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Relaxation or Stress Monitor Project

2015-01-23 14:35  
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This relaxation and stress monitor circuit tells you that whether your body is relaxed or not. It exploits the skin’s galvanic property to measure the stress level of the body.
Even though the circuit do not measures the stress level precisely, it can be used as an indicator to know the stress level.

About the stress

What is Stress?Stress is a physiological condition in which the body becomes excited to face an emergency situation. A number of physiological changes including increase in heart rate, breathing rate etc occurs during a high stress state. These hormone triggered changes makes the body alert to face the situation.

But a permanent stress condition is harmful to the body and may results in many health problems. Many relaxation techniques are now in practice to keep the body in relaxed condition. The circuit described here is useful to monitor the stress level before and after practicing a relaxation exercise.

Galvanic Skin Response GSR

The circuit exploits the Galvanic skin response as an indicator of stress. The electrical conductivity of the skin varies depending on the stress level of the body. In the relaxed state, skin offers high resistance about 2 Meg ohms or more but it reduces to 25K or less in a stressful condition. The physiological changes in the body during high stress causes sweating and leakage of water from the blood vessels in the skin. This makes the skin moist and electrical conductivity increases. This is specially noticed in the left palm and fingers.

Some info about the circuit

Front end of the circuit has a voltage sensor comprising two touch pads and a signal amplifier. The touch pads along with R1 and VR1 forms a bridge to give a variable voltage into the input of IC1 based on the conductivity of the skin. When the touch pads make contact with the skin of first and middle fingers of left hand, current passes from pad 1 to pad 2 through the skin. The skin between the touch pads act as a variable resistor and the current passing through it depends on the resistance of the skin, that in turn depends on the stress level.

The variable voltage level is sensed by IC1 and it gives an amplified variable output. That is the output of IC1 depends on the current flowing through the skin.TL071 is a JFET input op amp with sensitive inputs so that it can sense even minute current in its inputs. Resistor R2 and capacitor C1 gives sufficient feed back to IC1 to give maximum gain. Regulator IC7805 gives fixed 5 volts to the bridge for accurate measurement of skin conductivity. IC2 is the monolithic integrated circuit with ten current sinking outputs (pin18-pin10 and pin1). It gives a logarithmic LED display.

The input of LM3915 is very sensitive and its internal circuit is referenced at 1.25 volts. Each output from pin 18 to pin 1 sinks current when the input receives an increment of 125 milli volts. Resistor R4 determines the current flowing through the LEDs and thus its brightness. Pin 9 of IC is the mode selector and it tied to the positive supply to get a Bar mode display. If it left free, IC will give Dot mode display.

Relaxation and Stress Monitor Circuit diagram

relaxation and stress monitor circuit schematic

Touch pads

Touch pads can be two strips of conducive foil such as aluminium foil glued on self adhesive straps as shown in the photo. The foil side should be in contact with the skin after tying the strap.

ICTL071 and LM 3915 Pin out


Do some checking

After wearing the touch pads on left fingers, wait for few minutes to stabilize the circuit. Keep body and mind relaxed for some time. Adjust VR1 till one or two LEDs connected to pins 18 and 17 just glows. Adjust VR2 also to keep these LEDs glowing. This can be considered as the relaxed state provided your body is truly relaxed. Measure the stress level after a busy schedule and see that more LEDs light up indicating the high stress level. Since the skin conductivity is different in different persons, setting of VR1 and VR2 is necessary for each person.

LM3915
7805

TL071 datasheet
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