■ FIGURE 3. Parts layout for the PICAXE controller board.
pin 5 of the nine-way D female socket. If your computer
only has a USB port, you will need a USB to serial
Source code for both the 28X1 and 08M are available in
the download package and can also be obtained from www.
Magiclandelectronics.com. The PICAXE Programming software
is available as a free download from www.picaxe.co.uk.
• Minimum Setting: 10°F • Maximum Setting: 94°F
The various options available with this thermostat can be
set with the DIP SWITCH.
YELLOW LED: LOW AIR FLOW — Possible blower
motor/belt failure. Note: This LED won’t
go on until roughly five minutes after the
furnace starts or roughly one minute after
the A/C compressor starts.
ORANGE LED: SYS FAILURE — Indicates an apparent
system failure. This goes on when there
exists a 6°F temperature difference
■ FIGURE 4. Parts layout for the air flow board.
Trying It Out
Put all DIP switches to their OFF position and set the
SYSTEM switch to HEAT. Don’t connect the air flow
sensor board just yet.
At power-up, SmartStat-Plus will go through a startup
routine to check if everything is okay. The display should
initially flash SC (Smart Controller), and the LEDs flash on
and off in sequence. The display will then show the
temperature of the sensor in degrees Fahrenheit.
Press either the UP or DOWN button. The display
should show 70 which is the default Set temperature and
start to flash. Use the UP/DOWN buttons to set the
temperature. Set it for a temperature above the room
temperature, say 80. After a few seconds, the display
should stop flashing and the room temperature should be
displayed. Since the SET temperature is above the room
between actual temperature and set
RED LED: HIGH TEMP — Indicates a dangerously high
temperature (over 95°F) exists in area.
GREEN LED: FREEZE POSSIBLE — Indicates a room
temperature at or below 32°F. At this
temperature, RLY3 will be energized and its
FAN Switch: This switch has two settings — FAN and
AUTO. However, some systems don’t make
the fan control accessible.
ALARM Switch: Put this switch ON if you want an audible
alarm if the High temp LED lights.
Note: The SmartStat-Plus uses a 5 CPH (cycles/hour)
maximum system rate (for both heating and cooling), a
five minute compressor off-time, and a 40 second heater-off time. These features help extend the life of the
systems. To set it to other rates, modify the source code.
While SmartStat-Plus has a 5 CPH maximum system rate
to protect the heating/cooling systems, it does have a
built-in override if the temperature difference between
what the user sets and the actual temperature exceeds
3°F ( 2°C). If you find this override taking place, this
implies your situation has a combination of an oversized
furnace (or A/C) and poor insulation. You can solve this
problem by either getting a smaller furnace (or A/C) or
adding insulation to save energy. It is best to do both.