Humidity Sensor: Types of Humidity Sensors & Working Principle

Humidity is the presence of water in
air. The amount of water vapor in air
can affect human comfort as well as
many manufacturing processes in
industries. The presence of water
vapor also influences various physical,
chemical, and biological processes
Humidity measurement in industries
is critical because it may affect the
business cost of the product and the
health and safety of the personnel.
Hence, humidity sensing is very
important, especially in the control
systems for industrial processes and
human comfort.
Controlling or monitoring humidity is
of paramount importance in many
industrial & domestic applications. In
semiconductor industry, humidity or
moisture levels needs to be properly
controlled & monitored during wafer
processing. In medical applications,
humidity control is required for
respiratory equipments, sterilizers,
incubators, pharmaceutical
processing, and biological products.
Humidity control is also necessary in
chemical gas purification, dryers,
ovens, film desiccation, paper and
textile production, and food
processing. In agriculture,
measurement of humidity is
important for plantation protection
(dew prevention), soil moisture
monitoring, etc. For domestic
applications, humidity control is
required for living environment in
buildings, cooking control for
microwave ovens, etc. In all such
applications and many others,
humidity sensors are employed to
provide an indication of the moisture
levels in the environment.
RELEVANT MOISTURE TERMS
To mention moisture levels, variety of
terminologies are used. The study of
water vapour concentration in air as a
function of temperature and pressure
falls under the area of psychometrics.
Psychometrics deals with the
thermodynamic properties of moist
gases while the term “humidity’ simply
refers to the presence of water vapour
in air or other carrier gas.
Humidity measurement determines
the amount of water vapor present in
a gas that can be a mixture, such as
air, or a pure gas, such as nitrogen or
argon.
CHARACTERISTICS
Sensor characterisation is done based
on the n-point(usually 9)
characterisation of the sensor.
Characterisation is performed at a
specific temperature (25°C) and
excitation.
In 9 point characterisation method,
humidity levels are swept the through
the RH values and measuring the
corresponding dc output voltage for
the individual sensor: Values are taken
at humidity levels of 0%, 25%, 53.2%,
75.3%, 93.8%, 75.3%, 53.2%, 25% and
0%. Based on the characterisation
results, Best Fit Straight Line (BFSL) is
plotted and sensor characteristics are
specified in the datasheets.

• Acuracy
  Accuracy is specified based on the
  specific calibration curves for any
  individual sensor. It is specified using
  the linear Best Fit Straight Line (BFSL)
  and the non-linear 2 nd order curve.
  As an example let us consider a
  sensor with an accuracy of ±2% RH
  (BFSL). If the sensor has an output
  voltage of 0.689 V at 0%RH, an
  average slope(BFSL) of 0.036 V/%RH
  and offset of 0.662, then its BFSL
  accuracy error is given by (0.689 -
  0.662)/0.036 = 0.75% RH. As sensors
  accuracy is ±2% RH (BFSL), i.e. 0.072V,
  the sensor should always output
  0.662 ±0.072 V or a value in the range
  of 0.59 V to 0.734 V.
  
• Hysteresis
  Hysteresis is the difference between
  the two voltages to %RH conversions
  (using average BFSL slope) at each of
  the four duplicated points in the nine
  point characterization. Hysteresis is
  recorded in absolute %RH terms.
  The value taken is the largest %RH
  figure for an individual sensor over
  each of the four characterization
  points.
  
• Interchangeability
  Interchangeability defines the range of
  voltages for any population of sensors
  at this RH point.
  As an example let us consider a
  sensor from a particular company
  with an interchangeabilty of ±5% at
  0% RH. With an average slope (BFSL)
  of 0.036 V/%RH and offset of 0.662 V,
  ±5% RH is equal to ±0.18 V. This
  means that the output voltage for this
  device is 0.662 V ±0.18 V, or a range of
  0.482V to 0.842 V. When exposed to
  an RH of 0%, the output of the entire
  population of sensors will fall within
  this range.
  
• Linearity
  Linearity indicates the voltage
  deviation from the BFSL value and the
  measured output voltage value,
  converted to RH.
  
• Reliability
  Sensors are subjected to accelerated
  stress tests. If the tests causes the
  sensor to drift and report RH outside
  prescribed specifications, the sensor
  is considered a failed sensor. Based
  on such tests, reliability figures like
  MTTF(Mean-time-to-failure) and FIT
  (Number of Failures per billion
  operating hours) are specified.
  
• Repeatability
  Repeatability is the maximum
  variation between sensor outputs for
  repeated sweeps of humidity levels
  across the sensors’ measurement
  range under identical conditions.
  For example, if the point value is
  0.013 V using the 31 mV/RH slope this
  is 0.42% RH.
  
• Response Time
  Response Time is measured in “slow
  moving air” (less than 5 m/s).
  Typically, maximum time required for
  the output voltage of the sensor to
  rise to 63% of its final value or to fall
  to 37% of its final value when exposed
  to a step rise or fall in humidity is
  specified as response time
  
• Temperature Compensation
  Voltage output for an individual
  sensor at a given excitation and RH is
  affected by temperature. In many
  sensors, the temperature is measured
  and the effect of temperature of
  humidity measurement is reduced
  and this is referred to as temperature
  compensation.
  
• StabilityOutput voltage stability is the output
  voltage drift in time at the specified RH
  level converted to a %RH value.
  This figure is also generated through
  accelerated stress tests and is typically
  taken as the change in mean output
  voltage from a large batch of sensors
  in specific environmental conditions.

SELECTING A HUMIDITY SENSOR
As there no real physical standard for
relative humidity calibration, humidity
instruments are not specified
properly. And it makes it really difficult
for a user to compare the sensors
from different manufacturers. This
makes it mandatory for a user to go
deeper into the specifications and
attempt to verify the claims of the
instrument manufacturer. Various
sensor characteristics, viz., accuracy,
linearity, hysteresis, calibration errors,
long term stability of sensor and
electronics, needs to be examined
using the support documentation
from the OEM.
Source: Engineersgarage

Hydrogen Race Car Takes On Fossil- Fueled Rivals

For the first time a hydrogen race car
entered the annual Formula Student
competition held in the UK. Built by
the Forze hydrogen racing team of the
Dutch Delft University of Technology,
the Forze V race car is equipped with
a hydrogen fuel cell which powers two
electric motors.
The Formula Student (FS) challenge
invites teams from universities across
the world to build a car and race
them against each other at the
Silverstone race circuit. FS started in
1998 and aims to inspire young
engineers to go through the full cycle
of building a single-seat racing car,
including business plan, design,
presentation and -of course- building
and competing.
In 2008 Class 1A was introduced, a
special category apart from Class 1,
dedicated to low carbon emission
cars.
However, the Forze V team decided to
race their car- which only exhaust is
pure water- against its Class 1 gas-
guzzling brethren. They came in 29th
of 103 contestants.
Alistair Wardrope, a judge of the FS
races said that the Forze V ‘beat many
gasoline powered race cars on a level
playing field. Internal combustion
engines have been developed for over
a hundred years. The significance of
the achievement lies in Delft Forzes’
skill in adopting a new technology to
compete against a well developed
technology.’
The car weighs 280kg (617lbs) and is
powered by a 18kW hydrogen fuel
cell. Its top speed is 120km/h
(75mph) and accelerates from 0 to
96km/h (0-60mph) in less than 5
seconds. On a full tank of 600 grams
of gaseous hydrogen the car can race
for an hour. Its only exhaust product:
three liters of water (0.8 gal).
The Delft Force V team hopes their car
will convince people that fuel cells are
the long term solution for sustainable
mobility.
Source: Formulazero.tudelft.nlW

Operational princle of SCR

SCR working principle
The SCR is a four-layer, three-junction
and a three-terminal device and is
shown in fig.a. The end P-region is the
anode, the end N-region is the
cathode and the inner P-region is the
gate. The anode to cathode is
connected in series with the load
circuit. Essentially the device is a
switch. Ideally it remains off (voltage
blocking state), or appears to have an
infinite impedance until both the
anode and gate terminals have
suitable positive voltages with respect
to the cathode terminal. The thyristor
then switches on and current flows
and continues to conduct without
further gate signals. Ideally the
thyristor has zero impedance in
conduction state. For switching off or
reverting to the blocking state, there
must be no gate signal and the anode
current must be reduced to zero.
Current can flow only in one direction.
In absence of external bias voltages,
the majority carrier in each layer
diffuses until there is a built-in voltage
that retards further diffusion. Some
majority carriers have enough energy
to cross the barrier caused by the
retarding electric field at each
junction. These carriers then become
minority carriers and can recombine
with majority carriers. Minority carriers
in each layer can be accelerated
across each junction by the fixed field,
but because of absence of external
circuit in this case the sum of majority
and minority carrier currents must be
zero.
A voltage bias, as shown in figure, and
an external circuit to carry current
allow internal currents which include
the following terms:

The current I_x is due to

• Majority carriers (holes) crossing
  junction J₁
  
• Minority carriers crossing junction
  J₁
  
• Holes injected at junction J₂
  diffusing through the N-region and
  crossing junction J₁ and
  

Minority carriers from junction J₂
diffusing through the N-region and
crossing junction J₁ .
Similarly I₂ is due to six terms and I₃
is due to four terms.
The two simple analogues to explain
the basic action for the thyristor are
those of the diode and the two
transistor models.
1. Diode Model. The thyristor is
similar to three diodes in series as
there are three P-N junctions.
Without gate bias, there is always
at least one reverse biased junction
to prevent conduction irrespective
of the polarity of an applied voltage
between anode and cathode. If the
anode is made positive and the
gate is also biased positively with
respect to cathode, the P-layer at
the gate is flooded by the electrons
from the cathode and loses its
identity as a P-layer. Accordingly
the thyristor becomes equivalent
to a conducting diode.

Scr working
2. Two Transistor Model.
Imagine the SCR cut along the
dotted line, as shown in fig. a.
Then we can have two devices, as
shown in fig.b. These two devices
can be recognized as two
transistors. The upper left one is P-
N-P transistor and the lower right
N-P-N type. Further it can be
recognized that the base of the P-
N-P transistor is joined to the
collector of the N-P-N transistor
while the collector of P-N-P is
joined to the base of N-P-N
transistor, as illustrated in fig. c.
The gate terminal is brought out
from the base of the N-P-N
material. This construction has
been conceived merely to explain
the working of SCR, otherwise in
physical shape the SCR has four
solid layers of P-N-P-N type only.
Now we can see that the two
transistors are connected in such a
manner that the collector of Q₁ is
connected to the base of Q₂ i.e. the
output collector current of Q_t
becomes the base current for Q₂ . In
the similar way the collector of Q₂ is
joined to the base of Q₁ which shows
that the output collector current of Q₂
is fed to Q₁ as input base current.
These are back to back connections of
transistors in such a way that the
output of one goes into as input of
other transistor and vice-versa. This
gives net gain of loop circuit as β₁x
β₂ where β₁ and β₂ are current gains
of two transistors respectively.
When the gate current is zero or the
gate terminal is open, the only current
in circulation is the leakage current,
which is very small in case of silicon
device specially and the total current
is a little higher than sum of individual
leakage currents. Under these
conditions P-N-P-N device is said to
be in its forward blocking or high
impedance ‘off state. As soon as a
small amount of gate current is given
to the base of transistor Q₂ by
applying forward bias to its base-
emitter junction, it generates the
collector current as β₂ times the base
current. This collector current of Q₂ is
fed as input base current to Q : which
is further multiplied by β₁ times as I_Cl
which forms input base current of Q₂
and undergoes further amplification.
In this way both transistors feedback
each other and the collector current
of each goes on multiplying. This
process is very quick and soon both
the transistors drive each other to
saturation. Now the device is said to
be in.on-state. The current through
the on-state SCR is controlled by
external impedance only.
Souce: circuitstoday

12V BATTERY LEVEL INDICATOR CIRCUIT

This battery level indicator offers (5)
LEDs that light up progressively as the
voltage increases:

• Red: Power Connected (0%)
  
• Yellow: Greater than 10.5V (25%)
  
• Green 1: Greater than 11.5V (50%)
  
• Green 2: Greater than 12.5V (75%)
  

Green 3: Greater than 13.5V (100%)
Of course, you may select your own
colors if desired.
12 Volts Battery Level Indicator
Circuit Schematic

Operation of the battery level
indicator
D1 is the voltage reference zener. Tied
to this is a string of divider resistors
(R2-6) that set the various fixed
voltage levels. R7 & 8 form a voltage
divider to that reduces the battery
voltage by a factor of 3. U1 is an
LM339 quad comparator that
compares the various voltages from
the two dividers. The comparator
sections have open collector outputs
that simply function as switches to
operate the LEDs. D7 protects against
reverse battery connection.
The LEDs are biased to operate at
about 4mA which is quite bright if
modern LEDs are used. This current
can be adjusted simply by varying the
series resistors (R9 through R13). The
overall current drain as shown is
about 25mA which tends to be
wasteful for continuous operation. For
energy conservation, connect to
battery via a pushbutton (Push to
Test).
Printed Circuit Board
I did a www.expresspcb.com SMT
layout using 0805 size components,
1N753 zener and SOIC-14 IC. D7 is in
a SOT-23 package. These components
are about as small as I like to work
with. The layout has not yet been
carefully checked or built. Note that
surprises abound when constructing
prototypes.
The circuit board measures only 0.5” x
1.5”.
download the PCB layout
More recently, I located an
inexpensive SOT-23 zener with a 2%
voltage tolerance—this has not yet
been incorporated.
I have had good results with 0805 size
LEDs purchased from China on eBay.
They are both inexpensive and
BRIGHT!

The 12v battery level indicator unit in
the photo has no reverse polarity
diode and R2 is the calibration
potentiometer.
Source: Elektro symetrics

LM358 DATASHEET

The LM358 series consists of two
independent, high gain, internally
frequency compensated operational
amplifiers which were designed
specifically to operate from a single
power supply over a wide range of
voltages. Operation from split power
supplies is also possible and the low
power supply current drain is
independent of the magnitude of the
power supply voltage.
The LM358 and LM2904 are available
in a chip sized package (8-Bump
micro SMD) using National’s micro
SMD package technology.
LM358 pin diagram

LM358 Features

• Available in 8-Bump micro SMD chip
  sized package, (See AN-1112)
  
• Internally frequency compensated
  for unity gain
  
• Large dc voltage gain: 100 dB
  
• Wide bandwidth (unity gain): 1 MHz
  (temperature compensated)
  
• Wide power supply range:
  
• Single supply: 3V to 32V
  
• or dual supplies: ±1.5V to ±16V
  
• Very low supply current drain (500
  μA)—essentially independent of
  supply voltage
  
• Low input offset voltage: 2 mV
  
• Input common-mode voltage range
  includes ground
  
• Differential input voltage range equal
  to the power supply voltage
  

Large output voltage swing

HIGH TEMPERATURE INDICATOR CIRCUIT

In some areas there is a need of
temperature detectors which help
them to detect temperature and
indicate them whether the
temperature is low or not.
Below shown circuit is the simple high
temperature indicator circuit. It will
not show the current temperature
but if it exceeds some threshold
temperature it will detect and indicate.
You can use this circuit anywhere you
needed.

The components needed for the
circuit are:
3 Resistors
1 Temperature dependent resistor
5v battery
Two 1n4007 diodes
One Op-Amp
One LED
And the circuit working is actually very
simple, the Op-amp is connected as
Non-inverting comparator. And a
bridge circuit is made with the
resistors and a Temperature
Dependent Resistor. From above
circuit R1, R2 and R3 are normal
resistors but RT is Temperature
Dependent Resistor.
The bridge resistance and RT are
selected in such a way that as long as
temperature is less than threshold
value, the bridge is unbalanced by
making Voltage at B more than
voltage at A . Hence Vo = -Vsat and
LED is reverse biased and remains
OFF. When temperature becomes
more than threshold then Voltage at B
becomes less that Voltage at A hence
it drives Vo = +Vsat. Due to this, LED
glows and gives high temperature
indication.

iPhone 5 Dock Smaller

A smaller dock connector means
the iPhone 5 will be incompatible
with accessories made for Apple
products going back nearly 10 years.

Apple is preparing to ditch the thirty-pin
connector it has used since the early
days of its iPod media player. The
iPhone 5 will instead ship with a
smaller dock connector containing 19
pins. The design change, long
rumored for the new iPhone, means
that the iPhone 5 will be incompatible
with the bulk of existing iPhone
accessories.
Why the change ? According to
sources cited by Reuters, the dock
switcheroo is meant "to make room
for the earphone moving to the
bottom." Souce: InformationWeek

Apple iPhone 3GS now available for Rs 9,999

New Delhi: Apple iPhone 3GS may
seem to be obsolete, but it has got
the same user interface that the latest
iPhones have. And Aircel is offering
the phone for just Rs 9,999.
To grab this deal customers will need
to pay advance rental of Rs 3,000 in
addition to the cost of the phone
which will offer them 2 GB unlimited
(after 2 GB at a slower 128 Kbps
speed) plan for 12 months in 3G
circles, and in circles where only 2G
network is available users will get
1000 mins local and STD calls and
2500 local and national SMS along
with unlimited 2G data for 6 months.
The only catch is that the plan is
available only for the postpaid users.
While the plan is more attractive for
people where Aircel has 3G network
is available but never the less 2G offer
also seems exciting.
If you are in the market for a
smartphone with a budget of Rs
10,000, this is the offer that you can
safely go for. Aircel offers only
unlimited plans and have one of the
cheapest 3G plans in the country, so
even after 12 months (in 3G circles) or
6 months (2G circles) as the case may
be, users will get very attractive data
and voice plans.
The iPhone 3GS was till now available
for Rs 19,990 from Airtel and Aircel.
Source: IBN LIVE

Class-D Audio Amplifiers with Wide Supply Range and High Switching Rates

Texas Instruments has introduced
three analog-input class-D stereo
amplifiers for use in soundbars, after-
market automotive solutions, portable
audio docks, and LCD televisions. The
TPA3116D2, TPA3118D2 and
TPA3130D2 feature the industry's
highest programmable switching rate
and widest power supply range,
enabling audio output that is both
high quality and highly efficient. The
devices are pin-to-pin and software
compatible and can support a single-
layer printed circuit board (PCB),
enabling cost savings by allowing
designers to reuse a single layout for
multiple designs.
Key features and benefits of the
TPA3116D2, TPA3118D2 and
TPA3130D2:

• Supply range of 4.5 V to 26 V
  enables simultaneous operation of
  line power and battery power on
  the same board for longer battery
  life in soundbars and portable
  audio docks.
  
• Thermally efficient FETs allow 30 W
  x 2 operation without the need for
  a heat sink, or 50 W x 2 with a heat
  sink (TPA3116D2 only).
  
• Programmable switching rates of
  400 KHz to 1.2 MHz allow inductor
  filter-free operation and
  interference avoidance for clear
  audio with no distortions.
  
• Closed-loop design provides
  excellent power supply rejection
  ratio (PSRR) and slower edge rates
  for waveform switching to improve
  electromagnetic interference (EMI)
  performance.
  
• RDS(on) of 120 milliohms at 30 W x
  2 improves efficiency by decreasing
  resistance during operation.

Source: Elektro Projects

Intervalometer for Android tablet

An Android tablet and a few
accessories are enough to transform a
camera into the time machine to
explore, or rather to accelerate the
process by a time lapse. The interval
between shots is adjusted via a web
interface on a single PC connected to
the tablet via a wireless network.

Temperature and Humidity Meter

Digital thermometer or thermostat
projects have been around for many
years. This project, in order to be
appealing in such explored subject,
incorporates a number of interesting
features, never put together in one
single circuit and, some of them, only
available on professional equipment.
The first characteristic that is worth
noting is that the circuit, from now on
called Meter, also measures relative
humidity. As temperature and relative
humidity Measurements are taken,
the Meter keeps record of observed
maximum and minimum levels. Each
reading is also compared to user set
maximum and minimum limits which,
if exceeded, will trigger outputs that
may be used to drive external circuits.
Temperature measurements are
made in either Celsius or Fahrenheit
degrees. A digital clock keeps track of
time, day, weekday, month and year
which is an important information
when the Meter is collecting and
storing measurements in its
microcontroller EEPROM
The temperature and humidity meter
entire setup is configured and
developed using Microchip 18F 4525
controller with assembly
programming firmware support. This
project is having multiple Menu
configuration setup with Respect to
S1,S2,S3 switches, along with that
Analog signal configuration and low
battery detection and update
temperature relative humidity stored
in inbuilt EEPROM of PIC18F4525
controller and we can record time
intravel between minimum and
maximum measurement of
temperature and humidity. Visual
Basic is front end tool is used for user
friendly configuration to set maximum
and minimum temperature and
relative humidity and it’s having more
user friendly selected option like
temperature measurements are made
in either Celsius or Fahrenheit
degrees, program memory read
configuration and meter
configuration.
The Entire setup is design with
appropriate schematic, visual basic
frontend and various hardware
selections to design this meter.
Source: Elektro Projects

Silicon Vacuum Tubes

Get that warm tube sound in your
MP3 player!

Researchers at the University of
Pittsburgh have developed a
semiconductor device with a vacuum
channel etched in silicon for electron
transport, instead of a conventional
solid-state channel. This represents a
return to vacuum tube technology,
but on a much smaller scale.
Fast electronic devices need on short
carrier transport times, which are
usually achieved by decreasing the
channel length and/or increasing the
carrier velocity. In an ideal device,
carrier motion is ballistic with no
collisions, but it is difficult to achieve
ballistic transport in a solid-state
medium because the high electric
field used to increase the carrier
velocity also increases scattering.
Vacuum is an ideal medium for
ballistic transport, but vacuum devices
typically have low emission currents
and high operating voltages.
The researchers decided to combine
the strengths of vacuum and solid-
state technologies. They fabricated a
low-voltage field effect transistor with
a vertical vacuum channel etched into
a metal-oxide semiconductor
substrate. With a channel length of
approximately 20 nm, they measured
a transconductance of 20 nS/µm, an
on/off ratio of 500 and a turn-on gate
voltage of 0.5 V under ambient
conditions. Coulombic repulsion in
the electron sheet at the interface
between the oxide and the metal or
the semiconductor reduces the
energy barrier for electron emission,
leading to a high emission current
density of approximately 10 μA/cm²
with a bias of just 1 V.
More info
Nature Nanotechnology article

Man Controls A Robot With His Mind From 2000km Away

In the 2005 comic book series The
Surrogates (turned into a movie by
the same name in 2009) people
remotely operate humanoid robotic
substitutes with their minds. While
humans stay at home strapped to the
controls the surrogates go out to
stand in for their physical lives.
As tech innovation speeds up fiction
finds science hard on its heels. For
now an international team of
researchers has succeeded in mind-
controlling a robot located in France,
from a lab in Israel.
The robot was operated by research
assistant Tirosh Shapira. In the Israeli
lab Shapira’s brain activity was
measured by an fMRI scanner. At the
Béziers Technology Institute in France
a small robot mounted with a camera
awaited his instructions. Watching the
video feed from the robot’s point of
view, Shapira saw a member of the
French team urging him/the robot
forward with arm movements.
Shapira’s thoughts about moving his
leg were picked up by the brain
scanner, sent over the internet and
only with a small time delay, the robot
moved forward.
Shapira told New Scientist that he
wasn’t just amazed about the
technological feat they’d pulled but
also at how easy the human mind is
tricked: ‘At one point the connection
failed. One of the researchers picked
the robot up to see what the problem
was and I was like, ‘Oi, put me down!’’
Perhaps that could be explained by
watching the video feed with upmost
concentration but Shapira actually
identified with the robot. He describes
his reaction when the French team
unexpectedly put a mirror in front of
the robot ‘I thought, ‘oh I’m so cute, I
have blue eyes’, not ‘that robot is
cute’. It was amazing.’
Source: Newscientist.com
Photo source: News.softpedia.com