This version is outdated by a newer approved version.
This version (27 Apr 2017 14:00) was approved by Antoniu Miclaus.The Previously approved version (25 Apr 2017 15:35) is available.
This version (27 Apr 2017 14:00) was approved by Antoniu Miclaus.The Previously approved version (25 Apr 2017 15:35) is available.This is an old revision of the document!
Table of Contents
Activity: Heartbeat Measurement
Objective:
The objective of this Lab activity is to collect heartbeat information that is displayed using the Scopy tool. A circuit is designed and implemented that manages to filter and amplify the pulse.
Background:
A pulse oximeter measures blood oxygenation and can monitor heartbeat by clipping onto a finger tip. It does this by shining light through your finger and measuring how much light is absorbed. This goes up and down as blood is pumped through your finger. For the operation as a pulse oximeter-type optical heartbeat detector, a pair of IR LED and Phototransistor is used. The Led emits light through the finger and is detected by the phototransistor, which acts like a variable resistor conducting different amounts of current depending on the light received. The voltage variations above the phototransistor changes with the heartbeat. The small signal obtained is used as input for the circuit, obtaining the behavior of a pulse oximeter. In order to have a relevant output, the input signal must be amplified a and noise filtered by designing a relevant circuit for this situation.
Materials:
ADALM2000 Active Learning Module
Solder-less breadboard
Jumper wires
1 - OP484 precision rail-to-rail I/O op amp
1 - 100Ω resistor
1 - 500Ω resistor
1 - 1KΩ resistor
2 - 10KΩ resistor
3 - 100KΩ resistor
1 - 100nF capacitor
1 - 22uF capacitor
1 - 47uF capacitor
1 - Infrared LED ( QED-123 )
1 - Infrared Transistor ( QSD-123)
Directions:
On your solder-less breadboard construct the pulse measurement circuit (designed in ADIsim) as shown in Figure 1.
Figure 1 Pulse Measurement Circuit
The circuit amplifies the input small signal from the the phototransistor(Q1) with respect to ground. Some filtering techniques are added in order to solve the noise issues. There are several stages implemented with op amps before obtaining the final output(Vout):
- Virtual ground with op amp (X1) - The op amp is used as current source and fed with half of the power supply voltage (Vp), obtained through the voltage divider (R2,R6). The output is used as virtual ground (Vp → '+'; 0V GND → '-'). The purpose of this circuit is to maintain the signals at around Vp/2 and it is done by decoupling through the series capacitor(C1).
- Preamplifier(X2) - The input signals from the pulse oximeter are fed into a chain of 3 opamp stages. The first is a preamplifier. The pulse oximeter output is decoupled through the series capacitor to place it near Vp/2, and amplified using a 10kΩ negative feedback resistor(R4).
- Low-Pass Filter with Buffer(X3) - The 10kΩ resistor(R1) and the 22nF capacitor(C2) describe a RC filter that manages to cut the high frequencies (noise). The opamp serves as a unity gain current source / voltage follower that has high input impedance when measuring the output of the low-pass filter and reproduces its voltage with a low impedance output.
- Final Amplifier with Low-Pass Filter(X4) - Amplifies the signal with a gain ~1000 determined by the ratio of the 100KΩ resistor(R5) and the 100Ω resistor(R3). Low-Pass filtering components are provided by the 100nF capacitor(C3) across the negative feedback resistor.
Simulation:
Considering the circuit designed in ADIsim, two types of simulation are made:
- Transient - Connect at the input of the circuit a waveform generation source. Configure the source to generate a sine with amplitude of 1mV, frequency 3Hz and 500mV-1V offset. Observe the output signal amplitude in order to determine graphically the total gain of the circuit(Figure 2).
Figure 2 Output Voltage - Transient Analysis
- AC Sweep - Connect at the input of the circuit a AC Source. Configure the source to have a magnitude of 1mV. Observe the output signal in a chosen frequency domain (100mHz - 10kHz) in order to determine graphically in which frequency range the output signal has the biggest amplification (Figure 3).
Figure 3 Output Voltage - AC Sweep
Hardware Setup:
Use the variable positive power supply from the ADALM2000 module set to +5 V to power your circuit. Use scope channel 1 to monitor the voltage at the collector node of Vout.
The circuit implemented on the breadboard should look similar to the one in Figure 4. The blue LED represents the IR LED, and the grey one represents the Phototransistor.
Figure 4 Breadboard Pulse Measurement Circuit
Procedure:
Put the top of your finger between the IR LED(D1) and the Phototransistor(Q1). The emitter and the receiver should be alligned and pointing one to another.
Observe the voltage waveform seen at the the output of the 3rd stage op amp (X4). An example of output waveform is presented in Figure 5.
Figure 5 Pulse Output Waveform
In the Oscilloscope feature of the Scopy tool activate the measure feature in order to read the frequency of the obtained signal. To get the value of beats per minute(bpm) use te following formula:
- Heartbeat Measurement Resource Files
- Download: Circuit Schematic - ADIsim
- Download: Breadboard Circuit - Fritzing
Return to Lab Activity Table of Contents.
university/courses/electronics/electronics-lab-heartbeat.1493293632.txt.gz · Last modified: 27 Apr 2017 13:47 by Antoniu Miclaus