How I control my woodstove with a computer:
I built my first "barrel stove" in 1980 and got into computers a couple
of months later. It wasn't long before I wanted to marry the two in an
effort to get better temperature control and efficiency from the stove.
It took several incarnations of both the stove itself and the control
interface to arrive at the present setup. All told, the computer has
been "in charge" since about 1985 or '86.
First the stove: The final design is a stove made from a 55-gallon drum,
using a Sotz stove kit. The Sotz kit was best of all that we tried
because it has a door that seals well, and a draft control that is easy
to adjust. We improved the assembly immensely by drilling the door frame
between every mounting hole, thus doubling the number of bolts holding
it to the drum head. This prevents most warpage, giving better
"airtightness", which is critical to the succsessful operation of any
woodstove. Unfortunately, I don't believe that Sotz is in business
A later addition was the installation of a catalytic convertor in the
stack (also from Sotz). We even had a "Magic Heat" unit in the stack for
a while, but that was removed after it proved to be troublesome due to
excessive creosote buildup. With the drier firewood supply of later
years, it might be better, but we've not tried it lately. The chimney is
a double-wall solid-pack insulated, all stainless steel assembly. If you
are installing a woodstove, this is the kind of chimney you want, as it
is rated for 2" clearance to combustables, and the liner runs hot,
reducing creosote buildup. You should not consider a triple-wall
air-insulated stack for an airtight stove, and a masonary chimney isn't
much better. We've also experimented with a barometric draft diverter,
which is a big help in controlling the stove. They reportedly also
reduce creosote buildup in the stack, due to the dilution of stack
The draft control on the stove is mounted over a 2" diameter hole that
we put in the lower portion of the door, using a hole saw in the drill
press. It consists of 2 aluminum plates (1/4" thick), spaced apart with
3 3/4" coupling nuts, and mounted to the door with 3 more 3/4" coupling
nuts and screws. A 24VDC solenoid is mounted to the "outside" aluminum
plate, with the plunger at a right angle to and centered on the plate.
It has a 10-32 rod threaded into the plunger, running through a hole in
each plate, and has a one-quart paint can lid attached to the other end
with a double nut arrangement. There is a spring around the plunger to
close the draft when the solenoid is de-energized; applying power
retracts the "damper" (paint can lid) about 1/4" from the door. It makes
a fairly loud "clunk" when it opens, and a "clack" when it slaps against
the door upon closing.
The computer selected for the control was a Radio Shack Color Computer,
fondly referred to by its users as a "COCO". It was chosen for several
reasons, chiefly because of its built-in Analog-to-Digital Convertors.
The COCO uses 2 joysticks, each consisting of 2 potentiometers, giving
us 4 analog inputs. The input swings between zero and 5 volts DC, and
can be read from the COCO's BASIC interpreter. It also has a small
relay, meant to start and stop the cassette recorder used for mass
storage of programs and data. This relay can be accessed from BASIC,
through the MOTOR ON and MOTOR OFF commands.
This left us with the task of interfacing the stove's draft control and
some heat sensors to the COCO. The stove heat sensors are 2
thermocouples that are sold at the local hardware for use in gas
furnaces or water heaters. They're used to verify pilot flame before
opening the main gas valve, and sell for about $5.00 each. These
thermocouples generate a very low voltage that varies between
approximately 0 and 30 millivolts (30 thousandths of a volt) for the
heat range I'm interested in. Maximum operating temperature in the stack
is 1000 degrees Farenhite, and is verified by one of those magnetic
stack temperature gauges. One of the thermocouples is stuck through a
hole in the stack about 2" above the stack flange, the other is above
the catalytic convertor, and is used to tell when the convertor lights
off. The lower one is the only one actually used for control.
The temperature in the living space is sensed by a thermistor, a device
with a "negative temperature coefficient", that is, it decreases in
resistance with rising temperatures. I've also experimented with using
an LM-334 integrated circuit as a sensor. The Thermistor is coated with
clear Krylon, similar to a varnish, to make it waterproof (see below).
The little relay in the COCO wouldn't last long if it was handling the
current for the solenoid, so I simply drive another relay coil with it.
The second relay applies 24VDC to the draft control when it is powered
up by the internal cassette relay. There is an RC across the line going
to the solenoid to absorb some of the inductive kickback when
de-energizing the solenoid. Without it, the computer is quite prone to
Getting the thermocouple and thermistor voltages to the 0-5 volt range
is accomplished by a Quad CMOS OpAmp (LM324N). Only a "span" (gain)
potentiometer is used for the thermocouple amplifiers, but a "zero" AND
"span" pot are used for the room temperature portion. All pots are
15-turn PC board type. Each output from the OpAmp has a 5-volt MOV
across it to protect the computer from excessive voltages. There is a
single 20VDC regulated power supply for the OpAmp, and a 24VDC @ 2 amps
supply for the solenoid circuit. This whole mess is built from Radio
Shack parts, assembled on an "experimenters board" and installed in a
"project box" 8"W x 6"D x 3"H in size. There are several cables to plug
into the joystick and cassette recorder ports, and a terminal strip for
the thermocouple and thermistor connections. An LED on the front for
power indication and a pushbutton to operate the draft manually top it
all off. It sits on top of the "monitor" (an old 12" B&W TV). The
computer, monitor, and interface box is fully 20 feet from the stove.
There is a battery backup device connected through the cartridge port
that supplies 5VDC power in the event of power loss to prevent the need
for reloading the program via cassette recorder every time we get a
power bump. Although this was a commercial unit, it would be very easy
to design one from scratch. Just use a 6-volt gelcel and a voltage
regulator set for slightly less than the computer's power supply
voltage, and charge the battery with a few MA from an old AC adaptor.
This design was arrived at through experimentation, based on a couple of
books on OpAmps. The best one was from Radio Shack. I do have a degree
in electronics, but have not really worked in the field for quite a few
years, and my education predates Integrated Circuits.
The temperatures were calibrated as follows: I adjusted the stack
temperature to follow the magnetic stack gauge. (High Tech, huh?) The
room temperature was a bit more complex. I took 2 insulated drinking
mugs, filled one with ice water, the other with water at about 90-95
degrees, as measured with a thermometer. I'd move the waterproofed
thermistor from one to the other, adjusting the zero pot with it in the
ice water, the span/gain pot while it was in the warm water. A short
program on the computer would continously print the joystick reading to
the screen for calibration purposes.
Note that the COCO's joystick ports are equipped with 6-bit A/D
convertors, meaning that the measured values can be broken down into
only 64 parts (0-63), instead of the 256 parts that an 8-bit convertor
would have allowed. A bit of thought showed this to be no problem,
however. I simply multiply the joystick number by 15, giving me a range
of 0-945 degrees for the stack temperature. The room temperature reading
is set up to read only from 32 to 32+63, or 95 degrees. This is also
done in software, of course.
There is a setup screen displayed upon running the program, that allows
me to adjust the room setpoint, and "stack high" and "stack low"
setpoints. While running, the stack temperature is read 20 times if the
draft is "on" or 30 times if it's "off" and an average calculated. The
average is then compared to the stack setpoint and a MOTOR ON or MOTOR
OFF command is sent to open or close the draft. While calcuating the
average, the room temperature is read and compared to the setpoint; if
below, the stack setpoint is set to "high", if above, it's set to "low"
to idle the stove. The program then enters the looping phase for another
20 or 30 readings. Pressing "any key" will abort the control portion of the
program and return me to the setup screen.
There is also a safety circuit in the program: if the stack temperature
falls below 125 degrees, the draft is closed and the program returns to
the setup screen. There are 2 reasons for this. First is that most
failures will cause the stack reading on the computer to fall to zero,
which would normally cause the draft to be opened. Second reason is that
closing the draft when the fire is mostly burned down preserves the
coals, assisting is restarting the fire. A "startup" loop in the program
over-rides this portion so I can get the stove started without having to
mess with the manual draft, usually forgetting to close it. If I remove
line 1230 or REM it out as shown, this part isn't operational. That's
the way it usually runs, too, as the safety circuit was kind of a
Although it might seem like a modulated control would be better than my
ON/OFF draft damper, it works very well in practice. The stack
temperature swings are quite small once the stove reaches equilibrium,
being totally unmeasurable with the magnetic temperature gauge. About
the only disadvantage is the noise it makes when closing or opening, but
I did finally get used to it. The biggest advantage is that it is simple
to build and very reliable. The solenoid was some sort of surplus piece,
picked up at a "hamfest", a flea market for electronic items. All parts
in the interface box (and the box itself) are from Radio Shack, and
didn't probably total $40.00 at the time. The old TV "monitor" is seldom
even turned on, as I know which keys to press to change the room
setpoint, etc.
Following is the original program, with added comments for readability:
10 CLS                  ;set up defaults
20 RS=70                ;Room Setpoint
30 SM=15                ;Stack Multiplier
35 CM=15                ;Combustor Multiplier
50 SL=200               ;Stack Low setpoint
60 SH=500               ;Stack High setpoint
65 I=30                 ;How many times we're going to loop
70 SS=SL                ;Stack Setpoint=Stack Low
;this next block is the setup screen
100 PRINT@0,"1) CHG ROOM SETPOINT"               ;Setup/adjustment
110 PRINT@28,RS                                  ;screen
120 PRINT@32, "2) CHG STACK LO SETPOINT"         ;Print@ is equivilant
130 PRINT@60,SL                                  ;to Locate X,Y
140 PRINT@64, "3) CHG STACK HI SETPOINT"         ;There are 512
150 PRINT@92,SH                                  ;locations on a 32 x 16
160 PRINT@96,"4) GO TO STARTUP OPERATION"        ;display
190 A$=INKEY$:IF A$="" THEN 190 ELSE C=VAL(A$)   ;pick up the selection
191 IF C<1 GOTO 190                              ;error trap it
192 IF C>5 GOTO 190
194 PRINT@211,C                                  ;print it
195 ON C GOTO 200,300,400,500,1000               ;do something with it
210 INPUT RS                                     ;get the new RS
215 IF RS>80 THEN PRINT"TOO HIGH!":GOTO 200      ;error trap
220 CLS:GOTO 100                                 ;and return
310 INPUT SL                                     ;get the new SL
315 IF SL>SH THEN PRINT "TOO HIGH!":GOTO 300     ;error trap
320 CLS:GOTO 100                                 ;and return
410 INPUT SH                                     ;get the new SH
415 IF SH>700 THEN PRINT "TOO HIGH!":GOTO 400    ;error trap
420 CLS:GOTO 100                                 ;and return
;the following block is a "startup" routine to allow stove to
;get hot before entering main control loop
500 CLS:SS=SL:SA=O:S=0                           ;beginning of
510 FOR Z=1 TO I                                 ;"startup" operation
511 CJ=JOYSTK(1)                                 ;temperatures are
513 CT=CJ*CM                                     ;joystick reading X
515 SJ=JOYSTK(0)                                 ;the multiplier (15)
520 ST=SJ*SM                                     ;the joystick is a 6-
525 RJ=JOYSTK(2)                                 ;bit convertor (0-63)
530 RT=32+RJ                                     ;room temp offset
540 PRINT@0,"ROOM TEMPERATURE=";RT               ;display
600 S=S+ST                                       ;start building the
605 PRINT@505,Z;                                 ;average and print the
610 NEXT Z                                       ;loop number in the
615 PRINT@505,"    ";                            ;lower RH corner
620 SA=S/I                                       ;calculate average
630 IF SA<200 GOSUB 700 ELSE S=0:GOTO 1010       ;do we open or close?
650 S=0:GOTO 510                                 ;restart the loop
700 MOTOR ON                                     ;turn the draft on
710 PRINT@224,"DRAFT ON ";                       ;and display status
715 I=20                                         ;set to loop 20 times
800 MOTOR OFF                                    ;turn the draft off
810 PRINT@224,"DRAFT OFF";                       ;and display status
815 I=30                                         ;set to loop 30 times
; main control loop
1000 CLS:SS=SL:SA=0:S=0                          ;initiate defaults
1010 FOR Z=1 TO I                                ;how many times?
1020 SJ=JOYSTK(0)                                ;stack temp
1030 RJ=JOYSTK(2)                                ;room temp
1040 ST=SJ*SM                                    ;stack temp multiplier
1050 RT=32+RJ                                    ;room temp offset
1060 CJ=JOYSTK(1)                                ;combustor temp
1070 CT=CJ*CM                                    ;and multiplier
1080 PRINT@0,"ROOM SETPOINT=";RS                 ;display everything
1160 S=S+ST                                      ;build the average
1170 PRINT@505,Z;                                ;print the increment
1180 NEXT Z                                      ;and loop to 1010
1190 PRINT@505,"    ";                           ;blank the counter
1200 SA=S/I                                      ;calculate average
1210 IF RT<RS THEN SS=SH                         ;do we want the stove
1220 IF RT>RS THEN SS=SL                         ;hot or idling?
1230 REM IF SA<125 MOTOR OFF:GOTO 100            ;stop if stove is cool
1240 IF SA<SS GOSUB 700 ELSE GOSUB 800           ;turn draft on or off
1250 S=0:GOTO 1010                               ;and loop again
Bear in mind that if you attempt to duplicate this, YOU ARE ON YOUR OWN.
The above is represented as no more than a report on how I'm doing it,
not as instructions on how YOU can do it! No circuit diagrams will be
supplied (I'm not sure that I have anything even close to what is
running anyway!). I will answer specific questions via NetMail or in the
Jim Dunmyer, 1:234/2, Toledo's TBBS (313) 854-6001
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