Pulse oximeter

Pulse oximeter

Pulse oximeter

• Original patent

Your data

0 20 40 60 80 100 120-0.35

-0.3

-0.25

-0.2

-0.15

time

Sig

nal (

arb

units

)

Big deep breathsand then hold

Damn, I want some air

Gave up

Photodiode detection

photodiode

Detectorsignal

R 5

1M

+5 VU 2 A

L M C 6 4 8 4

1

3

2

411

O U T

+

-

V+

V-

Photodiode: Current is proportional to amount of light hitting the detector

Detection demo

We will do something else…

Hysteretic oscillator

+5 V

R 1

10K

U 2 A

L M C 6 4 8 4

1

3

2

411

O U T

+

-

V+

V-

+2.5 V

C 2

1uF

Sq. wave

R 2

10K

R 3

6k

Hysteretic oscillator

0 0.005 0.01 0.015 0.02 0.025 0.030

1

2

3

4

5

time (s)

V

outputV-V+

+5 V

R 1

10K

U 2 A

L M C 6 4 8 4

1

3

2

411

O U T

+

-

V+

V-

+2.5 V

C 2

1uF

Sq. wave

R 2

10K

R 3

6k

LED Driver

Redwire

U 2 A

L M C 6 4 8 4

1

3

2

411

O U T

+

-

V+

V-

IR LE D

Red LE D

Sq. wave R 4

1K

+5 V

Yellow wire

Pulse OX probe

2.5 V

Background

0 0.02 0.04 0.06 0.08 0.10

0.2

0.4

0.6

0.8

1

time (s)

Det

ecto

r (a

rb u

nits

)

Background + red

Detector sees the sum of the two curves

0 0.02 0.04 0.06 0.08 0.10

0.2

0.4

0.6

0.8

1

time (s)

Det

ecto

r (a

rb u

nits

)

Background + red + ir

Detector sees the sum of the three curves

0 0.02 0.04 0.06 0.08 0.10

0.2

0.4

0.6

0.8

1

time (s)

Det

ecto

r (a

rb u

nits

)

Convert driving signal to +/- 1

0 0.02 0.04 0.06 0.08 0.10

0.5

1

time (s)

Det

ecto

r (a

rb u

nits

)

0 0.02 0.04 0.06 0.08 0.1

-1

0

1

time (s)

Driv

ing

Multiply +/- 1 and detector

0 0.02 0.04 0.06 0.08 0.10

0.5

1

time (s)

Det

ecto

r (a

rb u

nits

)

0 0.02 0.04 0.06 0.08 0.1-1

0

1

time (s)

Driv

ing

Blue – bg onlyRed – bg + red LEDBlack - bg +red + ir

Driv

ing

* D

etec

tor

Integrate w.r.t time - background

0 0.05 0.1 0.15 0.2

-1

0

1

time (s)

Driv

ing*

dete

ctor

0 0.05 0.1 0.15 0.2 0.25 0.3-0.2

0

0.2

0.4

0.6

time (s)

cum

mul

ativ

e in

tegr

al

(t)) dt

Integrate w.r.t time - bg +red

0 0.05 0.1 0.15 0.2

-1

0

1

time (s)

Driv

ing*

dete

ctor

0 0.05 0.1 0.15 0.2 0.25 0.30

0.2

0.4

0.6

time (s)

cum

mul

ativ

e in

tegr

al

Integrate w.r.t time - bg +red+ir

0 0.05 0.1 0.15 0.2

-1

0

1

time (s)

Driv

ing*

dete

ctor

0 0.05 0.1 0.15 0.2 0.25 0.30

0.2

0.4

0.6

time (s)

cum

mul

ativ

e in

tegr

al

Converges to average difference between red and ir

0 0.2 0.4 0.6 0.8 10

0.02

0.04

0.06

0.08

0.1

time (s)

cum

mul

ativ

e in

tegr

al

We will keep the running integral by using a low-pass filter

Low-passed result

0 0.2 0.4 0.6 0.8 1-1

0

1

time (s)

Det

ecto

r *

+/-

1

0 0.2 0.4 0.6 0.8 10

0.1

0.2

time (s)

Low

pas

s

How do we implement a low pass filter in software?