Stress Meter

1.    INTRODUCTION

1.1            STRESS METER

Stress is very common condition for human beings. Stress is nothing more than a socially acceptable form of mental illness. The 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 skin varies in accordance with the emotional stress. Resistance varies inversely proportional to the stress. If the stress level is high the skin offers low resistance, and if relaxed resistance is high.

      The low resistance of the skin during high stress is due to an increase in blood supply to the skin. This increases the permeability of the skin and hence conductivity for electric current. This property of 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 level and drives 9 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 pad and convey the same to the circuit. The circuit is very sensitive and detects even a minute’s voltage variation across the touch pad.

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.

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 skin due to blood pressure of one’s body can be directly converted and transmitted in to analog voltage levels to give the visual indication of human stress using proper circuitry.

2.2 BLOCK DIAGRAM

2.3 BLOCK DIAGRAM DESCRIPTION:

The 6F22 9v dc battery gives 9v to the zener diode voltage regulator. The zener diode 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 nine 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 red 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.

LM3915’s 3dB/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 analog display.

3.  COMPONENTS OVERVIEW

3.1 DOT/BAR DISPLAY DRIVER

     The LM3915 is a monotonic integrated circuit that senses analog voltage levels and driver ten LEDs, LCDs or vacuum fluorescent displays, providing a logarithmic 3dB/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 be operate from a single supply as low as 3V or as high as 25V

          LED current drive is regulated and programmable, eliminating need for current limiting resistors. The IC contains an adjustable voltage reference and an accurate ten-step voltage driver. The high impedance input buffer accepts signals down to the 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 1dB.

Figure: 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 equipment.

          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 distortions which are usable, among other applications, inalarms and sensors.

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.

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 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

Figure: Inside a Light Emitting Diode

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)

So the resistor we need is: (13.4 – 3.6) / (30 / 1000) = 327 ohms (Approximately 330ohms).

3.4 DC BATTERY

A nine-volt battery, also called a PP3 battery, is shaped as a rounded rectangular prism and has a nominal output of nine volts. Its nominal dimensions are 48 mm × 25 mm × 15 mm. 9V batteries are commonly used in pocket transistor radios, smoke detectors, carbon monoxide alarms, guitar effect units, and radio-controlled vehicle controllers. They are also used as backup power to keep the time in digital clocks and alarm clocks. Nine-volt batteries are often constructed of 6 individual 1.5V AAAA batteries enclosed in a wrapper.

                                              Panasonic PP3 (9 volt) battery

The connector (snap) consists of two connectors: one smaller circular (male) and one larger, typically either hexagonal or octagonal (female). The connectors on the battery are the same as on the connector itself; the smaller one connects to the larger one and vice versa.

The PP3 appeared when portable transistorized radio receivers became common, and is still called a "transistor" battery by some manufacturers. The Eveready Company claims that it introduced this battery type in 1956.

The battery has both the positive and negative terminals on one end. The negative terminal is fashioned into a snap fitting which mechanically and electrically connects to a mating terminal on the power connector. The power connector has a similar snap fitting on its positive terminal which mates to the battery. This makes battery polarization obvious since mechanical connection is only possible in one configuration. The clips on the 9-volt battery can be used to connect several 9-volt batteries in series. One problem with this style of connection is that it is very easy to connect two batteries together in a short circuit, which quickly discharges both batteries, generating heat and possibly a fire. Multiple 9 volt batteries can be snapped together in series to create higher voltage.

      Inside a PP3 there are six cells, either cylindrical alkaline or flat carbon-zinc type, connected in series. Some brands use welded tabs internally to attach to the cells, others press foil strips against the ends of the cells.

Rechargeable NiCd and NiMH batteries have various numbers of 1.2 volt cells. Lithium versions use three 3.2 V cells - there is a rechargeable lithium polymer version. There is also a Hybrid NiMH version that has a very low self- discharge rate (85% of capacity after 1 year of storage).

Formerly, mercury batteries were made in this size. They had higher capacity than carbon-zinc types, a nominal voltage of 8.4 volts, and very stable voltage output. Once used in photographic and measuring instruments or long-life applications, they are now unavailable due to environmental restrictions.

4. THE STRESS METER CIRCUIT

4.1 CIRCUIT DIAGRAM OF STRESS METER:

4.2 OPERATION OF THE CIRCUIT:

This   stress   monitor     lets   you assess   your   emotional   pain. If the stress is very high, it gives visual indication through a light emitting diode (LED) display along with a warning beep.   The   gadget is small enough to be worn around the wrist. The   gadget   is   based   on   the   principle that the resistance of the skin varies in accordance with your emotional states. If the stress level is high the skin offers less resistance, and if the body is relaxed the skin 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.   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 detects   even   a   minute   voltage   variation across the touch pads. The circuit comprises signal amplifier   and   analogue     display   sections. Voltage variations from the sensing pads are amplified by transistor BC548 (T1),   which is configured as a common-emitter amplifier. The base of T1 is connected to one of the touch pads through resistor R1 and to the ground rail   through   potentiometer   VR1.   By   varying   VR1,   the   sensitivity   of   T1   can adjusted to the desired level. Diode D1 maintains proper biasing of T1 and capacitor C1 keeps the voltage from the emitter of T1 steady.

The amplified signal from transistor T1 is given to the input of ICLM3915 (IC1) through VR2. ICLM3915 is a monolithic integrated circuit that senses analogue voltage levels at its pin 5 and displays them through LEDs providing a logarithmic analogue display. It can drive up to ten   LEDs one by one in the dot and bar mode for each increment of 125 mV in the input. Here, we’ve   used only five LEDs connected at pins 14 through 18 of IC1. LED1 glows when input pin 5 of IC1 receives 150mV.LED9 glows when the voltage rises to 1.25mV and LED9 flashes and piezo buzzer PZ1 beeps when the stress level is high. Resistors R4 and R5 and capacitor C2 form the flashing   elements. Resistor R3 maintains the LED current at around 20mA. Capacitor C3 should be placed close to pin3 for proper functioning of   the   IC. Zener diode ZD1 in series with resistor R6 provides regulated 5Vto the circuit. The circuit can be assembled on a small   piece   of   perforated   board. Use be transparent 3mm LEDs and a small piezo buzzer for audio-visual indications. Enclose the   circuit in a small plastic case with touch pads on the back side. Two self-locking straps can be used   to tie the unit around your wrist. After touching on touch pads and observe the stress variations. Adjust VR2 if selectivity is very high.

4.3 ZENER DIODE VOLTAGE REGULATOR

A Zener diode is a PN junction that has been specially made to have a reverse voltage breakdown at a specific voltage. Its characteristics are otherwise very similar to common diodes. In breakdown the voltage across the Zener diode is close to constant over a wide range of currents thus making it useful as a shunt voltage regulator.

Figure 1 shows the current versus voltage curve for a Zener diode. Observe the nearly constant voltage in the breakdown region.

The forward bias region of a Zener diode is identical to that of a regular diode.  The typical forward voltage at room temperature with a current of around 1 mA is around 0.6 volts. In the reverse bias condition the Zener diode is an open circuit and only a small leakage current is flowing as shown on the exaggerated plot.  As the breakdown voltage is approached the current will begin to avalanche.  The initial transition from leakage to breakdown is soft but then the current rapidly increases as shown on the plot. The voltage across the Zener diode in the breakdown region is very nearly constant with only a small increase in voltage with increasing current.  At some high current level the power dissipation of the diode becomes excessive and the part is destroyed.  There is a minimum Zener current, IZmin, that places the operating point in the desired break down region and there is a maximum Zener current, IZmax, at which the power dissipation drives the junction temperature to the maximum allowed (typically in the 125 to 150 C range). Beyond that current and the diode can be damaged or destroyed.

     There is no specific value for IZmin although it is typically taken to be ten percent of IZmax. It is possible that a lower value could be used particularly at Zener voltages greater than around six.  This insures that the diode operating current is in the breakdown region and not in the soft transition region.  The ten percent value is also a historical rule-of-thumb for shunt voltage regulators in general.    A shunt regulator has to conduct current in order to be in regulation.  To prevent the current from going to zero, shunt regulators are often designed so that at the maximum load current there is at least ten percent of that current in the regulator.

Zener diodes are available from about 2.4 to 200 volts typically using the same sequence of values as used for the 5% resistor series –2.4, 2.7, 3.0 3.3, 3.6, 3.9, 4.3, 4.7, 5.1, 5.6, 6.2, 6.8, 7.5, 8.2, 9.1, 10, 11, 12, 13, 15, 16, 18, 20, 22, 24, etc.

All Zener diodes have a power rating, Pz.  From Watt’s law the maximum current is Izmax = P / V. Zener diodes are typically available with power ratings of 0.25, 0.4, 0.5, 1, 2, 3, and 5 watts although other values are available.

The purpose of a voltage regulator is to maintain a constant voltage across a load regardless of variations in the applied input voltage and variations in the load current.  A typical Zener diode shunt regulator is shown in Figure 2.  The resistor is sized so that when the input voltage is at VINmin   and the load current is at ILmax that the current through the Zener diode is at least IZmin.  Then for all other combinations of input voltage and load current the Zener diode conducts the excess current thus maintaining a constant voltage across the load.  The Zener conducts the least current when the load current is the highest and it conducts the most current when the load current is the lowest

Shunt regulators are normally only used for applications where the load power is not much (no more than a few watts) because under the worst case situation of no load the Zener has to dissipate the full load power. Shunt regulators have an inherent current

As with most voltage regulators, the source voltage must be slightly higher than the Zener voltage in order to keep the Zener diode in reverse breakdown. Otherwise, the output voltage will simply follow the input voltage.

As touched on earlier, both the diode and resistor must have power ratings high enough to handle all the current should the circuit suddenly stop drawing current. This means that this solution may be impractical for circuits that require high voltage or high current. An 8V regulator with a 12V source that requires 1A of current requires a 250Ω resistor rated at 4W and an 8V Zener diode rated at 8W. Typically, Zener diodes are only available at up to 5W, so in even a medium voltage/current application like this example, it may be more practical to use an integrated circuit regulator.

The Zener diode, with its accurate and specific reverse breakdown voltage, allows for a simple, inexpensive voltage regulator. Combined with the right resistor, fine control over both the voltage and the supply current can be attained. However, the low power ratings of standard Zener diodes and resistors make this solution impractical for high power devices.

4.4 COMMON EMITTER AMPLIFIER

In electronics, a common-emitter amplifier is one of three basic single-stage bipolar-junction-transistor (BJT) amplifier topologies, typically used as a voltage amplifier. In this circuit the base terminal of the transistor serves as the input, the collector is the output, and the emitter is common to both (for example, it may be tied to ground reference or a power supply rail), hence its name. The analogous field-effect transistor circuit is the common-source amplifier.

Common-emitter amplifiers give the amplifier an inverted output and can have a very high gain that may vary widely from one transistor to the next. The gain is a strong function of both temperature and bias current, and so the actual gain is somewhat unpredictable. Stability is another problem associated with such high gain circuits due to any unintentional positive feedback that may be present. Other problems associated with the circuit are the low input dynamic range imposed by the small-signal limit; there is high distortion if this limit is exceeded and the transistor ceases to behave like its small-signal model. One common way of alleviating these issues is with the use of negative feedback, which is usually implemented with emitter degeneration. Emitter degeneration refers to the addition of a small resistor (or any impedance) between the emitter and the common signal source (e.g., the ground reference or a power supply rail). This impedance R_E reduces the overall Trans conductance G_m = g_m of the circuit by a factor of g_mR_E + 1, which makes the voltage gain

So the voltage gain depends almost exclusively on the ratio of the resistors R_C / R_E rather than the transistor's intrinsic and unpredictable characteristics. The distortion and stability characteristics of the circuit are thus improved at the expense of a reduction in gain.

                                                                

FIGURE: Basic NPN common-emitter circuit                          FIRGURE: With emitter-resistance

At low frequencies and using a simplified hybrid-pi model, the following small-signal characteristics can be derived.

	Definition	Expression
Current gain
Voltage gain
Input impedance
Output impedance

If the emitter degeneration resistor is not present, . As expected, when is increased, the input impedance is increased and the voltage gain is reduced.

The bandwidth of the common-emitter amplifier tends to be low due to high capacitance resulting from the Miller effect. The parasitic base-collector capacitance appears like a larger parasitic capacitor (where is negative) from the base to ground. This large capacitor greatly decreases the bandwidth of the amplifier as it makes the time constant of the parasitic input RC filter where is the output impedance of the signal source connected to the ideal base.

The problem can be mitigated in several ways, including:

·         Reduction of the voltage gain magnitude (e.g., by using emitter degeneration).

·         Reduction of the output impedance of the signal source connected to the base (e.g., by using an emitter follower or some other voltage follower).

·         Using a cascade configuration, which inserts a low input impedance current buffer (e.g. a common base amplifier) between the transistor's collector and the load. This configuration holds the transistor's collector voltage roughly constant, thus making the base to collector gain zero and hence (ideally) removing the Miller effect.

·         Using a differential amplifier topology like an emitter follower driving a grounded-base amplifier; as long as the emitter follower is truly a common-collector amplifier, the Miller effect is removed.

4.5 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. Pins 10 to 18 are connected to LEDs 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 through an amplifier.

4.6 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 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.

LED DISPLAY

      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 RED light.

LED	THRESHOLD
1	80mv
2	110mv
3	160mv
4	220mv
5	320mv
6	440mv
7	630mv
8	890mv
9	1.25v

Table 4.1 LED Vs. Threshold voltage

4.7THE STRESS METER

3.8 THE STRESS METER LAYOUT

5. RESULT AND ANALYSIS

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 from 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.

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 LM 3915(which is dot / bar display driver) which can easily drive ten LED’s with a suggested input voltage. The touchpad which is 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 LED display. The high stress level is indicated by a warning RED light. The regulated power supply used in project gives an input voltage of 5v for the circuit to operate. A switch is used to on or off the circuit.

6.2 APPLICATIONS

Stress meter is widely applicable in various meters and indicators. It is used as

•   A sample 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   dB or 90 dB

•   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.