thermostat inside the fridge, instead, I spliced into
the appropriate wires on the back of the fridge. I did this
cause I wanted the fan inside running constantly. Downside is that
the relay contacts need to be beefy to handle the compressor current which
can be quite high at startup. Check the schematic for your fridge
to identify the correct wires if you go this route (schematics are usually
pasted to the back of fridges). A simple alternative
is simply to wire the contacts of the relay in series with the hot 120
VAC wire to a receptacle you can then plug the fridge into. Instead
of a conventional relay, a solid state relay can be used.
The controller has very little differential (difference in on and off temperatures). A differential is needed to prevent the compressor from cycling on and off too frequently which will shorten it's life. To add differential, I buried the thermistor in a walnut-sized piece of duct-seal and placed it away from the outlet of the fan inside the fridge. This results in about a 5 degF dT and decent cycles periods.
The controller will need to be calibrated. The easiest way is to put the thermistor into a cup of water at the temperature you want the fridge to turn on at and twiddle with the pot until the LED turns on. Mark the location of the knob attached to the pot. in case it gets bumped or, if you use a board mounted trimmer pot instead of a panel mounted one, seal its setting with a bit of hot melt cement. Another way to calibrate it is to look up the resistance of the thermistor at the desired setpoint temperature, temporarily wire a resistor of this value place of the thermistor and adjust the pot until the LED turns on. The thermistor page has data on the resitance vs. temperature of the Radio Shack thermistors I use as well as info on how to build a temperature probe.
Going
Further
The 50kOhm pot is a bit coase if you wish
to have various set points. For finer control, wire a smaller pot
in series with the big one or, better yet, use a smaller value pot
in series with a resistor of suitable value. Another way to have
several setpoints is to use a rotary switch to select one of several pots
or fixed resistors arranged as a voltage divider. Here's a diagrams
showing these sort of dividers.
It's left as an exercise for the reader
to calculate the resistances needed..