Sensiron and they sent me two of the SHT21 sensors
which are 3 mm x 3 mm surface-mount devices. I also
found some of the older and larger SHT11 sensors on
eBay.
The Sensiron website has source code for reading the
sensors which I incorporated into the project with only
minor modifications. For the LCD display, I selected the
Electronic Assembly DOG-M two line by 16 character
display with LED backlighting. This display — which has
been featured in other Nuts & Volts project articles — can
operate on 3.3 volts and supports an SPI interface that
requires only three I/O lines to connect to the
microcontroller. The amber backlight is turned on for a
few seconds every time a button is pushed. A ULN2003
Darlington transistor driver triggers the relays. The
ULN2003 has built-in diodes for shunting the back EMF
from the relays. One of the drivers controls the backlight
LEDs for the LCD display, since this requires more current
than the processor pins can provide.
There are four eight-pin connectors that connect to
the TCP/IP base board. Mounting the Sensiron sensors
directly on the PCB worked great at the workbench, but
when I started to enclose the thermostat in a case,
problems came up. The ENC28J60 gets warm, as does the
power regulator. Unfortunately, even with plenty of
ventilation holes drilled in the case, the temperature read
REFERENCES
ENC26J60 Datasheet
ww1.microchip.com/downloads/en/DeviceDoc
/39662c.pdf
Sensiron
www.sensiron.com
ULN2003A
http://focus.ti.com/lit/ds/symlink/uln2003ai.pdf
EA DOGM LCD
www.lcd-module.de/eng/pdf/doma/dog-me.pdf
SparkFun Electronics
www.sparkfun.com/index
PCB Manufacturing
www.silvercircuits.com
All the hardware and software described in this
article, along with additional detailed instructions, is
available at the article downloard link or at
www.jgscraft.com. Microchip development tools and
libraries can only be downloaded from the Microchip
website. You can contact me at jurgen@jgscraft.com.
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September 2011 41