exhaust flue pipe. The thermocouple
consists of nothing more than a tiny
drop of solder on the ends of two
heat resistant wires (Figure 10).
You want the thermocouple far
enough away from the fire that you
do not get much creosote build-up
and it is not wise to subject the
thermocouple to unnecessary excess
heat. Find a good location by using a
magnetic thermometer affixed to the
exhaust pipe. Pick a location that
has reasonable temperature
variations without being too hot for
safety reasons. The magnetic thermometer also will aid
you in setting up your high and low limit temperatures.
Your location may not show a range of 100-400 degrees
the way mine does, so you will have to adjust the software
accordingly. My thermocouple is mounted approximately
five feet away from the upper barrel. The temperatures
recorded here are the relative exhaust gas temperatures
(100-400 degrees), not the actual fire temperature. This
will not really matter because as the fire temperature goes
up, so does the exhaust gas temperature.
In (Figure 11), the thermocouple assembly is mounted
in the flue pipe. The thermocouple itself is not actually
visible in this figure. Slide the wire and thermocouple inside
a 3/8 inch copper tube for mounting purposes. Attach the
copper tube with thermocouple inside to the flue pipe
using a right angle bracket. Apply high temperature silicon
to the outside wire end of the copper tube to secure the
probe inside of it. With the probe mounted inside the
copper tube, exhaust gases and creosote are not directly
hitting the probe tip and gumming it up with carbon.
Mechanically this protects the
thermocouple tip, as well.
Dallas-Maxim recommends that
your thermocouple sensor wiring be
twisted to prevent electrical
interference. A simple method for
twisting a long run of wire is to
place one end of your paired wire in
a vise and clamp the opposite end
into an electric drill. Spin the drill
slowly and twist the wire into a
■ FIGURE 8 spiral. Do not over twist! Twelve
turns per inch is plenty. The
MAX6675 and thermocouple
schematic is shown in (Figure 12).
Stepper Motor Driver Board
I designed my own stepper motor driver PCB (Figure
4). This is a carry-over from an old project. You may easily
build your own PCB using a protoboard following the
schematic (Figure 13) or purchase a stand-alone stepper
motor driver chip, as listed in the Parts List. The stepper
coil drive transistors are mounted on the inside wall of the
wood project box (Figure 6) using transistor heatsinks.
Note in this schematic I sketched the power transistors
separately from the driver board for clarity purposes.
Wiring connectors are not shown in the schematic
going from the output transistor phases to the stepper coils.
A manual shut-off switch is installed for safety reasons and
to shut down the stepper motor during summer use.
Switches Stepper Motor
and Limit Switches
■ FIGURE 9
38
November 2008
The stepper motor I used is made by Slo-Syn
(Figure 14). It is a 1.8 degree (200 step/revolution)
unipolar, 5V, 1.5A motor with a 1/4 inch diameter
output shaft. Linkage arms were designed for the
■ FIGURE 10
■ FIGURE 11
