Stress Meter Project Report

ABSTRACT

Stress is a term that refers to the sum of the physical, mental, and emotional strains or

tensions on a person. Feelings of stress in humans result from interactions between

persons and their environment that are perceived as straining or exceeding their

adaptive capacities and threatening their well-being. The element of perception

indicates that human stress responses reflect differences in personality as well as

differences in physical strength or health.

This Stress meter allows to assess one’s emotional pain. If the stress is very high, it

gives visual indication through LED display along with a warning yellow light.

Stress meter is based on the principle that the resistance of the skin varies in

accordance with your emotional states. Resistance varies inversely proportional to the

stress. If the stress level is high the skin offers less resistance, and if relaxed,

resistance is high.

In an article “Stress and Mind Control”, 21/03/2008, Roberto Bonomi stated that

“When we speak of the fabulous relaxation capacity that mind control gives us, the

first thing that comes to our mind, is that we will be able to take off, the excesses of

nervous tension, the stress; and this is a great benefit. Because suppose that you could

measure stress in inches, and that you have stress zero when the meter is located in

zero.” Based on this our project is aimed to give a visual indication of one’s stress

through a light-emitting diode display along with a warning light.

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LIST OF TABLES

1. Table 3.1: LED Color Vs. Potential difference…………………...13

2. Table 4.1: LED Vs. Threshold voltage…………………………...19

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LIST OF FIGURES

1) Fig 2.1: Block diagram of stress meter………………………….… .. 6

2) Fig 3.1: Dot/Bar display driver……………………………….…. ..... 8

3) Fig 3.2: The piezo element ……………………………………….… .9

4) Fig 3.3: Piezo electric diaphragm……………………………….…. ..10

5) Fig 3.4: Touch pad……………………………………………..…… .10

6) Fig 3.5: Light emitting diode……………………………………… ...11

7) Fig 3.6: Inside a light emitting diode………………………………. ..12

8) Fig 3.7: Regulated power supply……………………………….….. ..14

9) Fig 3.8: Voltage regulator…………………………………………… 14

10) Fig 3.9: Circuit diagram of Stress meter……………………………. .15

11) Fig 4.1: Basic block diagram showing the operation of the circuit… .17

12) Fig 4.2: Role played by touch pads…………………………………. 18

13) Fig 4.3: Bridge Rectifier Circuit…………………………………….. 20

14) Fig 4.4: The Power supply generation………………………………..21

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Chapter 1

INTRODUCTION

1.1 STRESS METER

Stress is the very common condition of every human being. Stress is nothing more

than a socially acceptable form of mental illness. This Stress meter allows to assess

the emotional pain. If the stress is very high, it gives visual indication on a LED

display along with a warning light.

Stress meter is based on the principle that the resistance of the skin varies in

accordance with your emotional states. Resistance varies inversely proportional to the

stress. If the stress level is high the skin offers less resistance, and if relaxed resistance

is high.

The low resistance of the skin during high stress is due to an increase in the blood

supply to the skin. This increases the permeability of the skin and hence the

conductivity for electric current. This property of the skin is used here to measure the

stress level. Using suitable circuitry we can convert the amount of stress a human

being feels to a varying analog voltage.

The LM3915 is a monolithic integrated circuit that senses analog voltage levels

and drives ten LED’s, LCD’s or vacuum fluorescent displays, providing a logarithmic

3dB/step analog display.

The touch pads of the stress meter sense the voltage variations across the touch

pads and convey the same to the circuit. The circuit is very sensitive and detects even

a minute voltage variation across the touch pad.

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1.2 EVOLUTION

In an article “Stress and Mind Control”, 21/03/2008, Roberto Bonomi stated that

“When we speak of the fabulous relaxation capacity that mind control gives us, the

first thing that comes to our mind, is that we will be able to take off, the excesses of

nervous tension, the stress; and this is a great benefit. Because suppose that you could

measure stress in inches, and that you have stress zero when the meter is located in

zero.”

Based on this, our project is aimed to give a visual indication of one’s stress

through a light-emitting diode display along with a warning yellow light.

1.3 PURPOSE OF THE PROJECT

The purpose of stress meter is to assess the emotional pain of human being. The

stress can cause hair to fall, acne to break out and many other problems. These

manifestations of stress can cause even more anxiety. Stress causes cortical levels to

increase within the body, which increases oil production, which causes acne

breakouts.

So this stress meter is to solve all the problems caused due to stress by checking

the stress of an individual and taking care before any serious problem occurs.

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Chapter 2

BLOCK DIAGRAM AND PROJECT OVERVIEW

2.1 PRINCIPLE OF STRESS METER:

The stress meter is based on the principle that the variations

in the resistance of the skin due to blood pressure of ones’

body can be directly converted and transmitted into analog

voltage levels to give the visual indication of human stress

using a proper circuitry.

2.2 BLOCK DIAGRAM:

230 volts AC power Supply

TransformerRegulated power supply

Dot/BarDisplay Driver

Out PutStress Level Indication

High StressIndication

Input throughTouch Pads

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Fig: 2.1 Block diagram of stress meter

2.3 BLOCK DIAGRAM DESCRIPTION:

The 230 volts ac power supply is given to the transformer.

The transformer steps down the input voltage line and

isolates the power supply from the power line. A full wave

bridge rectifier circuit along with a voltage regulator is

used to give a regulated power supply to the circuit .The

input touch pads are used to sense the resistance of our skin

and this input is fed to the dot/bar display driver.

The dot/bar display driver accepts the input through the

touch pads which sense the small change in resistance the

dot/bar driver gives the output stress level indication

according the input. The output is indicated on a led

display .The ten led’s act like the stress level indicators

form zero stress level to high stress level on a scale of ten.

The high stress detected from the dot/ bar display driver is

indicated through a warning yellow light.

2.4 APPLICATION:

Each LED in stress meter operates with a 3dB difference

from the previous one, and a jumper is provided to allow

dot or bar mode. This project is an essential part of the

expandable analyzer and one meter circuit is used for each

frequency band. There are many other uses for a simple

LED meter. They are ideal as power meters on amplifiers,

can be used with mixers (including the high quality mixer),

preamps and any other application where it is important to

know the signal level.

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LM3915's 3 dB/step display is suited for signals with

wide dynamic range, such as audio level, power, light

intensity or vibration. Audio applications include average

or peak level indicators, power meters and RF signal

strength meters. Replacing conventional meters with an

LED bar graph results in a faster responding, more rugged

display with high visibility that retains the ease of

interpretation of an analog display.

Chapter 3

COMPONENTS OVERVIEW

3.1 DOT/BAR DISPLAY DRIVER:

The LM3915 is a monolithic integrated circuit that senses

analog voltage levels and drives ten LEDs, LCDs or

vacuum fluorescent displays, providing a logarithmic 3

dB/step analog display. One pin changes the display from a

bar graph to a moving dot display. LED current drive is

regulated and programmable, eliminating the need for

current limiting resistors. The whole display system can

operate from a single supply as low as 3V or as high as

25V.

LED current drive is regulated and programmable,

eliminating the need for current limiting resistors. The IC

contains an adjustable voltage reference and an accurate

ten-step voltage divider. The high-impedance input buffer

accepts signals down to ground and up to within 1.5V of

the positive supply. Further, it needs no protection against

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inputs of ±35V. The input buffer drives 10 individual

comparators referenced to the precision divider. Accuracy

is typically better than 1 dB.

Fig: 3.1 Dot/Bar display driver

3.2 THE PIEZO ELEMENT:

Piezoelectric diaphragm is a basic electronic sound

component. It has the advantages of simple structure, stable

performance and high reliability. It is not only the core

element of piezoelectric buzzers and the alarms, but also

used as shock sensors in many sensitive equipments.

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Fig 3.2 The piezo element

Basically, the sound source of a piezoelectric sound

component is a piezoelectric diaphragm. A piezoelectric

diaphragm consists of a piezoelectric ceramic plate which

has electrodes on both sides and a metal plate (brass or

stainless steel, etc.). A piezoelectric ceramic plate is

attached to a metal plate with adhesives. Applying D.C.

voltage between electrodes of a piezoelectric diaphragm

causes mechanical distortion due to the piezoelectric effect

Design Considerations:

These devices contain no electronics, and require external

circuitry to produce an audible tone.  Presence of the

feedback tab enables the designer to simplify the drive

circuit.  Voltage applied to the device produces mechanical

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distortions which are usable, among other applications, in

alarms and sensors.

Fig 3.3 Piezo electric diaphragm

The Touch Pad:

The Touch Pad is two tinned pads on the PC board. When

touched them with a finger, the resistance of the finger is

reduced by a factor of about 100 - 400 by the gain of the

emitter-follower transistor and this puts a HIGH on the

input pin of the chip. The input impedance of the chip is

fairly high (about 50k) but when you add a pull-down

resistor (to prevent stray signals being detected by the

chip), the impedance decreases. The answer is to add the

emitter-follower transistor.

Fig: 3.4 Touch pad

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3.3 LIGHT EMITTING DIODES:

A light emitting diode (LED) is a PN junction

semiconductor diode that emits photons when electrical

current passes through the junction in the forward

direction, the electrical carriers give up energy proportional

to the forward voltage drop across the diode junction, this

energy is emitted in the form of light.

Fig 3.5 Light Emitting Diode

LED’s are used in numerical displays such as those on

electronic digital watches and pocket calculators. By

definition, it is a solid-state device that controls current

without heated filaments and is therefore very reliable.

LED’s are highly monochromatic, emitting a pure color in

a narrow frequency range. The color emitted from an LED

is identified by peak wavelength and measured in

nanometers.  LEDs are made from gallium-based crystals

that contain one or more additional materials such as

phosphorous to produce a distinct color. LED light output

varies with the type of chip, encapsulation, efficiency of

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individual wafer lots and other variables. Several LED

manufacturers use terms such as "super-bright," and "ultra-

bright" to describe LED intensity.

Because LED’s are solid-state devices they are not subject

to catastrophic failure when operated within design

parameters. LED’s are current-driven devices, not voltage

driven. Although drive current and light output are directly

related, exceeding the maximum current rating will

produce excessive heat within the LED chip due to

excessive power dissipation. The color of an LED is

determined by the semiconductor material, not by the

coloring of the 'package' (the plastic body). LEDs are

available in red, orange, amber, yellow, green, and blue and

white colors. LED’s are specially constructed to release a

large number of photons outward. Additionally, they are

housed in a plastic bulb that concentrates the light in a

particular direction

Fig 3.6 Inside a Light Emitting Diode

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Design Parameters:

Never an LED should be connected directly to a battery or

power supply. It will be destroyed almost instantly because

too much current will pass through and burn it out. An

LED must have a resistor connected in series to limit the

current through the LED; otherwise it will burn out almost

instantly and try to avoid connecting them in parallel.

LED Color Potential Difference

Infrared 1.6V

red 1.8 to 2.1V

orange 2.2V

yellow 2.4V

green 2.6V

blue 3.0V to 3.5V

white 3.0V to 3.5V

ultraviolet 3.5V

Table 3.1 LED color vs. potential

difference

Equation to determine the required resistance:

Resistance = (Source Voltage – LED Voltage Drop) /

desired current

To drive an LED from a system, the following values are

used:

Source voltage = 13.4 volts (approximately)

Voltage drop = 3.6 volts (typical for a blue or white LED)

Desired current = 30 milliamps (typical value)

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So the resistor we need is:

(13.4 – 3.6) / (30 / 1000) = 327 ohms (Approximately 330

ohms).

3.4 REGULATED POWER SUPPLY:

In a typical linear power supply, AC line voltage is first

down-converted to a smaller peak voltage using a

transformer which is then rectified using a full wave bridge

rectifier circuit. A capacitor filter is then used to smoothen

the obtained sinusoidal signal. The residual periodic

variation or ripple in this filtered signal is eliminated using

an active regulator.

Fig: 3.7 Regulated power supply

To obtain a DC power supply with both positive and

negative output voltages, a center-tapped transformer is

used, where a third wire is attached to the middle of the

secondary winding and it is taken as the common ground

point. Then voltages from the opposite ends of the winding

will be positive or negative with respect to this point

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Care should be taken while connecting 78XX and 79XX

ICs.

Voltage Regulator:

The 7805 takes in a voltage between 7 and 30 volts and

regulates it down to exactly 5 volts. The first capacitor

takes out any ripple coming from the transformer so that

the 7805 is receiving a smooth input voltage, and the

second capacitor acts as a load balancer to ensure

consistent output from the 7805.

The 7805 has three

leads. If the 7805 is seen

from the front (the side

with printing on it), the

three leads are, from left

to right, input voltage (7

to 30 volts), ground, and

output voltage (5 volts). Fig 3.8 Voltage regulator

3.5 CIRCUIT DIAGRAM OF STRESS METER:

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Fig 3.9 Circuit diagram of Stress meterChapter 4

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CIRCUIT OPERATION

4.1 OPERATION OF THE CIRCUIT:

This stress meter circuit uses just one IC and a very few

number of external components. It displays the input level

in terms of 10 LEDs. The suggested input voltage can vary

from 12V to 20V.

The LM3915 IC contains an adjustable voltage

reference and an accurate ten-step voltage divider. The

high-impedance input buffer accepts signals down to

ground and up to within 1.5V of the positive supply.

Further, it needs no protection against inputs of 35V. The

input buffer drives 10 individual comparators referenced to

the precision divider. Accuracy is typically better than 1

dB.

A high input impedance buffer operates with signals

from ground to 12V, and is protected against reverse and

over voltage signals. The signal is then applied to a series

of 10 comparators; each of which is biased to a different

comparison level by the resistor string.

The LM3915 is extremely easy to apply. A 1.2V full-

scale meter requires only one resistor in addition to the ten

LEDs. One more resistor programs the full-scale anywhere

from 1.2V to 12V independent of supply voltage. LED

brightness is easily controlled with a single pot.

The following relations can be used to know the

approximate values of current and reference voltages.

V ref = 1.25 (1+R2/R1) + R2* 80UA

I (LED) = (12.5V/R1) + (Vref/2.2K)

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The outputs can drive LCDs, vacuum fluorescents and

incandescent bulbs as well as LED’s of any color. Multiple

devices can be cascaded for a dot or bar mode display with

a range of 60 or 90 dB. LM3915s can also be cascaded

with LM3914s for a linear/log display or with LM3916s for

an extended-range VU meter.

4.2 CIRCUIT CONNECTIONS:

The pins 2, 4 and 8 of the LM3915 are grounded. 6 and 7

pins are shorted and a resistor is connected across them

which is grounded. Pin 1 and pins 10 to 18 are connected to

LED’s to be driven by the IC. Pin 9 and 11 are shorted to

give a bar mode display.3 pin is given the input voltage.

Pin 5 is used to connect the touch pads.

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Fig 4.1 Basic block diagram showing the operation of the

circuit

4.3 ROLE PLAYED BY TOUCH PADS:

The touch pad which is a piezoelectric substance senses the

skin resistance when touched with a finger and acts like the

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input to the circuit. The output stress level is indicated on

the LED display. The high stress level is indicated by a

warning yellow light.

The following figure gives a clear idea of the principle

behind the stress meter and the role played by the touch

pads.

Fig 4.2 Role played by touch pads

4.4 THE LED DISPLAY:

Conducting pad (upper)

Top view

Conducting pad (lower)

Resistance between the pads varies based on wetness and dryness of the skin of the finger

Insulator strip (between the upper and lower pad)

Side View

sis

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The output is indicated on a LED display .The ten LED’s

act like the stress level indicators form zero stress level to

high stress level on a scale of ten. The high stress detected

from the dot/ bar display driver is indicated by a yellow

light.

LED THRESHOLD 1 60mV

2 80mV

3 110mV

4 160mV

5 220mV

6 320mV

7 440mV

8 630mV

9 890mV

10 1.25V

Table 4.1 LED Vs. Threshold voltage

4.5 POWER SUPPLY GENERATION:

The 230 volts ac power supply is fed to the transformer and

the transformer in turn is connected to a bridge rectifier

circuit.

When four diodes are connected as shown in figure, the

circuit is called a BRIDGE RECTIFIER. The input to the

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circuit is applied to the diagonally opposite corners of the

network, and the output is taken from the remaining two

corners.

On the positive half cycle of transformer secondary

supply voltage, diodes D1 and D2 conduct, supplying this

voltage to the load. On the negative half cycle of supply

voltage, diodes D3 and D4 conduct supplying this voltage to

the load.

It can be seen from the waveforms that the peak inverse

voltage of the diodes is only Vm .The average output

voltage is the same as that for the center-tapped

transformer full-wave rectifier.

With a resistive load, the load current is identical in

shape to the output voltage. Most loads are inductive and

the load current with these loads depends on the value of

load resistance and load inductance so they do not conduct

any current.

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Fig 4.3 Bridge Rectifier Circuit

During the negative half-cycle, the top end of the

transformer winding is negative. Now, D1 and D4 are

forward biased, and D2 and D3 are reverse biased.

Therefore, electrons move through D1, the resistor, and D4

in the direction shown by the blue arrows. As with the

positive half-cycle, electrons move through the resistor

from left to right. In this manner, the diodes keep switching

the transformer connections to the resistor so that current

always flows in only one direction through the resistor. The

resistors can be replaced with any other circuit, including

more power supply circuitry (such as the filter), and still

see the same behavior from the bridge rectifier.

Fig 4.4 The Power supply generation

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The output from the bridge rectifier is thus connected to

voltage regulator 7805 to generate 5 volts regulated power

supply to the circuit. The capacitors are used as filters to

smoothen the sinusoidal signals.

4.6 THE STRESS METER:

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Chapter 5

RESULTS AND ANALYSIS

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5.1 RESULTS:

The stress meter thus detects the resistance of skin which is

according to one’s mental stress and gives a visual

indication on a LED display. The LED’s on the stress

meter can be observed as stress level indicators form zero

to ten stress levels on a scale of ten. The high stress of a

person is indicated through a warning yellow light.

5.2 ANALYSIS:

Resistance varies inverse proportional to the stress. If the

stress level is high the skin offers less resistance, and if

relaxed resistance is high. The low resistance of the skin

during high stress is due to an increase in the blood supply

to the skin. This increases the permeability of the skin and

hence the conductivity for electric current.

The LED 1 glows by default when the circuit is on.

When a person touches the touch pad of the stress meter

with his finger, it senses the skin resistance and hence the

stress. On a scale of ten, stress levels from 0 to 10 can be

observed, where the LED 10 when on gives a warning

yellow light high stress indication.

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Chapter 6

CONCLUSION

6.1 SUMMARY:

In this project, is proposed a stress meter that indicates the

stress level of a human being based on one’s skin resistance

on a scale of ten. The circuit uses the IC LM3915 which is

a dot/bar display driver which can easily drive ten led’s

with a suggested input voltage.

The touch pad which is a piezoelectric substance senses

the skin resistance when touched with a finger and acts like

the input to the circuit. The output stress level is indicated

on the led display. The high stress level is indicated by a

warning yellow light.

The regulated power supply used in the project gives an

input voltage of 5V for the circuit to operate. A switch is

used to ON/OFF the circuit.

6.2 APPLICATIONS:

Stress meter is widely applicable in various meters and

indicators. It is used as

A simple LED meter.

Signal level indicator.

In Peak detectors.

Light, audio, and power meters.

Multiple devices can be cascaded for a dot or bar

mode display with a range of 60 or 90 dB.

28

LM3915s can also be cascaded with LM3914s for a

linear/log display or with LM3916s for an

extended-range VU meter.

.

.

6.3 BENEFITS:

The circuit is absolutely free from ambient light.

It is economical and a low budget project.

Not a complex circuit.

The components are easily available in the market

and replaceable.

Noise pulse do not have any effect on the

circuit.

LED’s can withstand the voltage even if no resistors

are connected across.

Can be used easily to regularly check one’s stress

level.

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APPENDIX

PIN DIAGRAM OF LM3915:

DEFINITION OF TERMS:

Absolute Accuracy: The difference between the observed

threshold voltage and the ideal threshold voltage for each

30

comparator. Specified and tested with 10V across the

internal voltage divider so that resistor ratio matching error

predominates over comparator offset voltage.

Adjust Pin Current: Current flowing out of the reference

adjust pin when the reference amplifier is in the linear

region.

Comparator Gain: The ratio of the change in output

current (ILED) to the change in input voltage (VIN)

required to produce it for a comparator in the linear region.

http://www.atmel.com/dyn/resources/prod_documents/

doc0368.pdf

Dropout Voltage: The voltage measured at the current

source outputs required to make the output current fall by

10%.

Input Bias Current: Current flowing out of the signal

input when the input buffer is in the linear region.

LED Current Regulation: The change in output current

over the specified range of LED supply voltage (VLED) as

measured at the current source outputs. As the forward

voltage of an LED does not change significantly with a

small change in forward current, this is equivalent to

changing the voltage at the LED anodes by the same

amount.

Line Regulation: The average change in reference output

voltage (VREF) over the specified range of supply voltage

(V+).

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Load Regulation: The change in reference output voltage

over the specified range of load current (IL (REF)).

Offset Voltage: The differential input voltage which must

be applied to each comparator to bias the output in the

linear region. Most significant error when the voltage

across the internal voltage divider is small. Specified and

tested with pin 6 voltage (VRHI) equal to pin 4 voltage

(VRLO).

Relative Accuracy: The difference between any two

adjacent threshold points. Specified and tested with 10V

across the internal voltage divider so that resistor ratio

matching error predominates over comparator offset

voltage.

LM3915 OUTPUT CHARACTERISTICS:

32

Output voltage Vs. Output current

BIBLIOGRAPHY

33

Journals:

1. Roberto Bonomi, “Stress and Mind Control”, dated 21/03/2008

Reference Books:

1. Joseph Edminster and Mahmood Nahvi, Electric circuits, Schaum’s Outline, 2003

2. Stanley G Burns and Paul R Bond, Principles of Electronic Circuits, International Thomson publishing, 1997

3. Richard C Jaegar and Travis N Blalock, Micro electronic circuit design, Third Edition Errata, 2008

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