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Analyse signal diodes, rectifier diodes, Zeners, Schottkys, photodiodes, LEDs... and more! By Tim Blythman Diode Curve Plotter Our new Diode Curve Plotter is a superb piece of test gear that is both practical and educational. It’s very versatile and fits in the palm of your hand. It automatically tests diodes in both directions and plots the resulting current/voltage curve on a colour LCD screen. It tests zener diodes up to about 100V, but it can also test LEDs, schottky diodes, regular diodes, transient voltage suppressors and more. T here have been many diode tester projects. Tyically, they were rather simple and only able to provide a measurement of, for example, a zener voltage. Also, they were often treated as an ‘add on’, and required a separate multimeter to display the result. This new unit utilises the same 2.8inch colour LCD touchscreen used in the Micromite BackPack from May 2017, but this time it’s being paired with an Arduino Mega board and a custom PCB, which provides the user test interface. What can it do? The main feature of the Diode Curve Plotter is that it performs a full bidirectional current/voltage (I/V) sweep of a connected diode (or other component) and display the results in a graphical form. 14 For zener diodes, the zener voltage and current are displayed on the screen, along with the zener impedance at that point. You can move the test point to get different voltage, current and impedance readings along the curve. The unit can produce up to 100V at up to 30mA for testing diodes, providing a wide testing range. For devices like LEDs, you can limit the test voltage and current to avoid damaging them during testing. It also has a specific LED testing mode, to make that job even easier. The plot data can also optionally be sent to a connected computer as rows of CSV (comma-separated value) data, allowing plots to be stored and analysed further if necessary. You can plot and analyse this data on your PC using just about any spreadsheet program. Getting back to the unit itself, cursors on its screen allow the operating point to be varied, by selecting either a voltage or current, so that the operating conditions can be examined across the range of the plot. For example, you could investigate how a zener diode performs at points away from the ‘knee’ of the zener curve. The hardware scans the diode in both quadrants. It shows the full plot on the display, but only the forward operating point conditions are displayed in detail. A ‘Reverse’ button allows the plot to be flipped so that the reverse characteristics can be checked without rerunning the test. This is handy if the diode is connected backwards, or to check its behaviour in both forward and reverse directions. The unit has adjustable current, voltage and power-limiting parameters. But given that each test takes a few seconds to complete, even if these limits are set slightly high, any Practical Electronics | March | 2020 over-current or over-voltage condition is quite brief and unlikely to cause any damage. The LED test mode is essentially a constant-current mode, which provides a set output current. It shows the forward voltage, power and voltage/ current ratio for the connected device. The current and voltage limits can be set to the nearest milliamp and volt respectively, so even unknown devices can be probed without risk. If a resistor is connected, the voltage/current ratio will, of course, correspond to the resistance, and thus the diode tester can even be used as a very basic ohmmeter. How it works The Diode Curve Plotter is a sandwich of three boards: an Arduino Mega or compatible board forms the bottom layer and provides the processing power. The LCD touch panel is the top layer, providing the display and user interface. In the middle, the custom PCB contains the other parts, which measure the parameters of the connected diode. By the way, the reason we are using an Arduino Mega rather than an Arduino Uno in this project is that we need the extra Flash memory space provided by the larger chip on the Mega. We are not using any of the extra pins. High-voltage generator As we noted, the unit can test diodes up to 100V, but it runs from 5V DC, so it needs a way to generate higher voltages to apply to the device under test (DUT). The circuit diagram of the Diode Curve Plotter is shown overleaf in Fig.1. Inductor L1, N-channel MOSFET Q1 and diode D1 operate as a standard boost converter which is driven by IC2, an LM311N comparator. The plotter mounted in a UB3 Jiffy Box, with a laser-cut front panel to reveal the touchscreen display. Practical Electronics | March | 2020 Features and specifications • Tests zener diodes, LEDs, TVSs, silicon diodes, schottky diodes and more • Colour touchscreen interface • Tests up to 100V/30mA from 12V DC supply (can run from 5V, including USB) • Automatically plots I/V curve in both quadrants • Reads out current, voltage, power and impedance at any point in the curve • Adjustable current/power limit for smaller devices with 0.4W and 1W presets • Simple LED testing mode • On-screen button to show reverse characteristics • Based on an Arduino Mega with custom shield It runs from a 5V DC supply, which is convenient, because that means you use a USB power bank, USB charger or even a PC/laptop USB port. But note that it may draw more than 500mA when testing higher-voltage, higherpower devices, so a computer USB port may drop its bundle under these conditions. A 1A+ charger or battery bank is recommended. The boost regulator draws power from the Arduino’s VIN pin, which is connected directly to its DC power jack. In case the unit is powered via the USB socket instead, the 5V supply flows through schottky diode D2 into the VIN rail, thereby powering the boost converter. IC2, the LM311 comparator, is used both as an oscillator to drive the gate of MOSFET Q1 and also as a current limiter. Note that an LM311 can only sink current at its pin 7 output, so pin D3 of the Arduino (‘BOOSTCTL’) must be high to enable the oscillator. This pulls pin 7 up via the 1kΩ resistor; normally, it is held low by a 10kΩ resistor, so Q1 is off by default. Since the Arduino’s D3 output is capable of generating a low-frequency PWM signal, we can switch the boost circuit on and off rapidly with a varying duty cycle to control the resulting boosted voltage. The circuit around IC2 is not a fixed oscillator; instead, it monitors the current passing through inductor L1 using the 1Ω 1W series resistor. When Q1 switches on, the voltage across the 1Ω resistor increases as the current through L1 builds and its magnetic field builds up, until the threshold set by the comparator’s resistor network is reached. At this point, Q1 is switched off. The 100kΩ feedback resistor provides hysteresis, allowing the current to drop a small amount before Q1 switches on again and the cycle repeats. The resulting waveform has a high duty cycle, as required for a boost circuit with such a high output/ input ratio. When Q1 switches off, the voltage at the end of inductor L1 that’s connected to its drain shoots up and so diode D1 becomes forward-biased, charging up the 1µF capacitor to a much higher voltage than the incoming supply. This voltage is divided by 100kΩ /3kΩ resistors, filtered by a 100nF capacitor and fed to analogue input ADC1 of the Arduino. The divider provides a voltage which is within the 0-3.3V range of the Mega’s analogue-to-digital converter (ADC). While the Mega’s ADC has a 0-5V range by default, we are using its onboard 3.3V regulator as a more precise reference, and so we can measure up to around 108V with this divider. Jumper JP1 usually feeds 3.3V into its AREF pin. This divider also discharges the 1µF capacitor so that its charge will not persist after the boost converter is switched off. With the capacitor charged to 100V, the 100kΩ resistor dissipates around 100mW, which is well within the ratings of a small 1/4W resistor. Note that the 13kΩ resistor connected to pin 2 of IC2 via a 10kΩ resistor sets the maximum inductor current (ie, in L1) which effectively determines the maximum voltage that the boost generator can produce, and also affects the maximum current that the unit will draw. So if you want to reduce the maximum test voltage (eg, to allow the unit to run from a USB port that 15 +5V +3.3V 1 0 0nF MOSI +5V +5V D/C MOSI SCK LED MISO T_CLK T_CS T_DIN T_DO T_IRQ SD_CS SD_MOSI SD_MISO SD_SCK 4 RLYCTL 5 LCDDC 6 LCDRST 7 9 4 6 2 5 ADC3 ADC1 IO 5/PWM ADC0 36k IO8 IO 9/PWM 12 ARDUINO MEGA OR EQUIVALENT A 14 D2 K 1N5819 15 TP3 IO 11/MOSI +5V +3.3V IO 13/SCK RESET A K +5V SDA TO LCD SCL IPA60R520E6 G LK1 VIN 1 1W VIN IC2: LM311 10k 10k 62 2 3 5 6 8 IC2 4 1 0 k A 100nF 10k 10k TP1 L1 100 H 100k 7 Q1 IPA60R 520E6 1 62 D1 1N4004 D S TP2 CON1 1 F 250V TEST TERMINALS OPTO1 PC817 2 D K 1 100 F 4.7k D1, D2 AREF 6x 1k 18 36k GND GND 17 1k 6 VIN IO 12/MISO 16 IC1b GND IO 10/SS 13 4.7k 5 7 470 11 1k 2 4 MISO 10 IC1a ADC2 IO 4/PWM 3 IO7 SCK +5V 8 1 IO 6/PWM LCDCS 8 ICSP IO 2/PWM IO 3/PWM 3 3 BOOSTCTL ADC 5/SCL ADC 4/SDA DC VOLTS INPUT RESET 5x 470 2 IO 1/TXD USB TYPE B CS IO 0/RXD TOUCHCS OPTOCTL GND 1 1 CON2 VCC IC1: LM358 MOD1 4  1M 1M 100k 30k 10nF RLY1 3 10nF 30k 1k 3k 1 0 0nF OPTO2 PC817 1 1 3 k 10k 2 4  IC1, IC2 3 100 SC ZENER/ DIODE /LED Zener/Diode/LED Curve Plotter CURVE PLOTTER 20 1 9 PC817 + 4 8 1 4 1 2 Fig.1: the Diode Curve Plotter is based on an Arduino Mega (MOD1), a boost regulator (IC2/Q1/L1), two optoisolators which operate as a controlled current source (OPTO1 and OPTO2) and a relay to reverse connections to the DUT (RLY1). The test voltages and current are fed back to the Arduino so it can plot the curve and display measurements. can only supply 500mA) then you can drop the value of this resistor to 12kΩ or even slightly lower, down to as little as 11kΩ. Test circuitry The test voltage is fed to the DUT via two optoisolators, OPTO1 and OPTO2, and relay RLY1. At the other end, the DUT is connected to ground via a 100Ω resistor. OPTO1 and OPTO2 are configured as a controllable current source, with both collectors connected directly to the high-voltage supply and both emitters to the DUT. Their phototransistors are connected in parallel to enhance the amount of current they can supply to the DUT. Their LEDs are connected in series, so that the effective current transfer ratio (CTR) is doubled. They are controlled by a PWM signal from pin D10 of the Arduino which is fed to the two 62Ω resistors. The 100µF capacitor smooths the PWM signal, in combination with those resistors, so that a steady, controllable current flows. 16 The modulated current goes to the DUT via relay RLY1. When its coil is energised, it reverses the connections to the DUT. The 100Ω resistor operates as a current shunt, allowing currents up to 33mA to be measured against the 3.3V reference voltage. The voltage across this shunt is monitored at the Arduino’s A2 analogue input. In practice, while testing a device, the unit sweeps the test voltage with the relay switched on, monitoring both the current and voltage, then performs another sweep with the relay switched off, so that current flows through the device in both directions during a single test pass. Measuring circuitry Four main parameters are measured by the Mega’s internal 10-bit ADC. Two have already been mentioned: the voltage on the 1µF capacitor and the test current, as measured using the shunt. The other two parameters measured are the voltages at each end of the DUT. Both are fed into 1MΩ /30kΩ voltage dividers, giving the same 108V maximum reading. Practical Electronics | March | 2020 These voltages are fed into the two halves of IC1, an LM358 op amp. By default, these are configured as unitygain buffers, with the 36kΩ resistor in the feedback path having little effect. In this mode, voltages up to 108V can be measured with around 0.1V resolution (108V / 210). But there is also a 4.7kΩ resistor and 1kΩ resistor from the inverting input of each op amp to two digital pins on the Arduino. These are initially left floating, and in this case, do not affect the op amp’s operation. However, if either is pulled low by its corresponding pin on the micro, that changes the op amp gain to either 8.66 (ie, 36kΩ÷4.7kΩ + 1) or 37 (ie, 36kΩ÷1kΩ + 1). This amplifies the sensed voltages, giving resolutions of around 10mV and around 3mV respectively, with the maximum readings being about 12.5V and 3V. So the gain is only increased when measuring lower voltages, to improve resolution. All ADC measurements are sampled 16 times and averaged to improve precision and stability. Any error due to input offset will be taken care of during calibration stages. Touchscreen interface The touchscreen plugs into header socket CON2. The screen is powered from the 3.3V regulated supply, while the backlight is powered from the 5V rail. The Arduino controls it over two SPI (serial peripheral interface) buses. One is used for updating the screen and one for getting data from the touch sensor. Their MISO and MOSI (data) and SCK (clock) lines are connected together to share the same set of hardware SPI pins on the Arduino, via its six-pin ICSP header. The screen and touch controller have separate chip select (CS) pins, at pins 3 and 11 on CON2, so the Arduino can select which one it is communicating with by pulling one of the two digital outputs (D7 or D2) low. These five lines, plus the data/control line on pin 5 of CON2, have 1kΩ resistors connected from each pin to ground, plus 470Ω series resistors between the LCD pins and the Arduino. These form voltage dividers, reducing the 5V swing on the Arduino outputs to a 3.3V swing, to suit the LCD electronics. The MISO line is driven by the LCD so no level shifting is needed, as the Arduino will read 3.3V as a high level. The remaining five pins on CON2, the interrupt request line from the touch controller (T_IRQ) and the four SPI control lines for the SD card socket are unused and so are left disconnected. Software operation While testing, the boost converter is modulated by the PWM signal from pin D3 to maintain a voltage at TP1 that’s slightly higher than the desired test voltage, but within the programmed limits. Then, the current through OPTO1/ OPTO2 is varied across the testing range. The difference between the voltages measured at both ends of the DUT, plus the current through the 100Ω shunt are recorded in an array. RLY1’s state is toggled to reverse the polarity of the DUT, and the test is repeated, after which the results are plotted in a graph on the screen. The Arduino Mega interpolates the data points to find the voltage and current at which the device power equals the selected operating point. The relevant figures are then shown in a small box on top of the graph, as well as drawing lines which show where that point is on the curve. The box display includes the voltage, current and power at the operating point, as well as the zener impedance, derived from the gradient of the voltage/current curve at that point. The raw I/V data is then dumped to the serial port, where it can be read by the PC and used for additional analysis. Practical Electronics | March | 2020 Screen1: the splash screen/main menu allows you to select between the two different types of tests (I/V Test or LED Test) and access the Settings and Calibration menus. Screen2: the typical result of the I/V Test run on a 75V zener diode. A 250mW operating point is identified and indicated on the graph. Screen3: here we have selected the I/V Test option with an LED connected to the unit and it has performed the measurements and plotted the graph. It’s showing that 10mW is achieved, a forward voltage of 2.05V and a test current of 4.89mA. The details of a second operating point can be analysed by touching the graph along the right-hand axis. If the graph is touched in the first quadrant (top right), then the current level is selected according to the vertical position of the touch. A second info box is displayed, showing conditions at the new operating point. 17 With more screen space available, the statistics are shown in a larger font and they include voltage, current and power at the instant of measurement, as well as the ratio of voltage to current. This will not correspond to the zener impedance, but will be a fair measurement of the resistance of a fixed resistor. Screen4: in the LED test mode, where the forward voltage, current, power and zener impedance are continuously updated. You can adjust the maximum voltage and current applied to the LED directly with the arrows below. Screen5: in the Settings screen, where you can select the type of device being tested, the maximum power and the target (nominal) power. The four buttons at the bottom change these values, then you press the Back button when finished. Screen6: the Calibration screen reads out one of seven parameters, as measured by the Arduino, allowing you to compare them to readings made with a DMM and calculate coefficients to provide more accurate measurements. Similarly, touching the graph in the fourth quadrant (bottom right) sets the voltage according to the horizontal touch position and displays a similar box. The LED test page is much simpler, and in this mode tests are run continuously. The current is modulated to the limit set on that page, and the voltage is maintained within these limits by controlling the boost circuit. 18 Construction All of the components mount on a shield PCB, which measures 99mm × 60mm, and is available from the PE PCB Service (coded 04112181). Use the overlay diagram in Fig.2 as a guide while building the board. Start by fitting the small (1/4W or 1/2W) resistors where shown. It’s a good idea to measure the value of each one before fitting, as the colour bands can sometimes be ambiguous. Solder diodes D1 and D2 in place next. They are different types and also oriented differently. Make sure that you don’t mix them up and that the cathode stripes face in the directions shown in Fig.2. You can then fit the larger 1Ω 1W resistor. Now install the seven capacitors, making sure that for the electrolytic types, the longer (+) lead goes into the pad marked with a ‘+’ sign on the PCB overlay diagram and the PCB silkscreen printing. Since the LCD stacks above this board, all components must project less than 12mm above the top surface of the PCB. If any of your electrolytic capacitors are 12mm high or taller, you will need to lay them over on their side when you fit them. Note that we give two options for the 1µF capacitor, a polyester ‘greencap’ and an electrolytic type. While both should work, we prefer using the greencap, despite the fact that it needs to be installed with its leads bent over to keep it under 12mm high. Greencaps have better performance than electrolytics. But either should work, so it’s up to you. Fit the two ICs next. They are different types but come in the same package – don’t get them mixed up. Fig.2 shows where they go and the correct orientation of each. Make sure the pin 1 notch or dot is facing as shown before soldering the pins. Then fit the two optoisolators, again taking care that their orientation is as shown. Now mount MOSFET Q1. You will need to bend its legs 90° to allow the body of the MOSFET to sit flat. Before attaching to the board with a 6mm machine screw and nut, check whether that screw will foul the USB socket on your Mega board once the two are plugged together. If so, you will need to omit the mounting screw. Once you’ve sorted that out, ensure the writing on the tab is facing upwards and then solder its leads. Telecom-style relay RLY1 is installed next, with its pin 1 stripe to the left as shown. Then fit inductor L1, which is not polarised, so its orientation is not critical. Ensure that it is not too tall when installed; it may need to be laid on its side to keep it under the 12mm limit. Then solder pin header JP1 in place. The four SIL pin headers for connection to the Arduino can all be snapped from a single 40-pin header. The easiest way to mount them to the board is to plug them into the Mega board, then slot the shield PCB over the top to ensure that everything is square and flush before soldering them in place. Note that they are inserted through the bottom of the PCB and soldered on the top side. Use the same technique to solder the 2×3 female header to the board; again, it is mounted on the underside. Note that you could use a stackable header set, such as Jaycar’s HM3208, rather than the standard pin headers specified. But that is likely to change the overall height of the unit, and it may no longer fit in the specified case. Use a similar technique to fit the 14-way female header which connects the LCD to this PCB. It goes on top of the Practical Electronics | March | 2020 board. Plug it onto the LCD header, then mount the LCD on the shield board using three 12mm tapped spacers and six 6mm-long M3 machine screws – don’t attach it in the upper-right corner; there is no spacer mounted near IC2. Then solder the header in place. The 2-way female header is used for CON1, which connects to the device under test. We found this type of header ideal for this purpose, as most smaller component leads simply plug into the sockets. However, you could chassis-mount banana sockets instead, and wire them back to the pads for CON1. You may wish to solder extension leads to the pins of CON1 before fitting it, to make the top of the socket level with the top of the LCD once assembly is complete. That allows it to project through the hole provided in the laser-cut lid. But if you do so, insulate the wires with short pieces of heatshrink tubing or similar, keeping in mind that there can be around 100V between them during operation. That completes the assembly of the shield board. Once this is done, double check your soldering. Given that the board can generate over 100V, you don’t want a small error on the PCB to feed that back into your computer. You may wish to use the USB Port Protector we described in May 2019. Unplug the shield/LCD assembly from the Arduino Mega now, as it’s best to keep the boards separate until the Mega has been programmed, especially if the Mega has previously been programmed for another project. Connect the Mega to a computer using an appropriate USB cable; most Megas have USB TypeB full-size sockets so you will need a Type-A to Type-B cable. Fig.2: this component overlay echoes the silk-screen printing on the PCB surface shown below – between the two you should have no problems constructing the shield. Installing the software To install the software on the Mega, you need the Arduino IDE (integrated development environment) installed on your computer. The IDE includes a compiler and serial programming software, allowing the source code to be compiled and sent to the Arduino. The IDE can be downloaded from www.arduino.cc/en/Main/Software We are using Arduino IDE version 1.8.5, but a newer version may be available by the time you check the download page. Since we have written many of the libraries for this project ourselves, we have included all the necessary files in the sketch folder. Download the zip file from the March 2020 page of the PE website and extract the contents to a suitable location such as your ‘Documents’ or home folder. Open the Zener_Diode_Tester.ino sketch file using the IDE. From the Tools menu, under Board, select ‘Arduino/ Genuino Mega or Mega 2560’. Then choose the appropriate serial port from the Tools -> Port menu. Click Upload or press Ctrl-U to start the compile and upload process. This may take a minute or two. Unplug the USB cable and place the jumper shunt over the two pins of JP1 so that it is closed. Plug your shield PCB onto the Arduino Mega and then attach the LCD to the top of the PCB (if it isn’t already attached). You are then ready for testing. Testing and touchscreen calibration Plug the USB cable back into the computer or if you have a 12V DC plugpack handy, use it instead. Ensure that the screen illuminates and displays the main menu page with the SILICON CHIP logo. The Mega can have a start-up delay, so don’t be alarmed if nothing happens for a few seconds. The sketch is written with a default touch panel calibration. Practical Electronics | March | 2020 Try pressing some buttons on the touchscreen and check that they respond as expected. If you find that they don’t, or the touchscreen calibration seems inaccurate, or it is not responding to touch at all, you will need to use our provided calibration sketch to calculate new touch panel calibration parameters. By the way, we’ve seen some 2.8-inch touchscreens which look more or less identical to others but the touch panel axes are reversed. If you have one of those, you will definitely need to go through the calibration process. To do this, open the AVR_LCD_BackPack_Touch_ Calibration.ino sketch and upload it to the Mega using the instructions above. Open the Serial Monitor from the Tools menu (or by pressing Ctrl-Shift-M) and set the baud rate to 115,200. Following the instructions on the screen, use ‘1’ (followed by Enter to send the command) to perform the calibrations. Then use ‘2’ to test that the new calibration is accurate. Copy the new calibration constants from the Serial Monitor to the clipboard, as shown in Fig.3. Now re-open the original Zener_Diode_Tester.ino sketch and open the backpack.h tab. Find the lines in the code which are shown in Fig.4. Replace the existing calibration constants with the new values you copied earlier, save the updated sketch and then upload it to the Mega. You are now ready for final testing. Checking voltages and currents Note as you read the following, that if you are using a computer USB port to power the unit during testing and calibration, some of the voltages mentioned below may not reach 100V and the unit may reset, due to the limited current capabilities of that port. 19 This photo, along with the one opposite, shows how the three boards are ‘sandwiched’ together – the Arduino Mega board on the bottom; the new shield board in the middle (green) and the 2.8-inch LCD touchscreen on top. It is designed to fit in a UB3 Jiffy box with a new laser-cut Acrylic lid. Note the connectors on the Mega board in the photo opposite – the USB on the left, and the DC power input at right. Start by clicking the Calibration button and use the Previous and Next buttons to scroll through the various items. In the top-right corner, the display shows where to connect your multimeter test leads to read the appropriate voltage. The second line indicates the name of the value being tested. The third line indicates whether the test terminals should be open or short-circuited. They should be short-circuited for the current test (eg, using your DMM in current measurement mode); otherwise, there is no path for the current to flow. The first item to be checked is the output of the highvoltage generator, and this should be up around 100V, with an ADC value in the 900s. If this is the case, the high-voltage system is working. If not, check (after powering off the unit) for wiring faults around the right-hand edge of the board, particularly around Q1 and L1. The next three items measure the voltage at the positive test terminal with various gain settings. Using a DMM, measure the voltage at the positive terminal of CON1 (shown in Fig.2 and on the PCB) relative to TP3 (GND) and check that you get a reading that’s close to the one shown on the screen. The three following items are the negative test terminal voltage at its three different gain settings. Use the same technique as above to compare your readings to those shown on-screen. Any significant deviation in these voltages from reality indicates a problem in the vicinity of IC1. The final item to check is the current reading, and as noted, it will only work if there is a path for the current between the test terminals. Switch your DMM to current (mA) mode and connect it across CON1. The displayed current should be around 30mA, with an ADC reading around 900. Compare this to the reading on your DMM. It should be close. If you don’t get any current reading or it is wildly off, then you may have a problem with the circuit around the optoisolators. To calibrate the unit, step through each reading and record the ADC value shown and an accurate measurement of the voltage (or current) using your DMM. Then divide the voltage or current value by the ADC reading, and write this value down. The unit is then calibrated by modifying the scaling values in the sketch itself. This part of the code is shown in Fig.5. It’s near the top of the file. Find those lines and change the values to those you wrote down. If the values you have are significantly different from the defaults, you may have a problem with your board, or you might have made a mistake in calculating these values. Performing this calibration adjusts the software to be accurate with the particular components on your board (eg, the exact resistor values). After the values have been edited, the sketch will need to be uploaded again, as per the earlier instructions, to allow the new values to take effect. Completing assembly Once you are satisfied that the unit is calibrated and working correctly, it can be fitted in its case. Start by removing the screws holding the LCD screen onto the tapped spacers, then temporarily unplug the screen. Now plug the shield into the Arduino Mega and secure the two together using the specified nylon machine screws and nuts, through the mounting holes near Q1 (adjacent to the Arduino SCL pin) and near the Arduino A5 pin. Next, plug the LCD back into the shield but don’t attach it using screws just yet. Slot the laser-cut lid panel over the LCD screen, then feed 10mm machine screws through Parts list – Arduino-based Multi Diode Tester 1 double-sided PCB coded 04112181, 99mm x 60mm (available from the PE PCB Service) 1 Arduino Mega R3 board or equivalent [eg, Jaycar XC4420, Altronics Z6241] 1 2.8-inch LCD touchscreen [ILI9341-based touchscreen LCD panel, 320 x 240] 1 UB3 Jiffy box (included lid not required) 1 3mm laser cut Acrylic lid [SILICON CHIP ONLINE SHOP Cat SC4927] 1 2-pin female header (CON1) 1 14-way female header (CON2) 1 2-pin header with jumper shunt (JP1) 1 6-pin, 2 8-pin and 1 10-pin header (to connect to Arduino) 1 2x3-pin female header (to connect to Arduino ICSP header) 1 DPDT relay with 5V DC coil and 250VAC-rated contacts, DIP-10 (RLY1) 1 12V 1A (or higher) plugpack with centre positive 2.1mm tip 1 100µH bobbin type inductor (L1) 2 M3 x 20mm nylon panhead machine screws 3 M3 x 12mm tapped nylon spacers Do not touch any component 4 M3 x 10mm panhead machine screws leads while the unit is 7 M3 x 6mm panhead machine screws operating. 100V is enough 2 M3 nylon hex nuts to give you a bite! 2 M3 hex nuts WARNING 20 Semiconductors 1 1N4004 400V 1A diode (D1) 1 1N5819 schottky diode (D2) 1 LM358 op amp, DIP-8 (IC1) 1 LM311N high-speed comparator, DIP-8 (IC2) 2 PC817 optoisolators, DIP-4 (OPTO1,OPTO2) 1 IPA60R520E6 700V N-Channel MOSFET (Q1) [mouser.co.uk / digikey.co.uk] Capacitors 1 100µF 10V electrolytic 1 1µF 450V electrolytic or 250V polyester ‘greencap’ 3 100nF MKT or ceramic 2 10nF MKT or ceramic Resistors (all 1/4W 1% unless otherwise stated) 2 100kΩ 2 36kΩ 2 30kΩ 2 1MΩ 1 13k 6 10kΩ 2 4.7kΩ 1 3kΩ 9 1kΩ 6 470Ω 1 100Ω 2 62Ω 1 1Ω 1W 5% Practical Electronics | March | 2020 the panel and LCD, into the three tapped spacers below. Use the fourth 10mm machine screw and single nut to hold the lid onto the LCD screen using the remaining mounting hole, in the upper-right corner. If you aren’t planning to read measurement data out to a computer via the serial port and you are using a plugpack supply, then you only need to make a hole in the box base for the DC power jack. Otherwise, you will also need to make a cut-out to access the USB socket. Make the holes in the lower half of the UB3 case using the drilling diagram (downloadable from March 2020 page of the PE website) as a guide. Finally, attach the lid to the top of the box using the supplied self-tapping screws. Using it This device will test just about any type of diode, including standard silicon diodes, schottky diodes, LEDs and unidirectional or bidirectional transient voltage suppressors. Having connected the device to both of the test terminals, press the I/V Test button on the screen. You should hear two clicks and the graph will be displayed. If you have inserted the component backwards, press Reverse to swap the plot around. You can also touch the graph on the touchscreen to display figures for various voltages and currents. Cursors appear to show the point being touched and the relevant information is displayed in a second box on the bottom left of the screen. Pressing Back returns to the main menu page. From there, press the LED Test button to start the LED test. The voltage and current limits are set using arrow buttons at the bottom of the screen. These are soft limits which are controlled by the microcontroller, so the readings may occasionally drift above these settings. If this occurs, a small red asterisk is shown to alert you to that fact. The high-voltage rail value is shown at the bottom of the screen, and the current device operating conditions are shown along the right-hand side of the screen. Press Back again and then press Settings to go to the settings page. This sets the various parameters for the I/V Test mode (the LED test mode has its settings shown on that screen, as explained above). The Previous and Next buttons scroll between various items, while the Up and Down buttons change the values of those items. There are seven settings available. The first allows you to select either a 400mW or 1W zener; it automatically sets the maximum and target power settings. If this is set to ‘Manual’ instead, the next two items can be used to set the maximum and target power manually. The following two items allow you to manually set a conservative current and voltage limit for I/V tests. When running I/V tests, the test is stopped if either of these limits is exceeded. The final two items set the scale of the graph. If, for example, the voltage scale is set to 10V, then the horizontal axis of the graph will span −10V to 10V. Any time the ‘Back’ button is pressed from the Settings page, the settings are saved to EEPROM. The program uses a clever update method so that EEPROM is not rewritten Practical Electronics | March | 2020 Fig.3: after running the touch calibration sketch and following the instructions, the highlighted text appears on the serial console. Fig.4: you then replace this portion of the main sketch with the text copied from Fig.3 above so that it uses the new touchscreen calibration. Fig.5: this is the section of the main sketch where you can change the calibration parameters, just below the comment reading ‘//Calibration constants’ unless necessary, so going into the Settings menu and then exiting without making any changes will not cause any wear on the EEPROM. In any case, the EEPROM is rated for at least one million rewrites per cell, so you would have to spend a very long time making changes before you’re likely to run into any problems with the EEPROM! Reproduced by arrangement with SILICON CHIP magazine 2020. www.siliconchip.com.au 21