DEPARTMENT OF ELECTRICAL ENGINEERING AND COMPUTER SCIENCE MASSACHUSETTS INSTITUTE OF TECHNOLOGY CAMBRIDGE, MASSACHUSETTS 02139

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1 DEPARTMENT OF ELECTRICAL ENGINEERING AND COMPUTER SCIENCE MASSACHUSETTS INSTITUTE OF TECHNOLOGY CAMBRIDGE, MASSACHUSETTS Spring Introductory Analog Electronics Laboratory Laboratory No. 6 Check off by Apr 1, Report due Apr 6. READING ASSIGNMENT 555 Datasheet & Applications Note - Philips (course website "Datasheet") Neamen 3 rd Edition: section 2.5 Photodiode Photodiode Characteristics and Applications - OSI Optoelectronics (course website "Reference") Acknowledgement: Special thanks to Texas Instruments for their generous donation of the Nellcor Pulse-Oximetry probe and kit. Objectives: Learn to use the ubiquitous 555-timer chip. Design and build a circuit for detecting nanoampere current generated by a photodiode in a pulse-oximeter probe. Note that this lab requires a checkoff for experiments 1 and 2 as noted. Lab Report: This lab report must be in pdf format and uploaded to the course website using your certificate under submit PDF. The report should be in one pdf file. NOTE: Your lab write-up should clearly show your schematics, your component values and your calculations in addition to answering the questions. Experiment 1: 555 Timer Chip In this experiment you will investigate the performance of the 555-timer. The 555 data sheet and app note are posted on the course website under Datasheet. The data sheet shows both the schematic for the 555 and some typical applications. Figure 1: Astable Oscillator using 555 Timer Chip 1. Construct an astable oscillator, operating from the +15 V supply in your lab kit, which produces an output of 10 khz with a duty cycle in excess of 0.1. Note that to avoid damaging your 555, you should not use resistor values less than 1 k in the timing portion of your circuit. With the frequency of your oscillator set to 10 khz, measure the duty cycle. Note: The 555, along with some other timer chips, generates a very big [ 150mA ] supply-current glitch during each output transition. Be sure to use a Lab. No

2 hefty (>20 F) bypass capacitor from the chip V CC pin to ground, physically near the chip. Even so, the 555 may have a tendency to generate double output transitions. Most of the CMOS versions of the 555 do not have this problem and also draw far less current, can swing rail-to-rail at the output, and can operate down to 1 or 2 volts V CC! 2. Without changing any of the component values in your circuit, reconnect all of your circuit to operate from the +5 V supply in your lab kit. Measure the frequency and the duty cycle and compare with the values you found in part 1. Q 1.1 Why do these values vary so little with supply voltage? 3. You can use the 555 chip to generate a sawtooth waveform, instead of the square wave available from output pin 3. One way to do this is to drive the capacitor with a current source (which will give a linear capacitor voltage with time) and to reset the capacitor (discharge it) with the 555 discharge connection. Design and construct such a circuit to generate a 10kHz sawtooth waveform with a reset time less than 1 % of the period of the sawtooth. Construct your current source using the 2N5462 FET in the configuration shown in Figure 2. The FET characteristics and the value of the resistor R determine the current supplied by this source. Note: to protect your 555 from damage when discharging the capacitor you should usually make sure that there is at least a 1 k resistor in the discharge path. However, this resistor will distort the sawtooth waveform if it is located as shown above [R B ]. You may relocate this resistor so that it is still in series with pin 7, but not in the charging path. You may also eliminate this resistor entirely if your timing capacitor is small enough so that the transistor saturation resistance will limit the discharge current to safe levels [.01 μf or smaller]. +15 V R [can be a rheostat] G S 2N5462 [get from stockroom or parts drawer in ] D I OUT Figure 2: JFET current source. 4. Using the 555 timer IC, design and construct a voltage-controlled sawtooth oscillator. Your design objective should be a frequency variation of about 100 Hz to 10 khz as the input voltage is varied from approximately 0 to 15 volts. You may use a pot to provide the adjustable control voltage One of the circuits you should consider is the voltage controlled current source [VCCS] on the next page. The output transistor of this circuit connects in place of resistor R A. You may find that this circuit works better with your 555 if you connect [only] the 555 between ground [to the 555 V CC terminal] and 15 volts [to the 555 ground terminal]. Checkoff 1: demonstrate operation of voltage controlled sawtooth oscillator. Lab. No

3 VOLTAGE-CONTROLLED CURRENT SOURCE [VCCS] V CC =+15 R E I E R 2 V IN A 6 2N3906 R 1-15 R L I out = I C 1. Feedback forces [+15V V IN ] across R E, because V + must equal V 2. If we ignore any offset voltage at the output of the op-amp, the only error comes from the emitter current not quite being equal to the collector current [due to I B ]. One can use a Darlington transistor or a JFET to reduce or remove this error. 3. This version of the VCCS does not work if V IN is an external voltage not referenced to V CC. 4. Example: R E = 100Ω, β F = 100, V IN = 5 V, 10 V, and 14 V: [15V-5V] / 100Ω = 100 ma for I E ; I C = 99 ma. [15V-10V] / 100Ω = 50 ma for I E ; I C = 49.5 ma. [15V-14V] / 100Ω = 10 ma for I E ; I C = 9.9 ma. 5. R 1 R 2 can of course be a potentiometer for ease of adjustment! Lab. No

4 Experiment 2: Pulse-Oximetry A pulse-oximeter is an instrument for measuring blood oxygenation (oxygen saturation) and heart rate. The medical jargon for this is non-invasive optical plethysmography. This is accomplished by measuring absorption of light through a finger. 1 The Nellcor probe consists of a pair of back-to-back LEDs in one half of the probe and a PIN photodiode on the other half. A PIN diode is diode with an intrinsic (meaning undoped) semiconductor region between the p-type semiconductor and the n-type semiconductor. It can be used to detect light. The red LED is 905nm wavelength and the infrared LED is 660nm wavelength. The probe clips on to a finger. To determine blood oxygen saturation, two different wavelengths of LED are used: 660nm (red) and 905 nm (infrared) for the probes in the lab. The two LEDs are alternately pulsed. Because hemoglobin (present in oxygenated blood) and deoxyhemoglobin (present in deoxygenated blood) have different absorption for different wavelengths of light, the oxygenation level can be determined by comparing the ratio of the two wavelengths absorbed by the blood. The oxygen level in blood, SpO2, is then determined by running the optical measurements through some calculations and then using a lookup table. Thus SpO2 determination is more suitable for digital processing. However, measuring blood flow is ideally suited for analog circuitry. As blood flows through the figure the absorption of light varies directly with blood flow. For this exercise you will design an analog circuit that detects the tiny (nanoampere-range) current from the photodiode with a transimpedance amplifier (an amplifier that converts a current to a voltage), runs it through a low-pass filter (LPF), and then further amplifies it so that it can be seen on an oscilloscope. The resulting waveform is termed photoplethysmogram (PPG). A plethysmogram is a recording produced by a plethysmograph with measures blood flow. The photodiode can operate in either photovoltaic mode or a photoconductive mode. In photovoltaic mode, current is generated. In photoconductive mode, the diode is reverse biased and the diode current is proportional to the light received. The circuit model for the photodiode is shown below along with the voltage/current characteristic curves. (HP Optoelectronics Application Handbook p 4.7) Lab. No

5 For photoplethysmogram, photoconductive mode is used because of higher speed, larger dynamic range and linearity. In photoconductive mode, the photodiode can be modeled as a reverse biased diode in parallel with a current source controlled by light. As with all reverse biased diodes there is a reverse leakage current which in this case we call the dark current. For our probe, the dark current is typically 5 na. Notice that the reversed biased diode curves are very similar to a bjt with light replacing base current as the controlling factor. Lab. No

6 You will implement the transimpedance amplifier (current to voltage converter) using a 353 JFET op-amp as shown using a 9v battery. The output voltage is simply R3 * (-diode current). Because of the dark current, the anode of the diode is biased at a low voltage to allow for a wider output voltage swing. C1 acts as a low pass filter. The dark current and photocurrent is typically in the 5-10 na range. R3 is typically 1-5 megohm. You will need to select the value based on the individual probe to achieve the specifications outlined in the next paragraph. Power for this circuit is supplied by a single 9V battery with 4.5 set as the virtual ground through a 2K resistor voltage divider (in the same manner as we did in the ECG circuit). Also, note that in the diagram above, the ground symbol refers to the negative terminal of the battery, not the virtual ground. In order to display the pulse waveform, three additional blocks are needed: (1) a low pass filter, (2) additional amplification, and (3) removing any DC offset as a result of the dark current. As discussed in lecture, removing DC offset can be implemented by two methods. With dark current compensation circuit, monitor the output from the LPF and compare it to 4.5V. Any difference is integrated over a 1-2 second interval to remove the offset from the dark current. Alternatively the output of the transimpedance amplifier can be fed into a high pass filter. In this approach, the corner frequency should be 0.1 hz or less. This approach removes the need for an adder circuit. Finally, additional amplification should provide a gain of db. Ideally the output voltage should be 200mv pk-pk with respect to virtual ground. These are suggestions. For this exercise you are free to implement any design. The design choice is completely up to you! A small PCB has been constructed to allow access to the LEDs and photodiode. There is a 200 ohm resistor on the PCB. You will need to add a 270 ohm resistor in series (total resistance 470 ohm) with the led for proper current. The connections are numbered as shown below: Checkoff 2: Demonstrate operation. Your output signal should be a reasonably clean signal. Lab. No

7 Question 2.1 Calculate the peak photodiode current generated based on measured values and include in your report. Your lab report for this lab should include an electronically generated schematic diagram (using any software) along with a discussion on your implementation as well as an explanation for component values. For this lab, the report must be uploaded to the course website under submit PDF. Optional: We have available a small number of parts to build a PCB for use with the Nellcor probe. (This was designed for 6.S03 to display a photoplethysmogram for Fourier analysis.) You may build a unit for yourself if you wish. See the instructor. Lab. No