A pulse oximeter uses light to measure the oxygen saturation level in a patient's blood. It works by passing light through tissue and measuring how much light is absorbed. The document describes the circuit design of a pulse oximeter, including an LC oscillator to generate a 5kHz signal, filters to isolate the desired frequency, amplifiers to boost the signal, and a comparator to generate a square wave output. It discusses the principles of operation, gain levels, sensitivity achieved, and challenges in simulating and implementing the circuit. The goal is to create a low-cost heart rate monitor using common amplifier and filter components.

1. PULSE OXIMETER
EVANDER LEMOS
ORCIDIO SANTOS
EE3113
PROFESSOR AUGUST ALLO

2. WHAT IS PULSE OXIMETER?
• Measure the Pulse rate
• Monitoring oxygenation
• Fitness monitoring
• Several Medical applications

3. Circuit Overview

4. Simplification:
Neutral Reference

5. Stage Breakdown

6. Principles of Operation
 LC Oscillator generates 5kHz signal
 Filters trim frequencies above and below desired signal
 High-pass filters double as amplifier stages to boost gain
 Constant Amplifier boosts gain significantly
 Comparator generates a square wave when signal
exceeds reference value

7. Frequency Sweep
0
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Gain
kHz
Gain vs Frequency
Filter A (band pass)
Net Gain (dB)
Filter B (high pass)

8. Band-Pass Detail
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Gain
100 Hz Increments
Bandpass Filter Gain vs Frequency
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9. Gain at 5kHz

10. Circuit Sensitivity
 Huge gain in target band
 Comparator Reference only 0.5V above Neutral
Reference
 Result: Comparator can trip with an input of just 36
microvolts above Neutral Reference
 Original Designer compensated with stronger filtering on
power supply
 Reducing the final gain stage or raising the Comparator
Reference would have also worked.

11. BLOCK DIAGRAM
INFRARED TX AND RX
PULSE SENSOR
SIGNAL CONDITIONING
STAGE
OUTPUT SIGNAL

12. COMPONENTS
• Photosensor APDS9008
• Op Amp MCP6001
• IR Diode
• Capacitors
• Resistors

13. PROPOSED CIRCUIT

14. EQUIVALENT CIRCUIT

15. CIRCUIT ANALYSIS
• Infrared TX and RX
• Photodiode operating in Photoconductivity mode
fbw= 1/(2*pi*Rload*Cj)
Max Reverse Voltage = …

17. CIRCUIT ANALYSIS
• Signal conditioning
Av = -Rf/Rin = (3.3x10^6)/(10x10^3) = 330 = 50 dB

18. CIRCUIT ANALYSIS
• Simulation

21. CHALLENGES
• Simulating the circuit
• Hardware implementation
• Limitation

22. Interesting Notes
 Voltage dividers cannot be separated from the amplifiers
 Circuit more sensitive to noise than anticipated
 Band-pass filter does not contribute gain

23. CONCLUSION
• The implementation of a heart monitor involves low cost amplifier and
filter components which can be find everywhere.

24. THANK YOU

25. Circuit Source
 Maxim Integrated
https://www.maximintegrated.com/en/app-
notes/index.mvp/id/3845