The objective of this Lab activity is to learn how to use a chain of amplifiers for gain and filtering, in a practical example, that aims to recover heartbeat information. The result of the system provides an relevant output that is displayed using the Scopy tool.
Going through this lab activity, the students will learn how to drive a IR LED and a Phototransistor, will be able to design and understand the behavior of a low-pass filter and explore functionalities provided by the operational amplifiers in different configurations.
Combining the previously mentioned electronic devices, the result of the activity will demonstrate how a real world application can be implemented with minimum software and hardware equipment.
A type of Heartbeat Measurement Device consists of an electronic circuit that monitors 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 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 change with the heartbeat and are acquired from the collector of the phototransistor. The small signal obtained is used as input for the circuit, obtaining the behavior of a heartbeat detector.
In order to have a relevant output, the input signal is passed through multiple circuits:
ADALM2000 Active Learning Module
1 - OP484 precision rail-to-rail I/O op amp
1 - 100Ω resistor
1 - 470Ω resistor
1 - 1KΩ resistor
1 - 10KΩ resistor
2 - 47KΩ resistor
2 - 1uF capacitor
1 - 47uF capacitor
1 - Infrared LED ( QED-123 )
1 - Infrared Transistor ( QSD-123)
On your solder-less breadboard construct the heartbeat measurement circuit (designed in LTspice) as shown in Figure 1.
Figure 1 Heartbeat Measurement Circuit
The LTspice simulation uses OP284s, included in the standard set of LTspice models. The actual circuit is constructed with the quad OP484FPZ from the ADALP2000 Analog Parts Kit, powered by ± 5V from the ADALM2000 module (a total supply voltage of 10V.)
In order to have a proper current that will not damage the IR LED, a resistor needs to be added in series to limit the current. Varying the value between in the operating range will change the intensity of the emmited signal of the IR LED. The following formula expresses the value of the forward current (IF) through the LED, based on the positive voltage supply +5V (VP), series resistance (R1) and forward voltage drop on the LED (VF):
To acquire information from the phototransistor (Q1) when is in contact with the IR light, a common-emmiter amplifier circuit is designed. This circuit generates an output which transitions from a high state to a low state when light in the infrared range is detected by the phototransistor. The output is created by connecting a resistor (R2), whose value was determined experimentally, between the voltage supply and the collector pin of the component.
The input signal from the heartbeat measurement setup is fed into a Differentiator Amplifier Circuit1) (C1, A1, R3). The capacitor blocks any DC content, C1 and R3 behaving as a high pass filter with the cut-off frequency Fc1 being determined by the following formula:
Besides filtering, this stage serves also as an amplifier taking as input the current (IA1), and generating at the output an inverted voltage (VA1) based on the negative feedback resistance (R3):
Active Filters contain active components such as operational amplifiers, within their circuit design. They draw their power from an external power source and use it to boost or amplify the output signal. Active Low-Pass Filter2) principle of operation and frequency response is the same as of a simple RC Low-Pass Filter, the only difference being that that it uses an op-amp for amplification and gain control.
This first-order low pass active filter (A2, R4, C2), consists simply of a passive RC filter stage providing a low frequency path to the input of a non-inverting operational amplifier.
The filter has the aim to cut the high frequencies that correspond to the noise signal. Taking into account that the heartrate does not exceed a value of 180 beats pe minute (bpm) and the dependecy between the bpm and the frequency is:
will result that frequencies that are higher than 3Hz should be cut. The RC Low-Pass Filter is designed for the mentioned frequency value using the formula:
The amplifier is configured as a voltage-follower (Buffer) giving it a DC gain of one, AV = +1.
The advantage of this configuration is that the op-amps high input impedance prevents excessive loading on the filters output while its low output impedance prevents the filters cut-off frequency point from being affected by changes in the impedance of the load. While this configuration provides good stability to the filter, its main disadvantage is that it has no voltage gain above one. However, although the voltage gain is unity the power gain is very high as its output impedance is much lower than its input impedance.
The configuration of the final stage represents a AC Op-amp Integrator with DC Gain Control3). In simpler words, the circuit has the aim to low-pass filter (R4, C2) the signal from the remaining unnecessary frequencies that are higher than the maximum frequency of the heartbeat and amplify through the inverting amplifier the useful signal with a gain (AV)determined by ratio between R6 and R5:
Considering the circuit designed in LTspice, two types of simulation are made:
Figure 2 Output Voltage - Transient Analysis
Figure 3 Output Voltage - AC Sweep
Use the variable positive and negative 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 Heartbeat Measurement Circuit
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 (A3). An example of output waveform is presented in Figure 5.
Figure 5 Heartbeat 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 convert the frequency into beats per minute(bpm) use the formula from laboratory directions.
1. Using the values and formulas provided in the laboratory directions compute the following parameters:
2. Using the oscilloscope, capture the voltage signal at the output of the first stage and try to compute the amplitude of the input current.
3. What role has the buffer in the second stage?
4. What parameters change if R5 is modified? How will the signal look on the Oscilloscope?
5. What parameters change if R6 is modified? How will the signal look on the Oscilloscope?
6. On the Heartbeat Output Waveform the signal is not centered on 0V, even though the DC component should have been removed. Why is this happening? What should be added at the output of the circuit to reduce the offset?
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