connection between the higher voltage line circuit and the lower
voltage microcontroller, which protects the microcontroller
from voltage spikes on the line side when the HVAC unit starts.
Additionally, the optical isolator limits -- if not eliminates -- noise in
the signal line to the microcontroller and can be used to convert
between two different voltage systems such as 12 VDC from a
sensor to 5 VDC to the microcontroller input. The optical isolator
can also pass AC signals which lend them to applications such as
audio systems. I taught robotics and automation at a community
college for 20+ years and always recommended optical isolators
on every signal line to the controller.
For those not familiar with optical couplers, I will give a little
information on their operation. In Figure A, the optical isolator is
U1, a 4N25 six-pin through hole DIP IC package. The 4N25 input
side has an LED which turns on when the LED is forward biased
by the input signal and emits a light signal inside the IC chip. The
phototransistor in the 4N25’s output circuit turns on when this
light is present, and in the circuit shown drops the voltage on
the signal line to the microcontroller to near zero. If you need an
active high signal to the microcontroller, you can insert an inverter
(better to program the microcontroller for the active low signal
thus reducing the parts count and its ensuing failure potential).
#2 In Figure A on page 16 of May 2016: The connections
to the 7805s seem to be wired on the wrong side of the switch
and the labels are incorrect going into the GPIOs. “Heat” should
be “Cool” and “Cool” should be “Heat.” Hopefully, the µC GPIO
inputs are 5V and not 3.3V.
Marc, you are quite correct. I accidently reversed the labels on
the microcontroller when drawing the schematic. I have corrected
the schematic in Figure B. See the reply above from Rick Swenton
for using optical isolators on the inputs to the microcontroller to
protect the microcontroller from voltage spikes and reduce noise
to the µC.
#3 At the end of your piece on battery life, I believe there’s
a typo. It shows: 250 mA - 2. 50, 100 mA - 0.32 and 50 mA - 0.43.
I believe the first should have been: 250 mA - 0.25.
Phillip, you caught my decimal malfunction. I decided on a
better way of presenting this information: Load Current (mA) - Amp
Hours (AH) 250 mA - 0.25 AH; 100 mA - 0.32 AH; and 50
mA - 0.43 AH. Thanks for catching my mistake.
Collector Plate Design
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Instrumentation and Controls
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Inertial Navigation System Basics
Inertial Navigation Systems
n FIGURE A. n FIGURE B.
August 2016 19