That way, you can just crimp the wires
under the screws.
I also picked up a plastic box about
15x10x5 centimetres to put everything
into, and 100 feet of two-conductor 22AWG shielded cable. The shield
serves as a third wire, wrapping the two
conductors and screening them from
nearby electrical noise (just in case). At
the sensor end it connects to the GND
leg, and at the other end it joins all the
other cable shields and connects to the
sensor GND to really stamp out
any threat of electrical noise getting
into the signal.
Assembling the Pieces Install the
car meter thermometer (the display) in
the lid of the box, together with the rotary switch and the on/off switch. Decide
where the box is going to be mounted
onboard. Having already chosen where
you want the various temperature sensors, measure out the cable lengths required to reach those places. Cut. Pare
back the outer insulation and strip the
ends two to three millimetres.
This is where you need a fine-tipped
soldering iron because going to
solder three fine wires to the three thin
legs of each sensor. The DS18S20
like excessive heat. If you solder a
wire to a leg in less than a second, get
help from someone who can. tricky
if you know the technique; easy if
you do. Or clamp an alligator clip across
the three sensor legs to absorb some of
the heat from the soldering iron.
To make sure the sensor legs
touch afterwards, I sleeved the leg wires
with heat-shrink, and then encapsulated the whole end of the cable in an outer sleeve of adhesive-lined heat-shrink.
As this tightens around the sensor and
cable, the adhesive melts, forming a watertight seal.
Now time to run the cables to the
various points to be monitored. Make
sure you label the cables where they
converge at the display box! Connect up
the circuit as per Figure X. When
all connected, close the lid, and tidy up
the cables with tie-wraps/zap straps.
When attaching a sensor to a metal
object (like a stuffing box or manifold) I
used a stainless steel hose clamp.
over tighten, as the sensor is only plastic. You could use a dab of silicon, but
the heat flow may be slower, although
under normal circumstances you
looking for rapid temperature changes,
but steady-state values. The system
responds in seconds rather than milliseconds, and has an accuracy of about
Total cost for the completed system, wires included, was less than $50.
Troubleshooting Suspect the obvious first. Check there is +12 volts at the
toggle switch, and +5 volts at the sensor
output wire. Are all the +5-volt wires,
and the Ground wires firmly (but separately) connected? I used a small terminal block to do this. Avoid twist wire
connectors/marrettes. They can come
loose in a vibration environment.
Next check the moving two
switches. Rotary switches encapsulated
in plastic like being overheated
during the soldering process, and may
jam or skip a position. The most likely
cause of a weird temperature reading is
sampling the wrong sensor. For
that reason, best to connect the sensors one at a time, then test by touching it to your skin and watch the display
change, before moving on to the next.
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