Overview

1. Soldering iron and solder
2. Single-edge razor blades
3. Wire cutters and needle-nose pliers
4. Volt-amp-ohmmeter (multimeter)
5. Heat gun (for shrinking heat-shrink tubing)

Part 1. Parts and Supplies

Materials

1. Flexible double-stranded wire or thin coaxial cable (here we use 2 m lengths per probe). Waterproof if needed.
   
2. Thermistors. We use BC2299-ND 10k ohm thermistors from Digi-key.com, which cost about $2 US each.
   
3. Resistors. We recommend precision (0.1% or less) 10k ohm resistors, for instance the RC55 Series metal film resistors (Part # 66-RC55LF-D-10K) from Mouser Electronics, which cost about $1 US each.
4. Heat shrink tubing (about 1/4 inch or 6 mm expanded), which is often available from All Electronics, a surplus and discount supply.

Part 2. Construction of thermistor temperature sensors

1. Cut the wire or coaxial cable to the desired length of your probes, keeping in mind that you will want to bury a few small loops of the cable nearest the temperature-sensing end at the same depth of soil to reduce the influence of temperature gradients traveling along the cable. Also keep in mind that a portion of the cable will be dedicated to a connection to a datalogger or other reading device.
   
   
2. Strip both ends of the wire, separating the two insulated wires from each other and stripping those ends to about 1/4 inch (5 mm). If you are working with coaxial cable (as shown to the right), strip off the first layer of insulation and open the braided shielding near the insulation to pull the inner core through.

3. Place a small length of heat-shrink tubing over the lead of the thermistor and then make a connection to one of the stripped wires by twisting the metal together. It is important to make a good mechanical connection before soldering.
   
   
4. Solder the connection, making sure that you do not heat the heat-shrink tubing to the point where it starts to contract significantly
   
   
5. Clip the soldered junction to 1/8 inch (2 mm) and fold the junction so that it is parallel to the wire and thermistor lead, allowing you to slip the heat-shrink tubing over this junction to prevent short-circuits.

6. Heat the heat-shrink tubing with a heat gun (shown below) to shrink the plastic until it forms over the junction. The tubing may not shrink completely, but will conform to the shape of the junction and thus will not slide up or down.
   
   
7. Connect the other lead of the thermistor to the other wire and solder the junction. Trim the connection as with the first connection, to about 1/8 inch.
   
   
8. At this point, you may want to cover the thermistor and wire with water-proof silicon sealant.
   
   
9. Slip a length of another section of heat shrink tubing over the entire assembly (you may have to start from the other end of the wire). If you have silicon sealant on the connection, you may want to force more sealant into the heat-shrink tubing.
   
   

10. Heat the heat-shrink tubing to contract around the probe and wipe off any excess silicon sealant. Set the probe aside for the sealant to cure.
    
    
11. The finished probe should look something like the lower photo to the left, small and compact. If the probe is to be used for air and shielded from the elements, you can partially expose the thermistor, only extending the heat-shrink tubing to the leads, in order to improve the response time.

12. Cut and strip both ends of three additional wires with three different colors of insulation. We like to use red for the positive power connection, black for the negative or ground connection, with an additional color (blue in the photo to the right) for the signal. The wires should be about 6 inches long (15 cm).
    
    
13. Connect the positive wire (red) to one of the probe wires, it doesn't matter which.
    
    
14. Connect the negative or ground wire (black) to one of the resistor leads.
    
    
15. Connect the signal wire (blue) to both of the remaining bare wires -- the other probe wire and the other end of the resistor.
    
    
16. Solder all connections and trim.

17. Slip a piece of heat-shrink tubing over one of the connections and shrink to prevent short circuits.
    
    
18. Use the heat shrink tubing to secure the other connections, laying the resistor parallel to the probe cable. These connections can also be filled with silicon sealant to prevent water entry.

19. The ends of the three wires should be separately twisted and then "tinned" with solder to keep the strands together. This is particularly important if they will be inserted into screw terminal blocks where repeated pressure will damage individual strands. Ends of the tinned wires should be trimmed to about 1/4 inch (5mm) in order to prevent short-circuits near the screw terminals.
    
    
20. The finished product can be neatly coiled and bound for transport into the field.