circuit boards and software for heart rate and galvanic skin

7/24/2019 Circuit Boards and Software for Heart Rate and Galvanic Skin

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Circuit Boards and Software for Heart Rate and Galvanic Skin

Response Measurements

Jon Spaulding

Synthetic Neurobiology, MIT Media Lab,

[email protected]

January 7, 2009

Abstract

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SYNTHETIC NEUROBIOLOGY MEMO No. 3

This undergraduate research project report demonstrates methods

developed to measure the heart rate and skin conductance of a

subject. Schematics and formulae will be presented. The method

for detecting heart rate is fairly straightforward, using a simply

operational amplifier and some single pole filters. The method

developed for GSR uses a logarithmic amplifier to provide a

reading for skin resistances up to 10 Mohm. This paper will also

briefly discuss the digitization methods used to transfer the

analog data to a computer as well as past methods for GSR

measurement which simply were not robust enough for our

purposes.

Undergraduate researcher (UROP) report


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1 Heart Rate Sensor

The EKG measures the electrical changes in the body through a heartbeat. Two elec-

trodes (one on each hand) measure the electrical signals on different sides of the body.

A third reference electrode (on the pinky instead of the traditional right leg) sets the

ground for the system. The EKG signal is very low frequency. A typical heart beat

can be from about 40 bpm to 150 bpm. This corresponds to about a 0.66-3 Hz range.This is nice as not very much noise is found within this range (so filtering is a much

easier process on the output). The EAGLE schematic corresponding to this design is

EKGv1.2. Electrodes were purchased from Mindpeak (printed screen image from the

order is included in the Images folder).

Figure 1: EKG Schematic

1.1 Circuit Description

The two hand electrodes are first run through an instrumentation amplifier. An in-amp

was chosen as it provides a very high common mode rejection ration (CMRR). This

means that all signals which are common to both inputs are greatly attenuated. This

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includes 60 Hz noise which is inherent in the environment. Without a high CMRR, the

signals would be mixed in with high noise. The instrumentation amp also provides a

gain of 5 which is set byR1

Gain= 1 +50K

R1(1)

After this point, our signal simply needs to be amplified and further filtered. The signal

will pass through a series of filters provided by a quad op-amp. Each of these filters is

a single pole, from the combination of a resistor and capacitor.

1.2 Amplifiers and Filters

After the instrumentation amplifier stage, we have a series of filters. These are all

simple first order filters designed to block out unwanted noise. This section makes

heavy use of the equation:

f3dB = 1

2RC (2)

where the 3dB point is the frequency at which the power is diminished by half (I.e.

amplitude down to 0.707 of peak). The 3dB points of the filters are found below. The

gain of each amplifier is defined by the classicR

2

/R1in an inverting form (4 inverting

stages result in correct signs).

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2 Skin Conductance

When an individual feels nervous or fearful, their skin will release sweat, which in-

creases the conductivity (1/resistance) of the skin. These changes in resistance can be

observed, and used as a measure of the state of the individual. These changes are a very

slow signal, less than 1 Hz typically. I have not layed out a GSR board for the current

design as of yet.

Figure 2: Current GSR Schematic

2.1 Circuit Description and Analysis

This design (see Fig. 2) makes use of the fact thatVGSoperates on a log scale basedon the current flowing through the transistor, which is in turn based on RL. If we usethe human skin asRL, then we have a device which can measure any skin resistance,based on this log scale (see derivation below). As skin resistance doesnt take on any

value, we can cap our measurement at say 10Mohm, using a second leg so that we can

use a differential amplifier to get better resolution for the system.

The derivation for the human sensor side is as follows:

IC=ISe

VBE

VThermal

VBE=KT

q ln

ICIS

VBE= KTq ln

VSVBE

RLIS

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VBE=KT

q ln (VS)

KT

q ln (IS)

KT

q ln (RL) (3)

AsV2 is probably larger thanVBE, we dropVBEfrom the right hand side, and end upwith (3). We then perform a similar analysis, with the variableRL held at 10 Mohmfor a reference value. Let us name these valuesVGSfor the human sensor side, andVREF for the fixed leg side. Running these into the op-amp configuration shown in

Fig. 2 allows us to yield a total output as:

VOUT =R2R4

(VGS VREF) (4)

By changing ourRL term, we end up changing VGS logarithmically. This change ismultiplied by a factor ofR2/R4when it gets to the output.

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3 PIC18F Hardware and Design

3.1 Hardware

Once I had my analog signals, I ran them through a PIC18F4550 chip, with an onboard

10-bit ADC and USB module. The PIC was powered through the USB port. The setup

I used can be seen in Fig. 3. To reset the chip, flip the two switches connected topins 1 and 37 (switches 1 and 2 on the 4-switch bank). The EAGLE package for this

board is found in the USB directory. All chips were obtained for free as samples from

Microchip. It should be noted that the D+ and D- ports are the USB connections. The

pins below it can be used for serial data transmission. BE VERY CAREFUL. If you

dont have the right capacitor values, the circuit actually wont work.

Figure 3: PIC18F4550 Schematic

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3.2 MCHPFSUSB Installation

Find and download the MCHPFSUSB package online. My code makes use of the CDC

example project. I use all the same framework, making modifications to the user.c class.

To get a PIC18F chip talking to the computer, use the driver found in the MCHPFSUSB

package. The computer should then recognize the chip as CDC Demo and work from

there. To edit the code and compile it, I used the MPLAB compiler from Microchip.

3.3 Software

The PIC makes use of the skeleton code provided in the Microchip MCHPFSUSB

CDC code package. The code will wait until a 1 has been sent to it from some other

system. At this point, it will turn on timers and enable interrupts for Timer0. Timer0

is set to fire an interrupt at 1 KHz. At each interrupt, one of the channels will be read

from. The channels will be interleaved (i.e. read from AN0, then AN1, then from AN0

again). This results in each channel being read at 500 Hz (for two channels), which is

more than enough for the waves we are looking for.

The ADC code comes from the adc.h package. It first opens the ADC using the

OpenADC command, followed by which channel the ADC reads from. It starts the

conversion, waits for the conversion to complete, then reads the result from the ADCinto a temporary int (either hr or gsr). There exists a buffer, which contains a letter

(either H or G depending on the channel) in the beginning. The ADC result is

added to the buffer, and a newline character ends the buffer. The entire buffer is then

placed on the USB line, and sent to the host PC. My user.c file can be found in the same

directory as this documentation, along with a compiled .hex version of my code.

4 Failed Designs (And Why They Failed!)

4.1 Wheatstone Bridge GSR

This particular design worked very well for a set skin resistance, or a small dynamic

range. However, it failed for skin conductances which varied by an order of magnitude.

Also, it correlated skin conductances on a 1/x scale, as opposed to a log scale. For these

reasons, the implementation described above was used instead.

4.1.1 Circuit Description

The GSR makes use of a wheatstone bridge to measure the change in conductance

of the skin. Two electrodes are placed on the same hand. One of these should be

connected to ground, this electrode acts as the driven right pinky (or driven right leg)

electrode for the EKG circuit. The other electrode can simply be attached to any fin-

gertip. This circuit makes use of a 5V supply, which needs to stay steady as battery

power diminishes. The power supply is a 9V battery, so a 5V DC DC converter is used

to power the circuit. This 5V power is dropped across the 1K and 9K resistors for a

voltage of 0.5V on the top of the wheatstone bridge. Each leg starts out with a 100

Kohm resistor. On one leg the first (non-ground) electrode is added. On the other leg,

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a 400 Kohm resistor goes to ground, where it connects to the driven right pinky elec-

trode. As the conductance of the skin between the two electrodes changes, the current

through that leg of the bridge will have to increase to increase to get a balanced bridge.

In effect, the voltages above the 400K resistor and at the top electrode vary with time,

based on the conductance of the human skin. These two points are run into an op-

amp, which amplify their difference by a factor of 22. This op amp circuit is the same

configuration used in the new GSR technique (although resistor values are different).

Figure 4: Old GSR Schematic

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5 Appendix A. Amplifier Math

This section is just a reminder for the op-amp math used in the differential amplifier

circuits found in the GSR system. This is just so I dont have to re-derive it every time

I want to use the circuit. The numbers [R1, R4] come from Figure 2 with the currentGSR amplifier. V1 is the voltage beforeR 1, andV2is the voltage beforeR4.

VOut = A[e1 e2]VOut e2

R3

=

e2 V2

r4

e2

1 +

R4R3

= VOut

R4R3

+V2

e2 =

VOut

R4

R3

+V2

R3+R4R3

e2 =VOutR4+R3V2

R3+R4

e1 = R2V1R1+R2

VOut = A

R2V1R1+R2

R4VOutR3+R4

R3V2R3+R4

Now if we letR2 = R3,R1 = R4, and A = large, we get it to simplify to:

0 = R2V1 R4VOut R3V2

VOut =R2R4

(V1 V2) (5)

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