Step 1: The Sensor
Thus this sensor is ideal for measurement where the result is to be interpreted by a human. A multimeter in it volts range will show the temperature with an offset of fifty. Zero degrees Celsius will result in 500 mV. 100 degrees Celsius will result in 1.5 volts.
Since I was making this to investigate the temperatures in and around a refrigerator, in order to troubleshoot the making of ice cream, I decided to encapsulate the sensor in plastic, it being easier to manage than metal.
The downside is that it might not resist heat very well, as the plastic tube becomes soft if immersed in boiling water. The plastic does not conduct heat very well and so the sensor takes some time to respond to the applied temperature.
If the above points are important to you, there is another instructable on the same subject: http://www.instructables.com/id/Waterproof-a-LM35-Temperature-Sensor/
Step 2: Gather materials
Microchip have a very generous policy towards hobbyists and they send you samples of anything they make just for the asking.
Step 3: Solder wires
The writing tip was removed from the refill and the tube washed out with some solvent.
Step 4: Enlarge the end of the tube
Step 5: Warning!
Fire is hot. It will burn you. The awl is a sharp instrument, and if you poke yourself with it might hurt you. If you heat it and poke yourself in the eye with it you will lose the vision in that eye.
So, regardless of what they say over there at MAKE, do not do any of those things. Stay safe.
Step 6: Thread wires through tube
Identify the wires at the other end and label them now, because, once the sensor is fixed inside, it is virtually impossible to tell them apart. It would be a good idea to measure and make sure no short circuits exist.
Using the diode check range of the multimeter, the three leads will either show 'open' or as a diode. A reading of zero ohms should be cause for consternation.
Step 7: Seal it in
Overheating the sensor might cause it to be destroyed, so it might be a good idea to check its functioning after this step.
Step 8: Extend wires from the other end
Paper clips bent into a "V" make great plugs to go into the sockets of a multimeter. Three AA cells in a four cell holder were used to power this sensor.
Step 9: Calibration
Immersing the sensor in a mixture of crushed ice and water gives an output of 500 millivolts, thus proving that my meter is accurate. Actually, the reading fluctuates a bit, this exact zero reading needs a bit of work with crushing ice and stirring it up to get to.
For occasional use, this sort of jury-rigged arrangement can be made to work. For a more professional instrument, an auxiliary circuit to subtract the .5 Volt offset could be incorporated, thus enabling the meter to show the temperature in degrees Celsius directly.
But that will be another project, all by itself.