Electronics - Build and test of Stylophone PDF

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Electronics - Build and test of Stylophone - Year 1 - Assignment 1...

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MO1S11 Build and Test Project Introduction Three laboratory exercises will be carried out by combining theory, testing and construction. Exercise 1 will be concerned with the following: • Understanding Ohms law • Learning the resistor colour code • Learning how to solder • Constructing a series resistor ladder • How resistors are connected in series to form new values • How resistors are connected in parallel to form new values • How a potential divider is used and how to calculate Voltages from a potential divider Exercise 2 will be concerned with the following: • Constructing an oscillator circuit that will form the central part of a Stylophone • Constructing a Power amplifier that will enable a speaker to be driven at audio frequencies • Constructing an active filter containing high and low pass elements • Testing the filter using National Instruments ELVIS Exercise 3 will be concerned with the following: • Examining the effect of the active filter on a square wave • Measuring the frequencies at the output of the Stylophone and comparing them with standard musical frequencies

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MO1S11 Build and Test Exercise 1 Ohms law For a single resistor the Voltage measured across the resistor and the current measured going through the resistor are related as follows:

V = IR

Figure 1: Ohm’s law for a single loop

Notice that the current is measured by an ammeter in series with the resistor and the Voltage is measured by a Voltmeter which is connected directly across the terminal of the resistor. Also notice the direction of the flow of current, the direction in which the source is acting and the direction of the measured Voltage. Rearranging Ohms law:

V I= R

R=

V I

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Colour code for practical resistors A standard colour banding code exists so that the value of a resistor can be determined. The code for resistors with 4 value bands and a tolerance band is as follows:

Figure 2: Resistor colour code

In preparation for the soldering exercise, use the resistor colour code to write down the 1st four bands for each of the following resistors: Designation Value 1st band 2nd band 3rd band 4th band R1 8.2k R2 820 Ohm R3 820 Ohm R4 1k R5 1k R6 1k R7 1.2k R8 1.1k R9 1.2k R10 1.2k R11 1.5k R12 1.5k R13 1.5k

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Soldering the resistor ladder The PCB you have been given contains an area designated “Lab exercise 1”. Refer to page three of the schematic diagram in the Appendix and also refer to the following diagram:

Figure 3: Resistor ladder as seen on the project PCB

Assemble the thirteen resistors using the resistors you identified on page 3 of these lab notes. To insert a resistor, 1st ensure that the legs of the resistor are clean. Bend the legs through 90 degrees so that the pitch is the same as that on the PCB. To bend the legs, use the fine nose pliers close to the resistor in order to relieve the bending force away from the resistor body (see picture). Do not allow the resistor body to take the bending force.

Figure 4: Bending the leg of a resistor

Use a solder temperature of 380 deg C. Avoid “dry joints”. All solder joints must be uniform and indicate the correct amount of heat and solder. Finally solder two tinned copper leads to the two pads of terminals J8.

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Resistors in series When resistors are connected in series you simply add the Ohmic values. Using the bench meter set on the resistance measuring range; measure the resistance across the two leads you soldered to connector J8. How does this compare to the sum of resistors R1 to R13? Is it within the tolerance indicated by the tolerance band on the resistors? Using the appropriate test leads for the resistance meter, measure the resistance across combinations of resistors as follows and record the results: Measure across R1 and R2 R4, R5 and R6 R9, R10 and R11 R7, R8 and R9

Measured value

Expected Value

Resistor in Parallel When resistors are connected in parallel (designated Ra and Rb) the combined value is given by:

Ra .Rb Ra + Rb

Figure 5: resistor in parallel

i.e. product divided by sum For example, a 2k resistor in parallel with a 1k resistor has a combined value of 1 x 2 /(1 +2) k = 0.66667k or 666.67 Ohms Knowing the sum of all 13 resistors (see previous page), what would be the combined value of this in parallel with 10k Ohms? What is the colour code for a 10k resistor? Place a 10k resistor in parallel with the two wires connected to J8 and measure the combined parallel value. How does it compare with your calculated value?

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Demonstrating Ohm’s law Using the resistor ladder you have already constructed, connect up the following circuit:

Figure 6: connections to the project board resistor ladder

The total series resistance has already been found (see page 5). The current I is measured using the bench meter set to the DC current range. Set the bench supply to 10V. Using Ohm’s law, calculate the expected value of current I and compare it to the measured value.

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Understanding a potential divider Connect up the following circuit (which is the same as the circuit above but without the current measuring instrument in circuit) but includes a Voltage measuring instrument:

Figure 7: Project PCB resistor ladder used for potential division

The bench meter is now set to the DC Voltage range and is shown connected to the negative end of the bench supply and to R13. The meter will read the following Voltage:

10V ∗ R13 resistorSu m i.e. the Voltage reading is the ratio of R13/(resistor sum) of 10V. (Resistor sum of R1 to R13 was found on page 5). Now move the Voltage meter connection point away from R13 to the connection point between R11 and R12. The meter will now read the following Voltage:

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10V ∗ ( R13 + R12 ) resistorSu m Voltage division depends on the ratio of the sum of the resistors being measured to the sum of all the resistors. Complete the following table and compare calculated Voltages with measured Voltages: Meter probe position

Between R13 and R12 Between R12 and R11 Between R11 and R10 Between R10 and R9 Between R9 and R8 Between R8 and R7

Measured Voltage

Ratio to calculate

R13 = resistorSum R13 + R12 = resistorSum R13 + R12 + R11 = resistorSum R13 + R12 + R11 + R10 = resistorSum R 13 + R 12 + R11 + R10 + R9 = resistorSum R 13 + R 12 + R 11 + R10 + R9 + R8 = resistorSum

Between R7 and R6

R 13 + R 12 + R 11 + R10 + R9 + R8 + R7 = resistorSu m

Between R6 and R5

R13 + R12 + R11 + R10 + R 9 + R8 + R7 + R 6 = resistorSu m R13 + R12 + R11 + R10 + R 9 + R 8 + R7 + R6 + R 5 = resistorSu m

Between R5 and R4 Between R4 and R3 Between R3 and R2 Between R2 and R1

R 13 + R 12 +R 11 + R10 + R9 + R8 + R7 + R6 + R5 + R4 = resistorSu m R 13 + R 12 + R 11 + R 10 + R 9 + R 8 + R7 + R 6 + R 5 + R4 + R3 = resistorSum R 13+ R 12+ R 11+ R 10+ R 9+ R 8+ R 7+ R 6+ R 5+ R 4+ R 3+ R 2 = resistorSum

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Calculated Voltage = ratio * 10V

MO1S11 Build and Test Exercise 2 Constructing the remainder of the board and testing the Active Filter Remove the two wires connected to J8 either by unsoldering them or cutting them close to the board.

Construct the power amplifier Locate the area of the board “Power Amplifier”: To an 8 Ohm speaker

Figure 8: Project PCB Power Amplifier section

The components required are: R15: 100 Ohms (brown black black black) R16: 3.3k (orange orange black brown) R17: 1k (brown black black brown) U3: (inserted using a 8 pin DIL socket) LM741 Operational amplifier C3 and C6: 100nF capacitors C7: 33nF capacitor Q1: BD135 NPN transistor Q2: BD136 PNP transistor Put the transistors Q1 and Q2 in last. Ensure that the transistors are not put in the wrong places, and ensure that they have the correct orientation. If you solder these devices incorrectly then you risk damage to the PCB trying to remove them.

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Solder two wires of the same colour (shown green in figure 8) to connector J6 as shown and solder these to an 8 Ohm speaker.

Construct the Stylophone Oscillator Locate the area of the board “Stylophone Oscillator”:

Figure 9: Project PCB oscillator section

Do not at this stage insert the wire link (R18) The components required are: R21: 10k (brown black black red) C1: 10nF capacitor C2: 100nF capacitor R14: 11k (brown brown black red) U1: (inserted using a 8 pin DIL socket) LM555 timer IC

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Construct the Active Filter Refer to the Filter schematic in the Appendix. Locate the “Filter” area of the board: View of area:

Figure 10: Project PCB filter section showing copper tracks

View of area without tracks shown:

Figure 11: Project PCB showing top view of filter section

Construct the circuit with: U2: a LM741 Operational amplifier Do not solder U2 directly to the board – use the 8 pin DIL socket provided. This is important since if you damage the device it can then be easily replaced. C4: 100nF capacitor C5: 10nF capacitor R20: 47k (yellow violet black red) R19: 100k (brown black black orange) Now locate the power supply connection area of the board:

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Complete the power supply

Figure 12: Project PCB power connections

Solder in the two diodes D1 and D2 (both type 1N4001). Make sure they are connected the right way around (study the above diagram!). Solder three tinned copper leads to the three connection of J2. It is a good idea to use different colours for the wires as shown above (if available). Use black for 0V, red for +12 V and blue for -12V as shown in figure 12. Note: for convenience we will be connecting to the ELVIS +/- 15V supplies. Next connect two tinned copper wires to J4 and a further two wires to J5. Again use different colours (if available) as shown in figure 13 (green and black to J4 and grey and black to J5).

Figure 13: Connecting wires to the filter section of the Project PCB

Make sure that both right hand sides of J4 and J5 are black since this is the signal’s ground connection.

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Filter circuit characteristic The filter has both a high pass and a low pass characteristic. A simulation of the filter is as follows:

Figure 14: Simulation of the filter used on the Project PCB

The filter has minimum attenuation (centre frequency) at 72Hz, a lower -3dB point at 14.8 Hz and an upper -3dB point at 369 Hz giving a bandwidth of 354.2 Hz. The pass band gain at 72Hz is -20.4dB.

Testing the filter circuit using National Instruments ELVIS In this section you will obtain practical results for the filer you have built and compare the results with figure 14. The ELVIS system will inject frequencies into the input of the filter (at connector J4) and measure the output of the filter (at connector J5). Using the Bode plotter ELVIS will display the filter characteristic. The characteristic obtained should be fairly close to the simulation given in figure 14. As ELVIS drives the input of the filter, you will be able to hear the output of the filter via the power amplifier and speaker you have already assembled.

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Connecting to the ELVIS system and set-up procedure Inspection and Set up Before connecting up your active filter, inspect the board for correct positioning and orientation of components. Check also that all solder joints are clean and there are no visible short circuits. Do not turn on the ELVIS system until all connections have been made and checked.

Power and Test Connections Connect the positive negative power and ground supply of the Project PCB to the NI ELVIS prototype board as shown in figure 15.

Figure 15: Connecting the Project PCB power supply to the Elvis prototyping panel

Note that the red wire is connected to +15V, the blue wire is connected to -15V and the black wire is connected to GROUND. A close-up of the connection points is shown in figure 16:

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Figure 16: ELVIS power connections close-up

The next step is to connect the filter to the ELVIS prototyping board as shown in figure 17.

Figure 17: Connecting the Project PCB filter to the ELVIS prototyping board

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Notice that grey and black from PCB connection point J5 go to ACH0+ and ACH0respectively and that green and black form PCB connection point J4 go to ACH1+ and ACH1- respectively. The input signal to the filter is derived from the ELVIS function generator. This is connected up as shown in figure 18.

Figure 18: connecting up the ELVIS function generator to the filter input

Figure 18 shows a red wire connecting the ELVIS function generator FUNC_OUT to the green wire on ACH1+. A close-up of the connection is given in figure 19.

Figure 19: ELVIS connection point FUNC_OUT

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Obtaining the frequency plot of the filter using ELVIS Run the program NI ELVIS to obtain a window as shown in figure 20.

Figure 20: ELVIS virtual instruments Select the Bode analyser and set up the measurement parameters as shown in figure 21.

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Figure 21: ELVIS Bode plot of the PCB filter

Notice that the measurement parameters are as follows: Frequency start = 5Hz Frequency stop =10kHz Steps per decade = 10 Peak signal amplitude (which drives the filter input) = 2V Display gain range from -50dB to -20dB To acquire the data press the Run button. Once the run has been completed the result should look like that in figure 21. Press Cursors on and take a measurement at the peak of the filters gain. Also find the two frequencies at which the gain has dropped by 3dB.

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MO1S11 Build and Test Exercise 3 Connecting up the Stylophone oscillator Remove the connections form the ELVIS system. Locate the area of the board “Stylophone Oscillator”:

Insert the wire link (designated R18). This is just a shorting link that connects the oscillator to the active filter. The oscillator is formed using a LM555 timer IC in Astable mode (see schematic page 2 included in the Appendix). Solder a wire for use as the stylus into the PCB pad marked J1 “stylus”. Connect the bench power supply to connector J2 (Red to +12V, black to 0V and blue to -12V). Set the current limit of both 12V supplies to 200mA. Touching the stylus wire to the keyboard pads should cause the notes to sound on the speaker. They should be fairly close to those of the chromatic scale starting at middle C:

Notes: C# is the same as Db, D# is the same as Eb, F# is the same as Gb, G# is the same as Ab, A# is the same as Bb.

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Testing the frequencies of the Stylophone Connect the bench frequency meter to the connector J5 (grey wire is the output signal) ensuring that the black wire of J5 is at ground (0V) otherwise you will short the power supply. For each note of the scale, measure the frequency and compare with the standard musical frequencies. Complete the following table: Note C1 C# D D# E F F# G G# A A# B C2

Standard frequency (Hz) 261.63 277.18 293.66 311.13 329.63 349.23 369.99 392.00 415.30 440.00 466.16 493.88 523.25

Measured frequency (Hz)

Percentage error

Notice that A = 440Hz is the standard for tuning instruments. This is the note that you will hear an Oboe play in order for the instruments of the orchestra to check their tuning.

Examining the effect of the Active Filter Disconnect the frequency meter from connector J4. Connect channel 1 of the oscilloscope to connector J4 (ensuring that the 0V reference is connected to the black wire). Connect channel 2 of the oscilloscope to connector J5 (ensuring that the 0V reference is connected to the black wire). When notes are played on the Stylophone you will see square waves on channel 1 (going into the filter) and rounded waves on channel 2 (coming out of the filter and driving the input of the power amplifier). The following examples have been obtained from simulation:

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Output for note C1:

Figure 22: Simulated output of filter being driven at the frequency of C1

Output for note C2:

Figure 23: Simulated output of filter being driven at the frequency of C2

It can be seen from figures 1 and 2 that the effect of the filter is to remove harmonics from the square wave that is produced by the Stylophone oscillator and produce a more “rounded” waveform. For the notes C1, A and C2 store the oscilloscope results using a USB flash memory (plugged into the oscilloscope) for reproduction them with suitable comments in your laboratory report.

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MO1S11 Build and Test Final Report and Assessment All lab reports should contain methods, conclusions and suitable comments. If any problems were encountered then provide suitable explanations with possible solutions. Your completed PCB will be inspected for soldering quality, neatness of construction and functionality. Marks will be awarded as follows: PCB construction and functionality: Build and Test Exercise 1: Build and Test Exercise 2: Build and Test Exercise 3: Concluding remarks:

30% 15% 25% 20% 10%

You may keep the final PCB construction. You may consider mounting the PCB into a box and powering it from 2 batteries. Connect two batteries and a 2-pole switch as follows:

Switch SW1 must be a double pole switch since each battery must have a separate switched connection. All parts (box, battery holders and switch) can be obtained from Maplin Electronics. See www.maplin.co.uk.

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Appendices Circuit Schematics

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PCB layout Top copper layer

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Bottom copper layer

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Silk screen

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