www.nutsvolts.com/index.php?/magazine/article/september2011_Schmidt
sensor, memory, and MAC
address chips since these
only come in surface-mount formats (see Figure
2). For the mockup, I used
LEDs to represent the
relays that will turn on the
fan, heater, and cooling
components. I wrote an
initial version of the
thermostat program,
combined it with the
Microchip TCP/IP stack,
and loaded it into the chip.
I verified that I can control
a thermostat from a web
page and a wall-mounted
interface. The LCD displays
current temperature and
humidity, along with the
set point and status of the
heater, fan, and air
conditioner.
■ FIGURE 1.
Design
• Relays
• Connector for HVAC control
While it would work, mounting the breadboard on the
wall is clearly not an option. Some additional design goals
are related to producing something nice that will not
offend the aesthetics of family members and will also be
easy to make. For ease of construction, the connectors,
switches, and display should be mounted on the circuit
boards. I have two heating and cooling systems in the
house, so I will be making two of them with
at least one or two extra to experiment
with. In order to control the thermostat
from a web browser or client, each
thermostat will include an embedded http
or web server. With this in mind, I laid out
all the major components for the project.
The web server section will need:
The initial design focused on a single circuit board.
However, while I was working on it, I started thinking of
some of the other cool projects for an embedded web
server. This — along with the impending large size of a
single board implementation — led to a double-decker
design. The TCP/IP base board will contain all the
• PIC microcontroller
• Web page memory
• MAC address chip
• Ethernet module with RJ45 connector
• RS-232 interface
• RS-232 connector
• Power plug
• Programming connector
The thermostat user interface will need:
• LCD display
• Switches
• Temperature and humidity sensor
• Relay driver
■ FIGURE 2.
September 2011 35
