■ FIGURE 8.
a toasty 95°F even though the room
temperature was only 75°F. I measured the
temperature at different locations on the
PCB but they were all high. The sensor
needs to be outside the case. I mounted
the SHT11 sensor on a pigtail and lowered
it through one of the ventilation holes.
Problem solved. This was not as elegant as I
had originally hoped, but was effective. The
updated version of the PCB has a four-position screw terminal for attaching
temperature sensors which allows me to
experiment easily with different sensors.
Finally, some of the leftover I/O lines
are routed to an auxiliary connector. You
can see the circuit diagram for the
thermostat daughterboard in Figure 7.
Figure 8 shows the populated PCB with an
attached sensor. The file
ThermostatBOM.xls (included with this
article’s downloads) lists the part numbers
and sources for both boards.
Next month, we will cover the physical
connections, construction issues, and the