I've a small project for the fridge. When I first installed the fridge I added a small electronic thermometer purchased on ebay. They are dirt cheap but with two main disadvantages. The readings are in Celsius and not Fahrenheit and they are battery operated. The Celsius readings are easy to get used to especially since this is for a fridge and the Celsius freezing point is zero. The batteries are a concern because they don't last long enough.
I'll start with the initial installation.
The temperature probe is on the end of a wire. I ran the wire through the grill then EVER SO CAREFULLY used a screwdriver to create a gap through the foam insulation along side the fridge cooling lines where they enter the body of the fridge. Then carefully side the temperature probe through this gap. The foam closed up and sealed the gap after the screwdriver was removed.
Used foil tape to cover the probe wire inside fridge. I made a slight adjustment after this picture. I moved the probe farther from the side so wall it would hopefully have a more accurate reading.
Reinstall the grill then cover the back of the thermometer with double sided tape and stuck it on the front grill.
How it looked when done.
That was 2 years ago. First problem was the tape didn't hold so I ended using a zip tie to hold it place. Second problem was battery Life. It was too short. Time for an upgrade.
Unmount the fridge from the van and haul into the shop. Remove the thermometer body from the grill. I don't want to mess with removing the probe from inside the fridge so I just cut the wire. Use a diamond coated file to carefully cut through the plastic grill.
Not done yet. I had to cut one more louver after this picture and even up the cuts on the right side.
Mount the thermometer in the grill. Looks good.
Now how to power it without batteries. First off check how much power it needs. I thought the the two batteries in the back of the unit were most probably in series. This takes LR44 batteries so that would give it 3 volts. Checked my assumption with a voltmeter. I was wrong. The batteries are in parallel. The unit uses 1.5 volts.
So I need to convert 12 volts to 1.5 volts. This is a LM317 Voltage Regulator. It has a range of 1.25 V – 37 V. It's voltage output is determined by the ratio of the two resistors in the picture.
When hooked up on breadboard it outputs 1.57 volts. Close enough.
Solder wires in place of the batteries.
Build the circuit and test it out with the thermometer. It's reading "Lo" since the probe is hasn't been reconnected.
Install back on the fridge. Here's Fox handing me the lead from the temperature probe so I can join the wires back together. I soldered them to get the least resistance in the splice.
The circuit board has to be insulated from the surrounding surfaces so I'll build it a small box. Keep my left over acrylic pieces for times like this.
Secure the circuit box in place with foil tape. When the LM317 is in operation it will convert the excess voltage to heat which must be vented. Since the thermometer draws just a little current there shouldn't be that much heat in this application but I've still left a couple of vent gaps in the top of the box.
Use taps to get power for the circuit board from the fridge plug.
Mount the fridge back in the van.
Let it run for a couple hours.