Building an LM35 sensor assembly

The LM35 is very susceptible to cable capacitance and mildly suceptible to induced noise. The simplest approach is to use a bypass capacitor and a low-pass filter. These components must be installed at the sensor end of the cable. There are many ways that this could be done. This page illustrates the best technique that we've found so far. This page assumes electronic soldering skills and equipment.

Components:

1. Small heat shrink tubing
2. (2) pcs medium heat shrink tubing
3. Large heat shrink tubing
4. Circuit board
5. 1uF capacitor
6. 0.1uF capacitor
7. 75 Ohm resistor
8. LM35 sensor
9. Sensor cable
10. RJ-11 connector

The sensor cable can be any four-conductor wire. Stranded conductors are preferred, and the wire gauge should be around 24awg. Standard telephone or alarm hookup wire works fine. If you use solid conductor wire, be careful not to bend the cable too often or sharply at the ends, and make sure that you use RJ-11 connectors that are specifically intended for solid conductor wire.

The small heat shrink tubing should be the smallest that will fit over the cable jacket. The large should be the smallest that will fit over the assembled circuit board. The medium should be halfway between. The medium heat shrink is not shown in the photo.

The circuit boards are double-sided with plated-through holes. They are cut from a general purpose prototyping board. The layout is important: three rows of four connected pads, and one row of three connected pads at right angles.

Prep

Strip the sensor cable outer insulation back about 3/4". Cut off the yellow conductor. Strip the remaining conductors about 1/8" and tin if they are stranded wire. Slide the small heat shrink and one piece of the medium heat shrink onto the cable.

Re-bend the leads on the 0.1uF capacitor (yellow) to a .2" spacing.

Check the circuit board for shorts between pads.

Step 1

Solder the cable to the circuit board. The cable is soldered from the 'wrong' side.

Step 2

Insert the components as shown and solder. Test the board for shorts.

Step 3

Insert the LM35 sensor. Using clip-on heat sinks, solder to the circuit board. Bend capacitors down flush to the board. Test for shorts.

Step 4

Crimp an RJ-11 connector on the other end of the cable. Insert into NFCS sensor input that's configured for an LM35 sensor and verify operation.

Step 5

Slide small heatshrink tubing up to circuit board and shrink with heat gun. Straighten board so that it's in line with the cable. Dip in sealant and let dry.

Step 6

Slide second piece of medium heatshrink over sensor as far as it will go. Shrink with heat gun and pinch end.

Slide large heat shrink over circuit board and shring with heat gun.

Slide first medium piece of heat shink (on cable) over small piece and shrunken large piece as far as it will go and shrink with gun.