Microelectronics final project report

Sound-Level Meter

System Design and

Development

Microelectronic Applications

Andrew Dilucia A10938

2

Contents

Introduction........................................................................p3

Experiment 1&2

Investigation of Light Emitting Diodes (LEDs).....................p4

Investigation of a Comparator..............................................p7

Experiment 3

Investigation of a Peak Detector Circuit..............................p10

Experiment 4

Investigation of a Digital to Analogue Converter................p13

Experiment 5

Investigation of a Switched Capacitor Filter........................p19

Experiment 6

Investigation of a PIC Microcontroller.................................p24

System Integration...............................................................p30

Conclusion..........................................................................p31

References..............................................................................p32

3

Introduction

For this laboratory programme I was tasked with creating a low cost, flexible sound level

meter for use within the entertainment industry as well as the health and safety industry. The

sound level meter should be designed around a PIC microcontroller (16F84 or 16F88) that

uses a programmable low pass filter. During the course of my investigations I was to build a

prototype and if I were successful, we could go ahead and mass produce the meter in order to

bring down the cost even further. This report has been created to document the various stages

of my investigations of building a prototype meter. Below is a block diagram of the proposed

overall system.

Overall system block diagram

(Cowey 2014)

As you we can see from the picture above, the overall system can be broken down into six

separate sub-systems. For my investigations I started with sub-system F (the LED display)

and continued backwards through sub-systems E to A. The prototype meter was created by

designing, building and testing individual sub-systems on bread boards and all experiments

were performed in the lab with the relevant Integrated Circuits (I.Cs). The information in this

report has been put together over a semester (following guidelines set out in the student

handout) and it includes information from both the lectures and practical sessions, plus

information gathered through student self learning.

4

Investigation of Light Emitting Diodes (LEDs)

The first subsystem I investigated was system F, a Light Emitting Diode (LED). LEDs are

part of the diode family and are semiconductor devices that allows current to flow through it

in one direction but not the other direction.

“A diode is a semiconductor component that lets current flow through it in one direction but

blocks current in the other direction depending on the polarity of the voltage applied to it”

(Frenzel, Jr, 2010, p50).

V-I Characteristics of a Diode

(Storr, 2014a, Accessed 2014)

From the V- I characteristics graph (as seen above), we can see that the diode switches on at

about 0.6V. This means that when the voltage applied reaches 0.6V or greater a forward

current begins to conduct through the diode.

A light emitting diode is similar to a conventional diode, but it is constructed with different materials. It has been stated that LEDs are Gallium Phosphide(GaP) or Gallium-Arsenide-

Phosphide(GaAsP) devices and when a forward bias voltage applied across the diode is greater than the voltage that turns it on, the LED gives out energy in the form of light (Green 1999).

5

LEDs are extremely common and are used throughout industry. They are perfect for the system I am developing because they have low power consumption and they instantaneously

switch-on, which is vital for our new products, with regards to accuracy. They also are low in price, which helps keep overall costs to a minimum and they last a long time, which is good

for reliability. The picture below shows LEDs I-V curve characteristics for the different colours and as we

can see the curves are different for each colour;

I – V Characteristics of an LED

(Storr, W. (2014b) Accessed 2014)

An LED must be connected in series with a resistor so the forward current is limited when it conducts current, otherwise it will overheat and stop working. An LED has some

specifications and they should be followed when using otherwise there will be problems.

"If more current passes through an LED than its maximum rating specifies, the LED burns up

like a marshmallow in a campfire" (Ross et al, 2010, p121).

So to begin with I needed to consult the Data sheet that came with the experiment instructions

to find out its maximum values. The LED we were using was the colour Red so I found out

that the LED I was using had a maximum operating current of 20mA, however it was

typically operated at 15mA and the LED had a turn on voltage of 2.2V. With this information

I could calculate the size of the resistor that would drop enough voltage to create the correct

size of current that was needed for the LED to work correctly which is 15mA or 0.015A.

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Using the formulas;

Vr = Vcc – Vled & R = 𝐕

𝐈

So if Vcc = 5V and Vled = 2.2V

Vr = Vcc – Vled = 5V – 2.2V = 2.8V

Then when using Ohms law and wanting to drop 2.8V and to use a forward current of 0.015A

R = 𝐕

𝐈 =

𝟐.𝟖

𝟎.𝟎𝟏𝟓 = 186.6 Ω

So the resistor that I needed to use needed to be at least a value of 186.6Ω. However the

nearest value we had in the stores was 220Ω and this is the resistor I used in this circuit. From

this information I could design a circuit to test this theory. Below is my circuit diagram.

LED test circuit diagram

From this circuit diagram I built the circuit using the correct components from the store and

began to test the subsystem. I did this by connecting a regulated 5V power supply to the

circuit. When the switch to the power supply was turned on the LED illuminated which is the

outcome I was looking for.

To conclude this circuit was easy to construct. So as it worked correctly first time it was a

success and it proved the circuit was designed and built correctly.

7

Investigation of a Comparator

The comparator is an electronic device, which compares two voltages or currents on its inputs

and switches to give an output according to which is larger.

Basic Comparator Schematic

(Wikipedia, 2014, Accessed 2014)

For my test circuit the comparator will be comparing two voltages to give an output that will

make the LED illuminate. For my overall system this will be the component that will switch

the LED on by comparing the sound level input in the form of a sine wave with the reference

level from the PIC microcontroller and the reference level would be set to the desired level

for the application the meter was being used for.

For my investigations I used an LM311 comparator IC and it is a monolithic, low input

current voltage comparator and this device is also designed to operate from a dual or single

supply voltage of 5V. It also has fast response times and strobe capabilities.

For this part of my investigations I had to design a circuit that would be combined with my

LED circuit from earlier to test the operation of a comparator. First I had to consult the data

sheet that was given with the investigation instructions to be aware of the pin layout and the

devices absolute maximum ratings to find out what`s the LM311s best operating conditions.

Below is the pin layout diagram for the LM311.

LM311 Pin Layout Diagram

(Texas Instruments Incorporated, 2003, Accessed 2014)

8

The picture below is the LM311s absolute maximum ratings,

LM311 Absolute Maximum Ratings


The picture below is the recommended operating conditions of the device.

LM311 Recommended Operating Conditions


I then had to find the simplest circuit suggested in the data sheet and modify it to incorporate

my LED circuit. On the next page is the picture of the simplest circuit.

9

Basic Comparator Circuit


From all of the information in the data sheet I could design a circuit to test the comparator.

Below is my circuit diagram.

Comparator test circuit diagram

From this circuit diagram I built the circuit using the correct components from the store and

began to test the subsystem. An LED was connected to the output of the comparator (Pin 7),

with a resister connected in series to a power supply of 5V to protect the LED. The emitter

out was connected to ground (Pin1). Two power supplies were connected in parallel to give

us a positive and negative Vcc (Pins 8&4). I then connected a 5V supply to Pins 5&6 and set

the threshold voltage at Pin 2 to 5V. I then connected a variable power supply to the inverting

input at Pin 3. Then starting at 0V the LED showed no change, so I increased the voltage on

Pin 3 and still no change. Then when the voltage was increased to greater than the pre-set

threshold of 5V the LED illuminated which is the outcome I was looking for.

To conclude this circuit was easy to construct. So as it worked correctly first time it was a

success and it proved the circuit was designed and built correctly.

10

Investigation of a Peak Detector

A peak detector is a circuit that is a combination of an op-amp, with a diode, a capacitor and

a resistor connected to the output. The diode is forward biased and this charges the capacitor.

At the highest level of the signal, the capacitor charges and holds this charge while the diode

is reverse biased. The circuit would be unresponsive to levels below 0.6V because the diode

has a switch on voltage of 0.6V so to compensate a negative feedback loop is used.

For my investigations I used a 741 op-amp. Below are pictures of the pin layout diagram and

absolute maximum ratings from the data sheet.

741 Op-Amp Pin Layout Diagram


741 Op-Amp Absolute Maximum Ratings


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As I mentioned earlier a diode, capacitor and resistor is connected at the output of the 741 op-

amp. The diode was used to chop the negative half of the cycle to leave to positive side. A

time constant was needed so that when the peak level was reached the LED would illuminate

long enough so that it could be detected by the human eye. For this a capacitor and resistor

were used to create this, so I had to choose values for these components.

First I had to choose a time constant that I wanted to achieve, which was 0.2s. Then a

capacitor value which was 1µF. From this information I could calculate the resistor needed to

achieve a time constant of 0.2s.

τ = 𝟏

𝐑𝐂 τ = RC τ = 0.2

C = 1µf CR = 0.2 R = 𝟎.𝟐

𝟎.𝟎𝟎𝟎𝟎𝟎𝟏 = 200kΩ

So for a time constant of around 0.2s I needed a resistor of 200kΩ, however the nearest

preferred value we had in the store was 220kΩ and this is the value I used in my circuit.

From this information I could design a circuit to test the peak detector. Below is a picture of

the circuit I used.

Peak Detector test circuit diagram

12

From the picture on the previous page you can see I again used two power supplies in parallel

to create +5V & -5V and this was connected to pins 7 & 4. As I mentioned earlier the diode is

connected to the output at pin 6 but is also connected at the negative input at pin 2 providing

a negative feedback loop. The capacitor and resistor are also connected at pin 6 (after the

diode) and ground. Next I connected this peak detector circuit with the comparator and LED

circuit from earlier between pin 2 on the 741 op-amp and pin 3 on the comparator. I then

connected to pin 3 on the 741 op-amp a function generator to create a 5kHz sine-wave with a

5V peak to peak amplitude.

I then observed that the output of the diode was chopping the negative half of the cycle,

leaving just the positive half. We also observed the overall output and could use the

oscilloscope to see the time delay that the time constant had created. This helped the sine-

wave decay slower through the capacitor making the output visible to the eye. Also when I

altered the DC reference voltage, the LED switched on when it reached and passed the

threshold of 0.6V. At the oscilloscope display I observed that at every cycle the capacitor

charges and discharges slightly to smooth the signal. Also the signal size is proportional to

the size of the amplitude of the frequency and at higher frequencies the capacitors discharge

is negligible.

To conclude this circuit was quite easy to construct. The Led illuminated when it was

suppose to and the readings on the oscilloscope was what I was trying to achieve. So as it

worked correctly it was a success and it proved the circuit was designed and built correctly.

13

Investigation of a Digital to Analogue

Converter (DAC)

Within electronic systems information needs to be converted between analogue and digital

states and to do this, Analogue to Digital Converters (ADC) and Digital to Analogue

Converters (DAC) are used.

“Circuitry is, therefore, required that is able to interface between the

analogue world outside the system and the digital system itself. The two interface circuits that

are necessary are the ADC and the DAC” (Green, 1999, p329).

For my investigations I focused on a DAC and this operates by inputting a binary number and

outputting an analogue current or voltage signal. Below is a block diagram that shows this;

DAC Operation Block Diagram

(All About Circuits, 2015, Accessed 2015)

The DAC I used was an MC1408 IC. This DAC is based around an R-2R resistor ladder and

the output is dependent on the binary input on each input switch.

14

First I checked the data sheet for the MC1408 to consult the absolute maximum ratings, so to

be aware of them so I did not damage the device. The picture below shows the absolute

maximum ratings.

MC1408 Absolute Maximum Ratings

(Philips Semiconductors Linear Products, 2003, Accessed 2015)

Then I had to find the simplest DAC circuit suggested in the data sheet. The picture below

shows this circuit.

Simplest DAC Circuit


15

Next I had to consult the DC electrical characteristics on the data sheet to be able to choose a

suitable power supply. The picture below shows the DC electrical characteristics.

MC1408 DC Electrical Characteristics


I then consulted the data sheet to identify the digital input pins and find which pins are the

MSB and LSB. The MSB is pin 5 and LSB is pin12, and are highlighted in the pin layout

diagram below.

MC1408 Pin Layout Diagram


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Next I had to choose adequate values for the resistors and capacitors in the circuit. The

resistors I chose were all 1kOhm and the capacitor which provides noise suppression for the

circuit was 15µf. I then had to choose an adequate reference voltage for the investigation. I

chose around 2V and to achieve this I had to design a potential divider circuit that would be

connected to between pin 14 and +5V. After some testing, the final design of the potential

divider gave me a VREF of 2.1V. From this information I designed a circuit to test the DAC

which is shown in the picture below. The potential divider circuit is highlighted in the red

section.

DAC test circuit diagram

At the output of the DAC, an Op-amp is required because the output signal is current and I

needed it converted to an output voltage. The Op-amp circuit (highlighted in blue) needed a

negative feedback resistor to create the correct output voltage and as you can see I also chose

a 1kOhm value for RO.

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With the values of the VREF and resistors, I could calculate that the outputs should be what I

was expecting them to be.

VREF = 2.1 Ro = 1kOhm R14 = 1kOhm R15 = 1kOhm

So to calculate the output of the DAC (Io);

IREF = 𝐕𝐑𝐄𝐅

𝐑𝐑𝐄𝐅 0.0021A =

𝟐.𝟏𝐕

𝟏𝟎𝟎𝟎Ω 0.0021A = 2.1mA

IREF = Io Io = 2.1mA

As I mentioned earlier I need to now convert the DACs output from current to voltage and

this is achieved with the use of a 741 Op-amp. So to calculate what voltage (Vo) is expected

at the output of the 741 Op-amp;

VOUT = Io x Ro 2.1V = 2.1mA x 1000Ω

2.1V = 0.0021A x 1000Ω Vo = 2.1V

As I mentioned earlier the DAC I used was an MC1408 IC but as we were not using a PIC

microcontroller for this part of my investigations, I had to simulate the binary inputs. I did

this by connecting a DIL switch with an in-line resistor at pins 5 to 12 on the DAC to

simulate either logic 0 or 1 (low/high) states. In the final overall design of the sound level

meter, the PIC will send the signals to the DAC and then the DAC sends a reference level to

the Analogue Comparator. Finally I connected a volt meter at the output of the 741 Op-amp

and applied the power supplies that were required as shown in the DAC test circuit diagram.

Next I needed to simulate the binary inputs and measure the output voltage preferably with

results for 0, ¼, ½, ¾, and of course a full output.

To achieve this, the binary inputs were 00000000, 01000000, 10000000, 11000000,

11111111. Using the individual switches on the DIL switch, I simulated these numbers and

recorded the output voltage for each number in the table on the next page.

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DAC Test Results Table

VREF Fraction Binary Input Output Voltage

0 MSB 00000000 LSB 0V

¼ MSB 01000000 LSB 0.51V

½ MSB 10000000 LSB 1.05V

¾ MSB 11000000 LSB 1.56V

Full MSB 11111111 LSB 2.1V

As we can see from the results table, each time I inputted a binary number the output voltage

increased by a quarter and this is the response I was expecting to achieve. From the results

table I then plotted the graph that shows the relationship between the output voltage and the

binary numbers (shown below).

DAC Test Results Graph

From this graph we can see that the relationship between the output voltage and the binary

numbers is of a linear nature.

To conclude this circuit was quite difficult to construct however, I was able to complete my

investigations successfully. At each binary input, the output voltage altered by the correct

percentage that was expected. So as it worked correctly it was a success and it proved the

circuit was designed and built correctly.

19

Investigation of a Switched Capacitor Filter

(LMF100)

For the sound level meter, I needed to investigate possible low pass filters to block any

unwanted frequencies on the input of the circuit. I needed to do this because I am only

interested in audible frequencies that human ears can hear ranging between 20Hz & 20kHz.

The filter I used was an LMF100 High Performance Dual Switched Capacitor Filter.

Switched Capacitor Filters are ICs and, are used throughout microelectronic applications

because they have a low cost compared to highly accurate conventional filters made to a

microelectronic scale and this would be good for keeping the overall cost and size of the

sound level meter to a minimum. Conventional filters rely on the ratio between resistors and

capacitors, where switched capacitor filters only rely on the ratio between the capacitors

values.

Switched Capacitor Filter Circuit Schematic

(Cheever, 2015, Accessed 2015)

The LMF100 Switched Capacitor Filters are extremely accurate and they are very versatile.

One output could be used to create either, all pass, high pass and notch functions, while the

other two outputs are equipped to perform low-pass and band-pass functions. They operate up

to 100kHz and can be used as a 2nd or 4th order filters.

20

First I checked the data sheet for the LMF100 to consult the pin layout diagram and the

absolute maximum ratings, so I did not damage the device. The pictures shown below are of

the pin layout diagram and the absolute maximum ratings.

.

LMF100 Pin Layout Diagram

(National Semiconductor Corporation 1995, Accessed 2015)

LMF100 Absolute Maximum Ratings


I then had to consult the data sheet to find the simplest circuit to base my circuit design on.

On the next page is the picture of the simplest circuit and its frequency response graph.

21

Simplest Switched Capacitor Filter Circuit


The circuit in the picture above is a 4th order 100kHz Butterworth Low-pass Filter which

means the cut off frequency (fc) is at 100kHz. For the system I am developing, this fc is way

too high for the applications we were thinking of using it for so I needed to modify this

circuit to make it applicable to my needs. In the final design of the product I would like to be

able to alter the cut off frequency depending on the different applications needed for the

product.

For this investigation I wanted a cut off frequency of 1kHz instead of 100kHz and to achieve

this, the clock input needed to be altered from 3.5MHz to an appropriate value because the

input clock frequency is proportional to the cut off frequency. To calculate this I divided

3.5MHz by 100.

𝟑.𝟓𝐌𝐇𝐳

𝟏𝟎𝟎 =

𝟑𝟓𝟎𝟎𝟎𝟎𝟎

𝟏𝟎𝟎 = 35000 = 35kHz

So to achieve a 1kHz cut off frequency the clock input must be set to 35kHz. Also four

values for resistors were needed to be altered in the simplest circuit from the data sheet. With

this information I could then design a circuit to test the filter. The circuit I designed is

illustrated in the picture on the next page.

22

1kHz Switched Capacitor Filter Test Circuit Diagram

I then constructed the circuit with the correct power supply and clock frequency set. The

clock frequency was created on the ttl output on a function generator using a square wave

output. The clock frequency was set to 35kHz and as the clock frequency basically sets the

cut off frequency, this made the filter a 1kHz filter.

I then entered a range of sine-waves with different frequencies ranging from 10Hz to 20kHz

at pin 4 of the LMF100 via the 56kΩ resistor. I then was able to view the output from pin 16

and record the readings in the table below.

Switched Capacitor Filter Test Results Table

Freq / Hz Vin Pk Vout Pk Gain Av dB

10 1 1 1 0

20 1 1 1 0

50 1 1 1 0

100 1 1 1 0

200 1 1 1 0

500 1 1 1 0

1k 1 0.75 0.75 - 2.50

2k 1 0.075 0.075 - 22.5

5k 1 0.004 0.04 - 48.0

10k 1 Unreadable N/A N/A




23

From the results in the table as seen on the previous page I could then plot a graph depicting

the frequency response of the filter. Below is a picture of the graph.

1kHz Switched Capacitor Filter Frequency Response Test Results

Graph

From this graph I could see that the filter had about a 3dB drop off, 2.5dB to be precise at

1kHz, which proved I had altered it correctly to work as a 1kHz low-pass filter. Also we can

see a drop of around 24dB per octave proving it was a forth order filter.

To conclude this circuit was quite difficult to construct however, I was able to complete my

investigations successfully. So as it worked correctly it was a success and it proved the circuit

was designed and built correctly.

24

Investigation of a PIC Microcontroller

Microcontrollers are a single integrated circuit which has a CPU, memory and input/output interfaces within it. It is like a microcomputer system in one small chip. It is suitable for various applications because of versatile input/output capabilities and with it being a single

integrated circuit the power requirements are low for a system. A microcontroller is an excellent bit of kit as you can connect it to a PC via a development board to program it with

C programming language and reprogram it with little effort other than altering some code.

"The great thing about a microcontroller is that you can simply alter a few lines of code or

reprogram it completely to change what it does; you don't need to swap out wires, resistors

and other components in order to get this flexible IC to take on a new personality"

(Ross et al, 2010, p161).

The first microcontroller I researched was the PIC16F84 but soon realised that with the

PIC16F84 it would be very difficult to produce a good solid square-wave signal. So then I

researched the PIC16F88 and this has pulse width modulation as a feature, so this is perfect

to produce the solid square-wave required. Below is a picture of the 16f88s pin layout

diagram and the absolute maximum ratings are shown on the next page.

PIC16F88 Pin Layout Diagram

(Microchip Technology Incorporated 2005, Accessed 2015)

25

PIC16F88 Absolute Maximum Ratings

(Microchip Technology Incorporated 2005, Accessed 2015)

For the programming of the PIC I used some software called Flowcode. This is a graphical

programming tool using flowcharts that simplifies programming and is especially useful for

inexperienced programmers. For my investigation I wanted to be able to push a button to

switch between states to alter the cut off frequency, level, period and duty cycle. The cut off

frequencies I was hoping to achieve were 1kHz and 200Hz. So to calculate what PWM time

period and duty cycle needed to create a cut off frequency of 1 kHz I used the formula below.

[ (PR2) + 1 ] x 4 x Tosc x (TMR2) = PWM Period

T = 𝟏

𝐟 f =

𝐟𝐨𝐬𝐜

𝟒 =

𝟒

𝐟𝐨𝐬𝐜

T = 𝟏

𝐟 Tosc =

𝟏

𝐟𝐨𝐬𝐜

fc = 35kHz (for fc = 1kHz)

26

TPWM = 𝟏

𝐟𝐜 =

𝟏

𝟑𝟓𝟎𝟎𝟎 = 28.57µsecs = 29µsecs

29µsecs = [ (PR2) + 1 ] x 4 x 0.25µsecs x 1

= PR2 + 1µsecs

PR2 = 29 – 1 = 28

PWM Duty Cycle = (CCPR1L:CCP1CON<5:4>) x Tosc x (TMR2)

14 = (“10 bit binary number”) x 0.25 x 1

“10 bit” = 𝟏𝟒

𝟎.𝟐𝟓 = 56

CCPR1L = 𝟓𝟔

𝟒 = 14

So for a cut off frequency of 1kHz the PR2 = 28 and the CCPR1L = 14. As we can see the

CCPR1L is around half the value of the PR2, so to simplify calculating these values for a cut

off frequency of 200Hz it is done as shown on the next page.

27

fc = 35kHz (for fc = 1kHz)

TPWM = 𝟏

𝐟𝐜 =

𝟏

𝟔𝟔𝟎𝟎 = 151µsecs

PWM Duty Cycle = 𝟏𝟓𝟏

𝟐 = 75.5 = 76

So for the separate frequencies, the required values needed are shown below.

State 1

Level 1 = 100 Period1 = 28 Duty1 = 14 PWM = 35kHz fc = 1kHz

State 2

Level 2 = 200 Period2 = 151 Duty2 = 76 PWM = 6.6kHz fc = 200Hz

When the PIC is in state one and level 1, the DAC receives 1 volt from the PIC and when the

PIC is running the program in state 2 and level 2, the DAC receives 2 volt from the PIC.

As I mentioned earlier in this chapter I used Flowcode to program the PIC16F88 and one of

the benefits of this software is that you can simulate the circuit to check that the code works

correctly. For my investigation I wanted to use two individual switches to switch between

states. There will be one to switch between frequencies and the other switch to switch

between levels 1& 2.

I opened Flowcode and selected the PIC16F88 and clicked ok. I then changed the clock speed

to 4000000Hz or 4MHz, the oscillator to XT and turned the watchdog timer off. I added two

switches, an LED array and a PWM to the panel. Then I added some constants that were the

values from the two states as seen above on this page to the sketch. Next I compiled the

flowchart adding the relevant components like switches, inputs and outputs etc.

On the next page is the flowchart I built in Flowcode that will be tested.

28

Flowchart Built in Flowcode

When the flowchart as seen above was built, I could run the simulation to check to see if the

code had been compiled correctly. When I activated the first switch the PWM changed and

when I activated the other switch, the levels changed between levels 1 & 2.

29

As the simulation test was a success I was ready to program the PIC16F88 via a development

board and test it while connected to the switched capacitor filter. So I clicked on compile to

hex, which in turn created a hex file. I then created a PPP file and could now upload the

program to the PIC via a USB and the PIC development board.

I then connected the development board to the switched capacitor filter at pin 10 to alter the

clock input when pushing a button on the development board. A function generator was

connected to the filters input to create a sine-wave and an oscilloscope connected to the

output.

When running the program on the development board and pushing the button I could see on

the oscilloscopes display that the cut off frequency changed between 203Hz & 1060Hz.

To conclude, this investigation was quite difficult to complete, however it was made easier

with the use of Flowcode rather than a generic C compiler program. Also the use of the

development board made things easier when testing the code. So as the investigation gave the

required results that were expected, it proves the PIC was programmed correctly.

30

System Integration

Through my investigations I have designed and tested each sub-systems starting with an LED

display. First I had to choose an appropriate resistor value to protect the LED and provide the

correct operating current to switch the LED on. Then I investigated the LM311 comparator,

which was connected to the LED circuit and would act as a switch, controlling when the LED

illuminated. Next was the investigation of a peak detector circuit with a 741 Op-amp. A

negative feedback loop was created, using a diode between the output and the negative input

on the Op-amp. With the diode on the output, the output signal was chopped and this was

what I was aiming for because I was only interested in the peak level. After the diode, a

capacitor and resistor were connected in parallel to the circuit to create a time constant so that

the LED illuminated long enough to be detected by the human eye. I then moved on to the

MC1408 DAC and this would convert the output of the PIC so that it could be used as an

analogue reference voltage for the comparator sub-system to compare with the sound level at

the input of the overall circuit. Next was the LMF100 switched capacitor filter and this is

needed to block any unwanted frequencies on the input because I was only interested in

audible frequencies detectable by the human ear. Next was programming the PIC 16F88

microcontroller, using a development board and Flowcode.

In the lab this is as far as I got but I would like to revisit this at a later date where I would

remove the DIL switch from the DAC sub-system and connect the outputs of port b on the

PIC in their place. Then the output of the DAC via a 741 Op-amp would be connected to the

comparator. I would then connect the PIC development board to the switched capacitor filter,

which in turn would be connected to the peak detector circuit, which is already connected to

the comparator and LED circuit. After all this, the overall system would be available for

some testing. At this stage I would set the PIC program going and observe if the correct

output of the LEDs on the development board were the same binary inputs I would be

expecting to see sent to the DAC. Then I would generate say a 1kHz, 1 volt peak to peak

sine-wave at the input of the switched capacitor filter. So then if the level of the sine-wave

was greater than that of the DAC output, the LED should illuminate.

31

Conclusion

To conclude, every investigation was completed successfully, therefore the project to design

and build a low cost sound level meter was a success. The PIC16F88 microcontroller used

within the system is very efficient for controlling the reference voltage and is low in price.

The other components within the meter are perfect for the project and also low in cost. This

also proves that the sound level meter was designed and built to the specifications.

Moving forward, after testing each sub-system and achieving the results I was hoping for, I

can think about transferring my overall circuit design via some software to a printed circuit

board. I also can start thinking about the design for the products casing and user interface.

Maybe if costs can be kept to a minimum, replace the LED with a LCD screen to give an

actual decibel reading.

In the future there would be further design needed so that the consumer could easily switch

between different reference voltages and to be able to control the clock input on the switched

capacitor filter, so that the cut off frequency could be altered easily for the different levels

required for various applications.

32

References

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Documents

Microelectronics final project report