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ADVANCED TEST SMD T EEZERS Part 2 by Tim Blythman This new design, introduced last month, adds many features to the SMD Test Tweezers concept. No longer only for testing passive components, the new Tweezers can also act as a voltmeter, logic probe, basic oscilloscope, square wave generator and serial protocol analyser. This final article has all the construction and usage details. T he Advanced Test Tweezers circuit is simple and the PCB compact. The new functions are provided by the substantially larger firmware hosted in a 16-bit PIC24 rather than an 8-bit PIC12 or PIC16. That’s due to the PIC’s hugely increased flash memory size, up from 7kiB to 256kiB, for only a couple of dollars more! This has allowed us to fit so many new modes, and enhance the existing ones, that a substantial part of this article will explain how to use them all. But before we get to that, we need to assemble the Tweezers. Gather the parts yourself and program a blank PIC using software downloaded from the Fenruary 2024 page of the PE website – see: https://bit.ly/pe-downloads – or you can buy a PIC already programmed when you buy the PCBs from the PE PCB Service. Construction Like the earlier Tweezers variants, we’re mainly using surface-mounting parts to keep it compact. The main change from the earlier versions is that the 28-pin micro has more closely-­ spaced pins than the 8-pin micros used before, but some passives are slightly smaller too. So you will need tweezers, flux paste, solder-wicking braid and a magnifier to complete this build. Use solder fume extraction or work outside if you don’t have one. Refer to Figs.5 and 6 (the PCB overlays) and photos as you go, which show where the components are mounted. The design uses an SSOP-28 package microcontroller and M2012/0805 passive components, so the pin 28 spacings are a bit tighter than the SOIC-8 and M3216/1206 parts that we used previously. Still, it’s eminently doable with patience and a fine-tipped soldering iron (or even a larger tip, if you know how to use it; and remember, flux paste is your friend). Start by assembling the main PCB and solder the microcontroller first. It’s easily the part with the finest pitch pins and is best dealt with if no other components get in the way. Apply flux to the pads on the PCB, then rest the IC in place, making sure pin 1 is aligned with the dot. Clean the tip of the iron and add some fresh solder, then carefully tack one pin and check with a magnifier that the pins are aligned on their pads and flat against the PCB. If necessary, adjust its position by remelting the solder and gently nudging it. Your life will be much easier if you get all the pins close to perfectly lined up with the pads now. Then, carefully solder each pin in turn, keeping the iron low on the pads, cleaning the tip and adding solder to it as necessary. You can apply more flux to the pins too. You can also drag-solder them if you know how. Check that the pins are soldered and that there are no bridges. If there are bridges, add more flux and use some solder wick to draw out the extra solder. Surface tension should leave a small but sufficient amount of solder attached to the pin and pad. If you haven’t previously done any work with parts this small, you might like to clean the excess flux away to make it easier to inspect your work as you go. Even if you’re experienced, it’s best to clean it up when you’re done and use a magnifier to verify that all the solder joints have adhered to the pins and pads, and that there are no hard-to-see bridges. The Advanced SMD Test Tweezers consists of the Main PCB (top and underside shown enlarged) and one of the Arm PCBs shown below (actual size). Practical Electronics | March | 2024 The remaining 14 passive components on the top of the PCB are all M2012/ 0805 size (2 × 1.2mm); none are polarised. The resistors should be marked with codes representing their values, but the capacitors will probably not be. If in doubt, the 10µF part is likely the thicker or larger capacitor. Apply flux to the pads for all the parts and solder the 10µF capacitor first. Like the IC, tack one lead, check that it is flat and aligned within its pads, then solder the other lead. Apply more flux and touch the iron to the first pad to refresh the joint. Use the same technique to solder the two 100nF capacitors, then the resistors, in the locations shown in Fig.5. There are only a few parts on the reverse of the PCB: two diodes and the cell holder, as shown in Fig.6. Solder in the two diodes now. Though small, the SOT-23 parts are pretty easy to work with and should only fit in the correct orientation. Then solder the cell holder. Make sure that the opening faces the edge of the PCB, as shown in Fig.6 and the photos. Use a generous amount of solder to ensure the connection is mechanically sound. It’s a good idea to clean any flux residue off the PCB now. Doing so at this stage means that the entire PCB can be immersed in a solvent before the switches are fitted, so it won’t get into their mechanisms. Your flux’s data sheet should recommend a solvent, but we find that isopropyl alcohol works well in most cases. Allow the PCB to dry thoroughly. The Advanced Tweezers can measure relatively high resistances, and traces of flux residue could affect the readings. Now is a good time to thoroughly inspect the soldering of the smaller surface-mounted parts, as it will be tricky to make any repairs once the OLED has been fitted. Look closely for solder bridges and check that IC1 is in the correct orientation. Solder the three tactile pushbuttons in place next. That should be easy, as they have relatively large pads. You can carefully wipe away any flux residue left behind with a cotton tip dipped in solvent. Pre-calibration The standard 1% resistors used give the Advanced Tweezers a useful degree of accuracy. Still, if you have access to an accurate multimeter, you can measure the exact value of the six ‘probing’ resistors to improve its accuracy. They are marked in red in Fig.5. These are the 1kW, 10kW and 100kW resistors along the side near the top of IC1. The four lower 1kW resistors also affect measurements in the Scope and Meter modes, but we’ve provided an automatic calibration for them that does not depend on their exact values. Measure and separately note the exact values of the six resistors. It’s much easier to do this now, before the OLED is fitted over the top. A menu will allow these values to be loaded into the Tweezers during the calibration stage. Programming IC1 If you don’t have a pre-programmed chip (we sell a programmed micro individually), you will need to program it using a programmer such as a PICkit 3, PICkit 4 or Snap. If you need to provide power to the chip (likely if you are using the Snap), you can temporarily insert a coin cell into the holder. The ICSP header, CON1, can be soldered in place for programming. However, we find it’s sufficient to insert a five-way header pin strip into the PCB pads, so you might like to try that. This way, the header does not get in the way when the arms are fitted. Gentle sideways pressure on the header during programming should keep the pins in contact with the plated holes. We recommend programming using the free MPLAB X IPE software. Select the correct part (PIC24FJ256GA702) and open the 0410622A.HEX file. Use the Program button to upload the HEX file to the device. The only indication that programming was successful will be a message like ‘Program/verify complete’ in the bottom window of the IPE. If you have fitted a cell, remove it now to complete the assembly. Fitting the arms and tips The arms must be fitted before the display to ensure that the OLED is spaced clear above the main PCB and clear of the arms. For the tips, we use the same arm design as the Updated Tweezers from May 2023, including the gold-plated header pins. Fig.7 shows this arrangement. The gold-plated header pins are easy to source, and as a bonus, they can also plug directly into prototyping gear like jumper wire sockets and breadboards. Fit the arms to the main PCB, then solder the tips, making it easier to align the tips to be the same length and parallel. Place the arms as seen in the photos. They connect to the CON+ and CON− pads and should have their copper tracks on the inside of the arms to reduce stray capacitance while they are held. They should only extend past the CON+ or CON− pads where they leave the PCB. This will keep the arms clear Figs.5 and 6: remember to measure the resistances of the resistors marked in red and thoroughly check the soldering for bridges before fitting the OLED. It will take a lot of work to get to the top of this PCB (shown at left) after the OLED is fitted. You can use the large pad at top right (light grey) to support the OLED module by soldering a short piece of stiff wire between the two. The cell holder and two dual diodes are on the reverse side of the PCB (shown at right). The diodes should only fit one way, but the cell holder can be reversed. Fit it in the orientation shown so a cell can be inserted from the side near the edge of the PCB. Both overlays are shown enlarged at 150% of actual size. Practical Electronics | March | 2024 29 Fig.7: this shows how the two arms attach to the main PCB. It is easier to solder and align the tips to the arms after the arms are fitted to the main PCB. The arms are shown parallel here, but it’s better to angle them as shown opposite. of other connections on the PCB, especially those for the OLED screen. Angle the arms slightly inward to achieve about 15mm of tip separation when at rest. This will allow the Advanced Tweezers to be used with axial leaded components too. You could set them closer if you only use them on surface-mounting parts. Use a small amount of solder to take the arms and adjust their positions as necessary. Then use a generous amount of solder on both sides of the arms and main PCB to ensure a good mechanical connection between them. Keep the pin headers side-by-side in their plastic holder until they are soldered, as this will keep them aligned. Use a generous amount of solder and ensure it flows into the holes on the arm PCB, giving more strength. Test the action of the arms and if necessary, use your iron to melt the solder and adjust them. OLED installation The final step is to fit the OLED module, MOD1. If the OLED does not have a header strip fitted, attach that first, ensuring that the pins are perpendicular to its PCB. The OLED needs to be fitted such that it cannot flex and touch any other part of the Tweezers, so space it about 1mm above the arms. You can use BluTack or similar to locate it squarely in place, and tack one lead to confirm. Check that there is clearance all around between the PCB and OLED. Then solder the remaining leads to their PCB pads. Initial testing At this stage, the Tweezers are complete enough to do a quick functional test. Insert the cell into the holder, and the OLED should light up in Meter mode, with a reading under 1V. Pressing S1 should cause the counter at bottom right to start flashing, and S2 will cause it to stop flashing. Pressing S3 will switch to the next mode (Scope). If something else happens, your Tweezers probably have a problem, so you should remove the cell and check the assembly. If the displayed voltage is wrong, check that the resistors all have the correct values and are in the right locations. Any of the switches not working could point to that switch not being soldered correctly. Any problem you spot might also be due to a soldering problem with IC1, particularly bridged pins or a solder joint that doesn’t contact both the pin and pad. If all is well, the assembly can be completed after removing the cell. The top-right mounting hole of the OLED is designed to be soldered to the main PCB using a header pin or similar. This will prevent the OLED from flexing at this end and coming into contact with the arms. You can now apply heatshrink tubing to the arms, taking care not to direct heat towards the OLED screen. Cover as much of the arms as possible from the main PCB to just before the tips. The back of the Tweezers is protected by a small sticker. You could design your own and print it out on a blank sticker or use the one shown in Fig.8 using artwork downloaded from the March 2024 page of the PE website: https://bit.ly/pe-downloads. Take care when operating the Advanced Test Tweezers The Advanced Tweezers make use of a coin cell. Even though we have added protections such as the locking screw, there is no reason for this device to be left anywhere that children could get hold of it. Also, the tips are pretty sharp and might cause injury if not used with care. Avoid applying voltages across the Tweezers test tips when it is actively driving them. While this obviously includes the Tone mode, remember that the pins are also driven in the I/V, Auto, Res, Cap and Diode modes. So be sure that the Tweezers are set to the Meter, Scope, UART or Logic mode before connecting to an external voltage source. If a glitch causes the Tweezers to reset, they restart in Meter mode to avoid further damage. 30 Removal of the coin cell is stopped by a Nylon screw and two nuts. Practical Electronics | March | 2024 The arrangement of the arms and tips is much the same as that for the Updated Tweezers, using the same arm PCBs (blue this time) and gold-plated pins as simple, practical tips. This photo shows operation in left-handed mode. If printing it yourself, it’s a good idea to laminate it. Cut along the border to make a shape to match the main PCB. Then use clear neutral-cure silicone or a similar adhesive to secure it to the back of the Tweezers. A small amount of glue on each of the arms and the back of the cell holder should be sufficient to hold it in place. Finally, fit the cell and secure it using the nylon screw and two nuts. Put the head of the screw at the front, on the same side as the switches, so the extra height of the thread at the back blocks the cell from being removed. Before using the Tweezers, we recommend performing some calibration steps, explained just below. We’ll also explain all the various modes and how to use them. In general, pressing S3 cycles between the various modes, and S1 and S2 have different functions depending on the mode. A long press (more than one second) of S3 changes between Settings and the normal operating modes. In Settings mode, pressing S3 cycles between the different settings, while S1 and S2 adjust the particular setting, as described on the screen. Calibration The calibration procedure has a few steps but is fairly logical. To enter the Settings mode, hold S3 for more than a second and release. #1 Handedness Screen 1: being configured for right- or left-handed operation doesn’t change the polarity of the CON+ or CON− connections, but the diode polarity icons will appear relative to the arms. The first page allows the display orientation to be set to suit either lefthanded or right-handed operation Practical Electronics | March | 2024 On all pages like this, S1 will increase the displayed value and S2 will decrease it. Brief presses will make single steps, but holding the button in will cause it to increment or decrement about ten times per second. #3 Internal reference voltage Fig.8: this sticker is for protecting the rear of the Advanced Tweezers PCB. just print the artwork, laminate it, cut it out and glue it to the back of the cell holder. You can of course design your own – see Screen 1. The setting is toggled by pressing either S1 and S2, and the change occurs immediately. All settings like this take effect immediately, so you can test them before being saved to non-volatile flash memory. There is also a Restore option to reload the initial defaults in case of a problem. Pressing S3 cycles to the next page. #2 Six resistor values Screen 2: while it will provide reasonably accurate readings without calibration, it is better to enter the exact values of the six most critical resistors (see Fig.5; as measured by a multimeter) on these screens. The following six pages set the values of the probing resistors you measured earlier, as shown in Screen 2. After the resistor value is an ‘L’ or ‘R’, indicating whether you are setting the value of the corresponding resistor on the left or right side of the main PCB. The values are adjusted in steps of 0.1%, ie, 1W for the 1kW resistors, 10W for the 10kW resistors and 100W for the 100kW resistors. Use S1 and S2 to adjust these values, and then press S3 to step to the next. Screen 3: diode and capacitor measurements will be most accurate if the internal bandgap reference is calibrated. Adjust it using S1 and S2 until the displayed cell voltage is correct. The BAT page (Screen 3) calibrates the internal reference, which is nominally 1200mV and is shown at the page’s bottom. The value on the second line is the calculated cell voltage based on the reference setting. Trimming this parameter is best done with a multimeter. Measure the actual cell voltage (which can be measured at pins 2 and 3 of the ICSP header) and adjust the displayed cell voltage until it matches. The voltage shown in Screen 3 is higher than might be expected from a coin cell, as we were using a 3.3V supply for testing. In this case, the reference voltage has been trimmed upwards by about 3%, from 1200mV to 1237mV. #4 Lead/tip resistance Screen 4: the lead resistance was close to 0Ω in our prototype, but this setting might be handy if you are working with breadboards and jumper wires with significant resistance. 31 The next page (Screen 4) sets the lead resistance, which defaults to 0W. Our prototypes had less than 1W of lead resistance and so were accurate enough; thus, you probably do not need to change this. You can test this by pressing the tips together on a mode that displays resistance. If you are connecting extra leads or jumper wires and breadboards, you can account for the higher resistance with this setting. #5 Auto calibration Screen 5: the AUTO SET tunes three calibration parameters by performing internal measurements with the tips open. It depends on the previous calibration settings being entered and correct. The next page (Screen 5) provides the option to AUTO SET several parameters, namely stray capacitance, Meter offset and CTMU trim. These require the tips to be left open and not connected to anything, and are only accurate when the previous settings (ie, the test resistances and internal reference voltage) have been calibrated. Hold the Tweezers as you usually would to take into account the stray capacitance of your hand. Then press S1 to start this process. It takes less than a second and you can review the values on the subsequent pages by pressing S3. #6 Stray capacitance Screen 6: stray capacitance can be tuned automatically or entered manually; it should be around 100pF. You can check it varies by setting it to 0pF and watching the value on the Cap screen. The stray capacitance of our prototype is around 100pF; check that you have a similar value, as seen in Screen 6. A vastly different value might indicate a problem; for example, you may have a resistor in the wrong location. 32 #7 Meter offset 1kW resistors and thus depends on its actual resistance too. So ensure these values are set before running the AUTO SET process. #9 OLED brightness Screen 7: Meter offset adjusts for any difference in the two 1kΩ/1kΩ dividers and is set by the AUTO SET page. The 16mV error is noise in the ADC measurement, being a single ADC step. The Meter offset adjusts the relative value of the four lower 1kW resistors; it is effectively the difference between the midpoints of the two voltage dividers shown in Fig.4 last month. The value at the bottom is the number of ADC steps used to adjust the reading. In Screen 7, you can see the actual Meter reading at top right. You can validate this by verifying that the reading hovers close to 0mV when the tips are open. The –16mV seen corresponds to a single ADC step, and thus the resolution in this mode. Screen 9: the OLED is one of the major drains on the coin cell, so a low brightness setting increases the cell life. We had no trouble using the Tweezers with the OLED set to quite a low brightness. On Screen 9, the display brightness can be set between 32 and 255, with 64 being the default. This setting is a compromise between display visibility and cell life. You should set this to the lowest level at which you can still read the screen clearly. #10 Screen blanking timeout #8 Current source trimming Screen 8: the CTMU’s current source is also trimmed by the AUTO SET page but has very coarse trimming, with 2% steps. You can observe this by manually adjusting the trim value on this page. The CTMU current source, used for capacitance measurements, can be trimmed via Screen 8. The lower value is the degree of trimming, with each step being a delta of about 2%. This is a hardware limitation and is a significant factor in limiting the accuracy of capacitance measurements. The value shown at upper right is the deviation of the measured current from its nominal value on the 550µA scale, while the lower number indicates the amount of trimming, with zero being the default. With a 2% deviation, the steps are around 11µA apart, so a setting within about 5µA of zero is optimal. Note that the Meter reading depends on the internal bandgap reference voltage being set correctly, as does the CTMU trim. The CTMU trimming procedure uses one of the Screen 10: with an option to disable the timeout in all modes, the timeout value is less critical than on the earlier Tweezers. The default is 30s, but it can be set from 3s to 99s to suit your needs. Screen 10 sets the display Timeout and is the countdown (in seconds) before the Tweezers enter their low-power sleep mode after the last button press. This value can be set between 3 and 99 seconds, with a default value of 30s. Note that the operating screens all have the option to freeze the timer so that the Tweezers can be used continuously when required. #11 Save settings to flash Screen 11: all calibration and operation parameters are live as soon as they are set. On this page, you can press S1, then S2 to save them to flash memory so you won’t have to repeat the calibration. Practical Electronics | March | 2024 Screen 11 gives the option to Save the calibration settings to flash memory. On this page, press and release S1 and then S2 to save the data. You should do this once the Tweezers are set up to your liking. #12 Restore settings from flash Screen 12: if the settings become corrupt, the Restore option will load defaults from a backup location. You can also load flash defaults by holding S3 while powering on the Tweezers. The Restore page (Screen 12) can be used to reload the default settings from a backup copy. These settings are put into use straight away. Although it would be very unusual, it’s possible for the saved settings in flash to be corrupted. This scenario might happen if, for example, power is lost while writing to flash. Such corruption can be detected by the micro and then trapped to avoid improper settings being used. If you get a ‘Flash Error’ message when powering up the Tweezers, then remove the cell and hold S3 in while reinserting it (giving a ‘No Flash’ message). This bypasses the loading of the settings from flash, after which you can use the Restore and Save pages to reload and rewrite the flash memory with uncorrupted data. You should then treat the Tweezers as if they have not been calibrated and repeat the calibration procedure. press on S3 at any time will also exit Settings mode. Operation During operation, the bottom line in all modes shows data that always has the same format. From left to right, it shows the current mode, the cell voltage and a countdown timer. If the timer is flashing, it has been paused and does not count down, allowing continuous operation. When the timer counts down to zero, the Tweezers will enter the low-power sleep mode with a blank display. Pressing any of S1, S2 or S3 will reset the timer and resume normal operation. #1 Meter mode Screen 14: the initial Meter display mode, which can read up to 30V with both negative and positive polarities (with respect to CON+ and CON−). The resolution is 10mV to 9.99V and 0.1V above that. The Tweezers start on the Meter screen, which displays the measured voltage between the probe tips. Screen 14 shows the Tweezers in Meter mode, connected to a fresh 9V battery. Pressing S1 in this mode will pause the sleep counter and pressing S2 will resume it. As is typical, any button press will also reset the sleep counter. Pressing S3 cycles to the next mode. the selected parameter by cycling between the available options. You can see which parameter is selected as it will be flashing. These include the vertical axis maximum (voltage), trigger mode (RISE, FALL, BOTH or AUTO), trigger level in volts, timebase per division and whether the vertical axis minimum is 0V or the negative of the maximum. Pressing S1 also cycles through the countdown timer. Note that while it is selected, the countdown timer is paused. The trigger point is fixed at the first time division, which is marked by a more solid vertical graticule. Thus, one time division is displayed before the trigger point and three after. A tiny arrowhead also marks the trigger voltage level to the left of the grid area. Due to the slow update speed of the OLED display, the trace is not displayed live. Instead, a sample set is taken, spanning around two full screen widths. It is checked for trigger conditions and an appropriate portion is displayed. If no trigger is found (or AUTO trigger mode is selected), the first screenful of samples taken is displayed, along with a ‘WAIT’ message. If a trigger is found, then the trigger point is aligned with the graticule and ‘TRIG’ is displayed. Since a complete sample set at some of the longer time divisions can take several seconds, it can be a while before data is displayed. #3 UART serial decoding #2 Scope mode #13 Exit settings Screen 13: as well as using this screen, you can also leave the Settings pages at any time by pressing and holding S3 for more than a second. A brief press of S3 will take you back to the first Setting. Last, Screen 13 shows the final Exit page that allows you to press either S1 or S2 to return to the operating mode, while S3 will return to the first Settings page. Note that a long Practical Electronics | March | 2024 Screen 15: Scope mode is handy, even though there are only 100 horizontal and 48 vertical pixels in the trace area. It samples at up to 25kHz, is suitable for audio use, and has adjustable trigger settings. The Scope mode is shown in Screen 15, with a nominally 100Hz 6V peakto-peak waveform. this was fed to the Tweezers using a second set of Advanced Tweezers which were set to the Tone mode. This has various parameters to set; pressing S1 cycles between the parameters, while S2 adjusts Screen 16: we find the UART Serial Decoder indispensable at times. Like the Scope mode, it is highly configurable in terms of baud rates, bit depth and data polarity. This shows the TXT view. The next mode is the Serial Decoder, labelled ‘UART’ (see Screen 16). The bottom text shows the current settings, which are similar to those in Scope mode. S1 cycles between the parameters (including the sleep timer) while S2 adjusts the selected, flashing parameter. The first setting is the baud rate, which includes standard rates from 110 to 115,200 baud. The second setting is the format, which can be eight bits with odd, even or no parity, or 33 Reproduced by arrangement with SILICON CHIP magazine 2024. www.siliconchip.com.au The underside of the Advanced SMD Test Tweezers (shown larger than actual size) is mostly empty, with only the battery holder and two diodes present. #4 I/V plotter Screen 17: the Serial Decoder also offers a hexadecimal mode, useful for seeing binary data and control codes. Framing or parity errors are shown, which can help to determine the data format. nine bits with no parity. These are shown as 8O, 8E, 8N or 9N and are followed by a choice of one or two stop bits. The idle logic level is next and can be HI or LO, followed by a choice of text or hexadecimal (‘TXT’ or ‘HEX’) display output. Screen 16 shows TXT mode, which works much like a serial terminal and will handle line feed, carriage return and tab characters. The text will scroll up as lines are filled at the bottom of the screen. The text seen here is actually a decoded square wave; hence, the same character is repeated. HEX mode does not handle any control characters but displays both ASCII and HEX representations, also scrolling up as needed. Only HEX mode can display the full range of 9-bit data, and it also indicates parity (‘P’) and framing (‘F’) errors. Screen 17 shows the same data as Screen 16 but in HEX mode. The decoding depends on the PIC24FJ256GA702’s hardware UART and logic levels, but since the I/O pins are behind the protective resistors, this will work fine with any logic levels of around 3V or higher. Even non-TTL voltage levels, such as legacy RS-232 (which can swing between –15V to +15V) should be successfully decoded by choosing a LO idle level, since –15V is the idle level. 34 #5 Logic Analyser Screen 18: while Diode mode cannot report dual diodes such as bicolour LEDs, the I/V Plotter shows both polarities. The current and voltage scales can be zoomed in for more detail. Screen 19: the Logic Analyser shows whether it detects a high, low or high impedance logic level. A scrolling chart also shows a brief history, making it easier to see transients and repeating patterns. Screen 18 shows the I/V (current vs voltage) plotter, designed to characterise passive components. This uses much the same scheme as Meter mode, applying a voltage via different resistor combinations to probe the component at different operating points. Six readings are taken, including the voltage and current at each point. This is limited to about ±3V due to the cell supplying the test current; the current can be no more than around 1.5mA due to the minimum 2kW resistance. Like in Scope mode, the vertical and horizontal scales can be adjusted by using S1 to cycle between current (vertical), voltage (horizontal) and the timeout counter. S2 cycles between the available values. The horizontal scale can be set to 1V, 0.5V, 0.25V, 0.1V or 0.05V per division, while the vertical scale can be 1mA, 500µA, 200µA, 100µA or 50µA per division. The corresponding scale values are displayed in mV and µA, respectively. The 0V/0A origin is always at the centre of the display, and the I/V display updates continuously, so it is well-suited to sorting through piles of unmarked parts. Screen 18 displays what what the Advanced Tweezers indicate for a yellow LED with a forward voltage of around 1.7V. Pressing S3 again switches to the Logic Analyser, as shown in Screen 19. Sensing is done by alternately probing with high and low voltage levels via one of the 100kW resistors. A voltage that follows the probing voltage is assumed to be high impedance. It shows 1, 0 or Z at the left of the screen to indicate a logical high, low or high-impedance level. A horizontal scrolling display also shows about one second’s worth of history to allow brief transients or waveforms to be discerned. Here, we see a high-level signal that is interrupted by brief low pulses. Like in the Scope and Meter modes, S1 and S2 will pause and resume the countdown timer, respectively. #6 Tone Generator Screen 20: like Scope mode, the Tone Generator is handy at audio frequencies or as a simple clock generator. It can produce square waves at five different frequencies and four different amplitudes. Screen 20 shows the Tone Generator. Unlike most of the other mode settings, Practical Electronics | March | 2024 which are retained between uses, the tone is turned off when it is not being used to avoid interfering with other modes. It can be toggled on and off by pressing S2 when the ON/OFF indicator is flashing. There are choices of 50Hz, 60Hz, 100Hz, 440Hz and 1kHz. Only square waves are produced. There are four output (peak-to-peak) levels, which are nominally 300mV, 600mV, 3V and 6V. The 300mV waveform is produced by toggling one output via a 10kW resistor and dividing that with a 1kW resistor to ground. The 600mV selection drives two outputs similarly, but with opposing phases, to achieve the necessary swing. The 3V and 6V outputs are fed to the tips directly from one or two pins respectively, without the divider. The level selections assume that the supply is at 3V and the load resistance is relatively high. Under other conditions, the voltages could be different. Because of the way they are generated, the 300mV and 3V outputs also have a DC offset that the other two modes do not. So, you can use the 3V mode to drive a clock signal into 3.3V logic (or 5V logic, if it accepts a 3V signal swing), or you can use the 300mV and 3V modes to feed in an audio signal via a series capacitor (in the circuit, or added), which will remove the DC offset. #7 Component measurements Screen 21: the Auto screen is only one of ten pages but encompasses and surpasses the abilities of its predecessors. It shows resistance, capacitance, diode polarity and forward voltage. Finally, we come to the modes that can be used directly to read off the values of passive components. These are similar to the older Tweezers variants but have wider measurement ranges. The Auto mode performs readings for resistors, capacitors and diodes, and displays the readings for all three. You might get readings for more than one component type, as there is no algorithm that will always correctly determine what has been connected. Screen 21 shows Auto mode with no components connected. A high resistance and low capacitance are displayed. In Auto mode, pressing S1 Practical Electronics | March | 2024 As shown last month, a header pin is used to act as a reinforcing spacer at one corner of the OLED. This prevents the assembly flexing and causing a short between the two PCBs. will pause the countdown timer while S2 will resume it. The subsequent Res, Cap and Diode modes concentrate on just the one component type and display it in a larger font. These are seen on Screens 22-24, respectively. The maximum resistance that can be displayed depends mostly on leakage currents in the circuit. However, above 40MW, it will not achieve the stated 1% accuracy due to there being insufficient resolution at this end of the scale. We have specified much the same range for capacitor testing as the Improved Test Tweezers. Above these ranges, leakage and other factors make it difficult to achieve the stated accuracy, especially for electrolytic capacitors. The Advanced Tweezers will report up to 2000µF, but you should not rely on readings above 150µF. Since this is well above the typical range for the MLCC (multi-layer ceramic capacitor) types that we typically use for SMD designs, we don’t expect this will be much of a concern. Remember, many capacitors are manufactured to tolerances as wide as ±20% (sometimes even +80, –20%). The diode test current is higher than the earlier Tweezers due to the 1kW test resistors. In the standalone diode mode, the forward test current (CON+ positive and CON− negative) is supplied between samples, so LEDs should be seen to light up when connected in the forward direction. Conclusion The original SMD Test Tweezers and the subsequent Updated SMD Test Tweezers are compact and handy devices. By adding a more powerful and better-provisioned microcontroller, we have added numerous extra features in creating the Advanced SMD Test Tweezers, including improved passive component measurement and many new modes. The PIC24FJ256GA702 is a substantial upgrade over the tiny 8-bit, 8-pin parts we previously used; we are not even using half of its resources or features in this design. These new Test Tweezers can replace a basic voltmeter, logic probe and even oscilloscope in some situations, making them an indispensible general-purpose test instrument. We expect that the Advanced SMD Test Tweezers will be both popular and useful, not just for the numerous test and measure modes, but also as a tool during SMD assembly. Screen 22: the Res screen provides the same resistance information as the Auto screen but in a larger font, which is handy for checking and sorting through different resistor values. Screen 23: the Cap screen works similarly, displaying just the measured capacitance in large text. It’s perfect for working out which part is which among a pile of unmarked SMD capacitors. Screen 24: the Diode screen is similar to the diode display on the Auto screen but a bias is applied from CON+ to CON− between tests. This lets you quickly check the polarity and operation of LEDs. 35