16 May 2016
Actually, though, I think I would approach the design
differently for a reduced parts count. For each of the
two wires coming from the thermostat, I would go with
just one diode, and then two resistors to form a voltage
divider to give five volts. That would result in pulsating
DC for a high signal at the microcontroller which I would
simply watch for in a software loop, having a duration of
1/60th of a second.
Judy May W1ORO
Judy, thanks for your recommendations on the
furnace data acquisition. I have reproduced Figure A
from the February 2016 Q&A as Figure A here. Since
capacitors are cheaper than three-pin regulators, I would
go ahead and parallel the 33 µF capacitors if there is
room in your enclosure, to protect the 7805s from power
surges on the input and output sides, plus the input side
capacitors help level up the fluctuating DC signal from
the rectifiers. From my college instructor days, I leave a
few exercises for the student (reader).
For monitoring the thermostat line, a scan time of
1/60 second would work since the rule of thumb is to
scan at least 1/10 the frequency of the changes of the
monitored parameter; HVAC units usually run for several
minutes before turning off (at least at my house). As far
as thermostat terminal markings, you always need to
check the manual for your particular unit since some
manufacturers have different terminology.
Single diodes will work, but the full bridge rectifier
and three-pin regulator will give a better signal (I’m an
engineer too, so I always try to cover all of the bases, but
if it works, it works). Your design will be cheaper and
more reliable with the lower parts count.
Re: Question about a Previous Q&A Question (R/C
In reference to the January 2015 Q&A article, I am
still confused. The article is telling me to use pin 5 which
is a SERIAL CLOCK, and pin 26 on both units which is
a CHIP SELEC T. So, how am I communicating between
the two Raspberry Pis?
On the Raspberry Pi, several of the pins can be
used as General-Purpose Input/Output (GPIO) or other
functions to allow the microcontroller to perform more
functions with fewer pins to simplify the product for
the end user. For the 40-pin Raspberry Pi, pin 5 can be
GPIO03, SCL1 (serial clock 1), or I2C (Inter-Integrated
Circuit — used to communicate between I2C compatible
IC chips). Pin 26 can be GPIO07 and SPI CE1 (Serial
Peripheral Interface Chip Enable 1). The functions of the
multifunction pins are selected in the program you write
for your particular application. Refer to the programming
manual for your particular model of the Raspberry Pi for
details on programming. The R/C robot will require a fair
amount of programming to implement.
The January 2015 Q&A question also asked about
using a CB radio in place of the Kiatronics transmitters
n FIGURE A.