ENGR 072 - Electronic Circuit Applications,
Fall 2021

Homework due dates will be on the course moodle page; assignments will be turned in on moodle as a single pdf. Late homework will be penalized 25%, and won't be graded after 1 week.

We will go over any questions on the homework the day before it is due (assignments are due Thursday at 9:00 a.m. unless otherwise specified). If your name (i.e., the first three letters of your email address) are next to the problem, you are responsible for presenting an outline to the solution in class (not the entire solution, but enough to get people working in the right direction). If you are not sure how to do a problem assigned to you, please come ask - but do it at least a day in advance, and not right before class.

Before doing assignment 0, you may wish to read my E11/E12 review, in particular 1st order step responses.

Solutions are here.

• Asn 0: Review Problems (link) Do problems 1 sab, 4 nah, 5 sam, 7 jan, 9 lbl, 10 kde (there is no need to do the other problems).
  Note: the solution to problem 9 is the solution to the "Relaxation Oscillator" of the second lab. You may find the result of problem 10 useful to solve problem 9.
  We will go over assignment on Friday 9/3, and it will be due Tuesday 9/7 at noon.
• Asn 1: (due 9/16) From text, 1.48 bde, 1.50 jde, 1.57 qgu (hint: see 1.65 for a way to do subtraction, then modify it for addition), 1.65 uhe,  1.68 sji
• Asn 2: (due 9/23) From text, 2.8 rka (Rezhwan), 2.30 rke (Becca), 2.47 ykh (Youssef), 2.78 ame (Rey), 2.93 dme (David).
• Asn 3: (due 9/30) Problem A dna (Dorothy), String of LED's mng (Minh), From text 4.4 jod (Jonathon), 4.9 cst (Chris)
• Asn 4: (due 10/21) Problem B ato (Aiden), From text 4.66 twa (Tristan), 4.80 parts a,b,c jwe (Judith), 4.90 yya (Gordonson)
• Asn 5: (due 10/28)
  1. Problem C sab (Saeed)
  2. nah (Nader) If we want a PWM with ripple equal to one part in 256 of the input voltage, and the PWM frequency is 1kHz, what should the time constant of the output circuit be?
  3. sam (Suleyman) The ADC on our MSP430 has 10 bits. If the reference voltage is Vcc=3.3V, what is the resolution (i.e., the smallest voltage change)?
  4. jan (Jordan) The 3.3V power supply is usually not very accurate (+/-5% or so), so for more accurate conversions, the ADC10 has a 1.5 and 2.5V reference (see page 535 of the data sheet). Assuming the sensor is perfectly accurate, and the ADC10 reference voltage is set to 1.5V (and is also perfectly accurate), how accuarately can you measure tempearture in Celsius (see page 550 of the data sheet).
     
     
     
     The next 3 problems concern the image shown which was adapted from: "Comparators: The Unsung Heroes Of Electronics" at https://sound-au.com/articles/comparators.htm.
     • Remember that the output of the comparator can be modelled as a switch that is a short circuit to ground when low, and is an open circuit when high.
     • Hint: What does the voltage across the non-ideal inductor look like?
     • There are many ways to do these problems, but they will all use arguments/calculations similar to those used for the pwm input to the RC circuit that we used in class.
     • Assume an ideal diode.
     • Have fun.
  5. lbl (Nora) - Inductor fun: We want to be able to accurately control the current in the inductor using PWM. Qualitatively explain how this works. What are the restrictions on the PWM frequency for a small ripple current?
     
  6. kde (Kevin) - More inductor fun: What is the average (DC) current through the inductor.
  7. bde (Biruh) - More more inductor fun: If the PWM frequency is 240 kHz, what is the peak to peak magnitude of the ripple current through the inductor at 50% duty cycle?
• Asn 6: (due 11/11)
1. jde (Julio) Problem D
2. qgu (Stacy) Problem E
3. uhe (Vlad) Use the code below as the basis for a program that toggles the LED every time you hit the push button on the LaunchPad using interrupts on the push button input. Add code at the two locations that are indicatated. Also add comments to the code.
4. sji (Simon) Why is the variable pbFlag declared to be volatile?
   

#include <msp430.h> 

#define LED BIT0
volatile int pbFlag=0;  // flag that gets raised to indicate interrupt.

void main(void) {
  WDTCTL = WDTPW + WDTHOLD; // Stop watchdog
  // Add your code to set up ports.

__bis_SR_register(GIE);  // enable interrupts.

  while (1){
     if (pbFlag) {
         pbFlag = 0;
         P1OUT ^= LED;  
     }
  }
}

// Port 1 interrupt service routine
#pragma vector=PORT1_VECTOR
__interrupt void Port_1(void) {
  // Add your code to set pbFlag to 1 and clear interrupt flag
}

5. rka (Rezhwan) From text 1.69
6. rke (Becca) From text 1.73
7. ykh (Youssef) From text 2.117a
• Asn 7: (due 11/18 9:00 a.m. - Late homework will not be accepted because I will be posting solutions before exam)
  1. ame (Rey) Show that the circuits in Figure 17.40 have identical transfer function and explain why the second circuit uses one fewer op-amps.
  2. dme (David)17.25
  3. dma (Dorothy 17.29
  4. mng (Minh) The circuit below is from MAKE magazine and is designed as a lie detector using galvanic skin response. Resistors marked 3M3 are 3.3 MΩ.
     • You should assume the baseline resistance of the skin is about 200 kΩ.
     • V+ = 5V (this is the positive supply).
     • V- = 0V (ground).
     • Assume the constant drop model for all diodes. D1-D3 drop = 0.6 Volts each, when on.
     • LED1 drop = 2.8 volts when on.

  

  Two electrodes labeled SKIN-1 and SKIN-2 are attached to two fingers and forms a voltage divider to ground (V-) through R4. If you experience stress, you sweat a little and the resistance between the electrodes decreases because of the increased conductivity at the skin-electrode interface. This is filtered (IC1A) and then amplified (IC2B). If the signal at the output of the amplifier is high enough, the LED turns on.
  a) What is the voltage at the non-inverting terminals of the op amps? Explain your reasoning.
  b) If the skin resistance has been constant for a long time, what is the baseline voltage at the output of IC1B? Explain your reasoning. Is the LED on or off?
  c) Find the transfer function between points A and B and verify the cutoff frequencies.
  d) Plot the overall frequency response (i.e., the Bode plot, just magnitude) between A and C.

  5. jod (Jonathon) The diagram below shows the results of the device in action as different questions are asked.
     a) From the circuit diagram explain why a drop in resistance increases the output voltage of the amplifier (Pin 7, IC1B).
     b) If the skin resistance starts at 200 kΩ and then drops suddenly, how low must the resistance go to light the LED (i.e., assume a step change in resistance)? State any assumptions.
     
• Asn 8: (not to be turned in, solutions are posted)
  1. Problem F
  2. Problem G
  3. Find the transfer function for a Butterworth low pass filter with the following characteristics
     • No more than 3dB of attenuation below 3kHz.
     • At least 50dB of attenuation above 8kHz.
  4. Design a circuit to implement the filter above. Use the unity gain Sallen Key low pass filter with 10kΩ resistors
                        
     
     
  5. from text 2.101, 2.102, 2.105, 2.117 (b,c), 2.127, 17.77