Download Biofeedback Tension Monitor

BIOFEEDBACK TENSION MONITOR
Archana.R III SEM EC LourdMata Engineering College, Trivandrum
Circuit description
The circuit is used to measure the level of tension through analogue and digital displays.
Concept
The circuit is based on the skin’s electrical conductivity. The skin galvanic response depends on the resistance of the skin which varies
depending on the stress level. In the fully relaxed state, skin offers around 2 Meg Ohm or more resistance which reduces to around 500
Kilo Ohms in the fully stressed condition. As a result of various physiological mechanisms during stress, blood flow to the skin
increases and blood vessels becomes more leaky. This increases sweating. As result, resistance of the skin reduces and it conducts more
electricity. Thus the electrical conductivity is directly proportional to the stress level and skin resistance and electrical conductivity are
inversely proportional.
Circuit components
1. Touch pads
Touch pads are used to make contact with the skin. It can be two small aluminium sheets. When the touch pads are shorted with
the middle and first fingers of the left hand (skin of left hand responds more to stress), current flows through it to the input of
the signal amplifier.VR1 reduce current to touch pads.
2. Inverting Amplifier
To amplify the feeble current flowing through the touch pads and skin, a high gain inverting amplifier is used. IC CA3130 is
used as a Resistance-to Voltage converter amplifier. This IC is a CMOS version with gate protected P-channel MOSFETs in the
inputs. It has very high input impedance and low input current (5 pA) requirements. The current passing through the touch
pads is given to the inverting (pin2) of IC1 through resistor R1. The non-inverting input (pin3) of the amplifier is connected to a
potential divider comprising R2, VR2 and R3. VR1 adjusts the input current to pin3. Feed back is provided through VR3 which
adjusts the gain of the amplifier. With this arrangement- R1 and VR3- the amplifier can give very high gain. Thus the skin
resistance is converted by the amplifier into corresponding output voltage.
3. Analogue display
A very sensitive moving coil meter having a coil resistance of 100 ohms is used to get an analogue readout. The meter is
connected to the output of IC1 so that it can give full detection when the IC1 give full output voltage when the touch pads are
free. This condition is treated as high resistance of the skin without any flowing current. In short, when the touch pads are free,
IC1 give high output (high resistance – Relaxed state). If the touch pads are in contact with the skin, depending on the skin
resistance and current flow, needle of the meter move down wards to 6 or 4 readings. This happens when the output voltage of
IC1 reduces as the current the flows through the touch pads to the inverting input (pin2) of IC1.
4. Transistor Amplifier
It is used to stabilize and amplify the output voltage of IC1, a high gain transistor amplifier
(T1) is used. Resistor R5 restricts the base current of T1 and VR4 adjust the gain. Resistor R6
limits the collector current of T1 and capacitor C1 keeps the balance of Collector-Emitter voltage of T1 for stable operation.
Output from T1 is given to the input of the voltage analyzer IC2 through VR5.
5. Voltage analyzer
IC LM 3914 is used to analyze the voltage from T1. It is a Monolithic IC with 10 active low outputs. Each output of IC2 from pin
18 to pin 10 sinks current when its input pin 5 gets an increment of 125 mill volts. It has a series resistor chain inside and the high
end of the chain is connected to pin6 and low end to pin 4. By adjusting VR6 and VR7, it is easy to adjust the current through the
internal resistor chain so that the outputs sink current in a linear fashion. Resistor R8 connected to pin 7 of IC2 determines the
current flowing through the outputs.
All the 10 outputs of IC2 are connected to the positive rail through resistors R9 through R17 (1.2K) Pins 11 and 10 are ORed
using diodes D1 and D2 to give display 9. Pin 9 (display mode selector) is connected to pin 11 to get dot mode display when the
pin 11 becomes high. Outputs of IC2 are used to get digital read out.
6. Analogue –To – Digital Conversion
The analogue signals from IC2 are converted to digital signals using IC3 and IC4. IC3 74LS 147
is a Decimal-to-BCD counter IC. It converts the analogue output signals from IC2 to BCD signals. The BCD outputs from IC3 are
available from the output pins 9, 7,6,14 pins of IC3. IC4 (74 LS04) is the Hex inverter which inverts the BCD signals. IC5
(74LS247) is the true BCD to 7 segment Decorder / Display driver IC. It converts the BCD signals and drives the 7 segment
display so that digital read out is obtained.
7. 7 Segment Display
To get a digital readout, Common Anode type 7 segment display is used. Its cathodes are connected to the outputs of IC5 white
the anodes pin 3 and 8 are connected to the positive rail. The display can give digital readings from 4 to 9 in this arrangement.
That is, when the skin resistance is too high, the display shows reading 6(indicating low skin conductivity) and in the stressed
state, the display shows digit 9(indicating high skin conductivity)
Power Supply
Power to the circuit is derived from a 9-0-9 500 mA step-down transformer with diodes IN 4007 for AC rectification and 470 UF
capacitor for smoothing. Regulator IC 7806 is used to give regulated 6 volts for the circuit.
POWER
SUPPLY
DIGITAL
DISPLAY
BCD TO
7 SEGMENT
DECORDER / DRIVER
HIGH GAIN
INVERTING
AMPLIFIER
TRANSISTOR
ANALOGUE
DISPLAY
VOLTAGE
ANALYSER
AMPLIFIER
INPUT
SENSOR
BIOFEEDBACK TENSION MONITOR
BLOCK DIAGRAM
DECIMEL
TO
BCD
COUNTER
HEX
INVERTER
VR1 47K
Touch Pads
1x2 cm Aluminium Foil
R2
47K
3
VR2
1K
T1
IC1 CA 3130
BC 547
7
R5 560 R
6
IC1
R3
470R
VR3
C1
10 UF
IM GAIN
16
3
18
17
IC 2 16
VR6 LM 3914 15
10K
14
13
5
12
11
6
10
VR7
9
10K
VR4 1M
METER
VU
Meter
R18
4 3
13
D 5 16
6
R19
12
12
13 IC3
1 74LS
147
2
3
4
5
6
C
3
B
7
5
IC4
74LS
4
04
6
A
9
8
9
R20
C
2
IC5
11
R21
B
74LS 10
247
1
9
A
15
7
R22
R23
14
D1 IN 4148
a
7
b
6
3
8
FND
LTS 542
c
4 Common
Anode
d
2
e
1
f
9
g
10
R24
10
R 18 - R24
100 R
D2 IN 4148
8
7
2
2
1
11
4
VR5
10K
14
D
14
1
4
R4
22K
S1
R7
680 R
R6 1.2K
2
R1 100K
R9 - R 17 2.2K
R9 R10 R11R12 R13 R14 R15 R16 R17
7
8
Hex Inverter
BCD-to- 7 Segment
Decorder / Driver
8
R8
680 R
Signal
Input
High Gain
Inverting
Amp
Analogue
Display
Transistor
Amp
Decimal -to-BCD
Counter
Voltage Analyser
Biofeedback Tension Monitor
IC 7806
1
Diode
IN4007
9
P
Vin
2
+
0
N
9-0-9
500 mA
Transformer
Power Supply
Circuit diagram
3
GND
100R
230V
7806
Vout
9
470 UF
25V
Diode
IN4007
470UF
25V
LED
Green
6 V DC
-
Digital Display
+
6V DC