Nuts and Volts - September 2011

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.