18 August 2016
actual tuning fork gyro fabricated on a substrate.
Wouldn’t it be wonderful if we could buy a complete
MEMS INS encased in a single integrated circuit chip?
Figure 19 shows the block diagram of a Maxim MAX
21100 integrated circuit Inertial Measurement Unit (IMU
— the sensor portion of the INS). The MAX 21100 IMU
(which sells for less than $6) incorporates accelerometers,
gyros, and ancillary electronics which take the gyroscope,
accelerometer, and external magnetometer raw data as
inputs and provides the device orientation information
as output. This orientation data could be used by a
microcontroller or PC to calculate a vehicle’s position,
speed, and altitude using the appropriate user-generated
I hope I have given you enough insight into the world
of navigation and the operation of an Inertial Navigation
System so that readers can get started incorporating the
IMU into their drones or mobile robot projects.
n FIGURE 19. Block Diagram of Maxim MAX 21100 Inertial
Furnace Data Acquisition
#1 I would like to comment on your recent response to
Furnace Data Acquisition. Your circuit as drawn will not work. The
left sides of the bridge rectifiers need to connect to terminals Y
and W, respectively, and not to Rh and Rc. If wired as shown, the
rectifiers will have 24VAC on them all the time and will not be
going on and off with the thermostat signal.
There are many ways to monitor the thermostat signals.
While your circuit will work, I would not have done it this way. I
would use an optocoupler. With an optocoupler, the connection
between the heating system and the Arduino (or whatever
you use as a controller) will be completely isolated. There is
no direct connection between them as the signal is coupled
by light. Further, the optocoupler output is open collector. The
pull-up resistor can be tied to any voltage you need (such as 3. 3
volts) without changing any circuit components. My circuit also
provides noise immunity as any noise in the heating system will
not pass through the optocoupler. Who knows if the heating
system relays have back EMF diodes on them? Almost any
optocoupler will work here.
If you want to monitor both heating and cooling, you need
to build two of these. The one shown in Figure A is for heat. The
second one would connect to the Y wire instead of the W wire.
Many installations do not have a C wire at the thermostat. It is
usually black or blue. If you have an electronic thermostat and
you want it powered by the heating system and not running on
batteries, you either need a C wire or you would have to run a
new power wire from an AC adapter to the thermostat. If your
thermostat does not have the C wire, you can install this circuit
near the heating system where the C wire is available.
A word of caution about that jumper between Rc and Rh.
That jumper should only be installed on a system that provides
both heating and cooling from the same single system with one
transformer. In the case of having separate heating and cooling
systems with two transformers, the heating system will provide
the Rh wire and the cooling system will provide the Rc wire. In
this case, the jumper should be removed. If you leave the jumper
connected, it will cause the transformer secondaries to be in
parallel. If the transformers are out of phase, they will burn up
because you will have a 48 volt short circuit. Many thermostats
come with this wire already installed.
Another note is concerning the optocoupler. If you plug in
the optocoupler with the circuit powered up, you will blow it.
Without U1 connected, the capacitor C1 will charge to about 34
volts. If you plug in U1, there may be a puff of smoke. With U1
connected, the capacitor will charge to less than one volt.
Rick, you must be looking
at the schematic in the February
2016 Q&A. I corrected the
schematic using the Rh and Rc
terminal connection in the May
2016 Q&A (Figure B in this issue’s
Q&A) and confessed that I am not
an HVAC person. Optical isolators
(another name for optocouplers)
are a great way to couple
power line operated circuits to
a microcontroller. The optical
isolator breaks the electrical
Figure A. Circuit provided by Rick Swenton