■ BY PETER BEST
NOT YOUR FATHER’S
THIS MONTH, WE’LL EXPLORE a useful
application that employs the services of UDP
to send detailed temperature and humidity
data over an LAN in your home or through the
routers and hosts that make up the Internet.
■ PHOTO 1. The temperature/humidity monitor uses
0603 and 0805 SMT passive components to keep
the size down. If you were to leave off the
CompactFlash Wi-Fi card interface and the
Sensirion SHT15, this would be an every-day-run-of-the-mill microcontroller hardware design.
To add some spice to this
project, we’ll do it with a unique
sensor and Wi-Fi technology. Just
in case you’ve ever wanted to
implement a microcontroller real-time
clock, we’ll do that too.
Ever want to write some bit-bang
code to communicate with a device
external to your microcontroller? We’ll
do some big time bit banging in this
project. And, this isn’t your father’s
temperature/humidity monitor. Our
temperature/humidity creation is
accurate to within ±2% for humidity
readings and ±0.3% for temperature
measurements. Plus, the temperature
can be displayed as degrees Celsius or
degrees Fahrenheit with two digits of
precision behind the decimal point.
This month also marks the beginning of nerdvilla.com. In addition to
supplying you with the Design Cycle
project printed circuit board (PCB)
layout files, www.nerdvilla.com will
provide a means for you — the Nuts &
Volts projecteers — to get professional
ready-to-roll Design Cycle project
printed circuit boards, complete kits of
Design Cycle projects, and any of the
unique parts that may accompany
Design Cycle projects.
The purpose behind www.nerd
villa.com is to provide me a way to
74 August 2006
help all of you get Design Cycle
projects off the ground with minimal
investment and minimal effort.
Nerdvilla.com is simply a portal I am
providing for you to enter and obtain
Design Cycle project hardware. All of
the logical and graphical Design Cycle
project stuff that regularly gets put
onto the Nuts & Volts website will still
be there for you, as well.
The temperature/humidity monitor hardware behind this month’s
Design Cycle is shown in Photo 1.
Six subsystems make up our Wi-Fi
temperature/humidity monitor. The
first subsystem consists of a
PIC18LF6722, which is the central
hub for all timekeeping, measurement,
and transmission activity.
The PIC drives the second temperature/humidity monitor subsystem
that happens to be a regulation RS-232
serial port built around a 3.3-volt Sipex
SP3232 and associated charge pump
capacitors. The third temperature/
humidity monitor subsystem is found
within the confines of a CompactFlash
card, which houses all of the necessary
electronics and logic to bring 802.11b
Wi-Fi capability to the temperature/
humidity monitor. A 32.768 KHz clock
crystal working in conjunction with the
PIC’s internal TMR1 timer module
forms a real-time clock, which makes
up the temperature/humidity monitor’s fourth subsystem.
The fifth temperature/humidity
monitor subsystem is represented by a
six-pin RJ- 11 jack. The RJ- 11 jack is the
portal for programming and debugging the PIC18LF6722 via the latest
version of Microchip’s MPLAB and
MPLAB ICD2. The sixth and final subsystem is the Sensirion SHT15 itself.
Instead of the normal voltage regulator circuitry, regulated power for our
temperature/humidity monitor is
provided by a 3.3V-DC wallwart,
which is unassisted by any on-board
voltage regulation circuitry.
I can extol the virtues of the
temperature/humidity monitor circuitry using words but nothing can top
seeing a circuit concept schematically.
Thus, Schematic 1 is a graphical depiction of all of the temperature/humidity monitor’s subsystems.
On the software side, the temperature/humidity monitor’s source code
was crafted using the HI-TECH PICC-
18 C compiler. The HI-TECH source is
ANSI compliant and can easily be
converted to your favorite C compiler.
If you don’t have a PIC C compiler
already, the minimum price tag for PIC
C compilers is in excess of $200
unless you can get your hands on a
freebie GNU version.