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SMD Test Tweezers By Tim Blythman This clever little device is made from just 11 components. Yet it can measure the values of many SMD resistors and capacitors, plus show diode and LED orientations and measure their forward voltages. It’s quick and easy to use, and is powered by an onboard button cell, with a high-contrast OLED screen to show the readings. W orking with SMD parts can be tricky. Reading component markings can be a strain on the eyes, if the component is even marked! Devices like SMD capacitors are totally anonymous and, once removed from their packaging, almost impossible to tell apart. These SMD Test Tweezers make it easier by telling you all about a component by simply picking it up. In some cases, these SMD Test Tweezers can also measure the properties of a component once it has been soldered to a board (although, depending on the circuit configuration, sometimes the readings will not be accurate). As time passes, fewer electronic parts are available in through-hole variants and increasingly manufacturers are building products mostly or entirely from SMDs. They are smaller and cheaper than through-hole parts, can be mounted on both sides of a board (often with internal traces ea es a ecifica i running underneath) and are also less sensitive to shock and vibration. Of course, while parts being smaller can be advantageous, it also presents problems when working with them. Certain tools, such as tweezers and a magnifier, are indispensable. Once you’ve had a chance to try out our SMD Test Tweezers, we think you will be adding them to your bag of SMD tricks! The tweezers SMD parts are very awkward to read with a multimeter. On many occasions, we’ve been pressing multimeter probes on to the ends of an SMD part, trying to get a reading, only for it to fly off and never be found again. Tweezers provide a much more natural way to do this, and as you don’t need to apply much pressure, there is less chance of the part taking flight. Even better, since tweezers are a convenient way to pick up and handle s dentifies and meas res resistors, capacitors, diodes and s ompact disp a reado t ns rom a sing e ithi m coin ce , aro nd fi e ears o stand ie A to power on and o isp a s own ce o tage when no component is connected an meas re components in circ it nder some circ mstances an per orm tho sands o meas rements e ore the ce is e ha sted esistance meas rements 10W to 1 W iode meas rements po arit and orward o tage, p to a o t V apacitance meas rements 1n to 10 16 such parts, if we incorporate the measuring tool into the tweezers, it can tell you what part you are handling while you are in the process of placing it on the board. The SMD Test Tweezers measure whatever component is present between its tips, so there are no extra fiddly movements to make. You pick up the part, and the screen displays its assessment. The Tweezers automatically detect the difference between resistors, capacitors and diodes, including many LEDs. With a maximum applied current of 0.3mA at 3V, there’s virtually no chance of causing damage. The Tweezers can measure resistances from around 10W to 1MW and capacitances from 1nF to 10μF. These ranges are slightly limited, but increasing them would significantly complicate the design, and a large percentage of SMD components fall within those ranges. The Tweezers also check diode polarity and forward voltage. If an LED is picked up, it will also be illuminated dimly so that you can check the colour. The forward voltage measurement is limited by the 3V available from the small coin cell that powers it. We’ve got no doubt that this tool will find much use in the hands of even our most SMD-savvy readers. Design We set out to make this tool compact, so it uses a tiny 0.49-inch (12.5mm) diagonal OLED screen. This is the same module we used in the Shirt Pocket Practical Electronics | October | 2022 Audio DDS Oscillator in the September 2021 issue. We’re also using a small 8-pin microcontroller, a PIC12F1572 in the SOIC package. It is a compact and capable part that puts some older 8-pin PICs to shame. And it’s cheap too. The design uses one small PCB to house the main operating parts, including the microcontroller, while another pair of PCBs form the arms. We added some custom brass tips to our prototype, but this is not absolutely necessary. Another option is to purchase premade tweezer test leads that can be combined with the main PCB to give a similar result. Circuit details The complete circuit for the Tweezers is shown in Fig.1, and it is extraordinarily simple. The test functions are provided by a 10kW resistor connected between pins 2 and 5 of IC1. Pin 5 also connects to one of the Tweezers arms and thus to the device under test (DUT). The other Tweezers arm connects to IC1’s pin 3. All the tests are done by placing different voltages on pins 2 and 3, then using the micro’s internal ADC (analogue-to-digital converter) to measure the voltage on pin 5 relative to the cell voltage. The cell voltage is also measured by using it as a reference to measure the micro’s internal 1.024V reference. CON2 is a 4-pin header that connects to the OLED module. This uses an I2C serial interface which is provided by pins 6 and 7 of IC1. The I2C pull-up resistors are fitted to the OLED module, so they are not needed in our circuit. The PIC12F1572 does not have a hardware I2C peripheral, so these pins are driven ‘manually’ by the software. We’ve chosen pins 6 and 7 so that if IC1 needs to be programmed, it can be done before the OLED module is fitted, which would otherwise interfere with the programming signals. Microcontroller IC1 is powered by coin cell BAT1, which is bypassed by a 100nF capacitor. IC1’s MCLR pin is pulled up to its supply voltage by a 10kW resistor so that it operates normally as long as power is applied. CON1 is an in-circuit serial programming (ICSP) header, with its pins connecting to IC1’s pins 4, 1, 8, 7 and 6 respectively. You can use it to program IC1 in-circuit if needed. That is not necessary if you purchase a pre-programmed PIC chip. Component sensing The IOTOP and IOBOT designations on the schematic denote the normal Practical Electronics | October | 2022 SMD Test Tweezers Fig.1: the Tweezers circuit is remarkably simple; it uses just one resistor and three microcontroller pins to perform all its tests. An I2C OLED display keeps the pin count within the limits of the tiny 8-pin microcontroller. Once the OLED screen is fitted, it will be tricky to access these parts, so check that everything is as it should be before proceeding further. With the four components fitted to the PCB, it should look something like this. IO states of these pins. When idle, pin 2 is pulled high and pin 3 is pulled low. This matches the designations of CON+ and CON−. On each measurement cycle, IC1 measures its internal 1.024V reference relative to its supply rails, and calculates the cell voltage based on this. This might be used later to calculate diode forward voltages; if no component is detected, the cell voltage is displayed. The next test is to see if a capacitor is present. Pin 2 is taken low, and a series of samples are taken of the voltage at pin 5, until pin 5 is below half the cell voltage, or 255 samples have been taken. If IC1 doesn’t see the voltage fall like a capacitor discharging, it reports that it does not identify a capacitor. This can also happen if the capacitance is too low (which causes the voltage to drop faster than IC1 can make its measurements) or too high (which causes the voltage to not change enough over the sample period). The capacitance is calculated based on the voltage drop and the time taken, although an approximation is used to avoid the computationally-expensive log function; our code comes within a handful of bytes of filling the available program space. The accuracy of the approximation is only significant at values near the upper measurement limit. Given that many capacitors are only specified to within 20%, this is sufficient for most purposes and will be adequate to tell components apart unless they are very close in value. The capacitance test is done first, as it means that the time since the last sample can be used to ensure that the capacitor is as close to fully charged as possible. Note that you should not connect a charged capacitor to the Tweezers (or any similar meter). If it is charged to more than a few volts when it is connected, or the polarity is reversed, it could easily damage the microcontroller 17 IC1. Even if it doesn’t, it will probably not be measured correctly. If a capacitor is not detected, then the idle state is restored for 200μs (to allow the voltage to settle). The micro then takes a measurement of its pin 5 voltage, flips the polarity for another 200μs, takes another measurement and then flips the polarity back. The algorithm averages 16 samples at each polarity to improve accuracy. Every second raw ADC measurement is adjusted to account for the fact that it was taken with reversed polarity. If the two voltage measurements are close, then the part is assumed to be a resistor and the value is reported according to the voltage divider formula (see Fig.2). If one value is close to full rail and one value is not, then the part is probably a diode of some sort, and the forward voltage and direction are reported. This can include LEDs, silicon and schottky diodes. The LED portion of phototransistors and opto-isolators should also show a diode reading. Bi-colour LEDs and other diode networks may not be detected, as they will conduct and not appear open-circuit in the reverse direction. If you’re smart, you can probably identify bipolar transistors by connecting the tweezers across their suspected base and emitter pins and identifying the junction polarity; it should be detected like a diode. LEDs connected with their anodes to CON+ and cathodes to CON− will be forward-biased by the idle current and supplied with a few hundred microamps of current, which should be enough to light them dimly and indicate that they are working. The Test current is quite low due to the 10kW resistor, no more than around 300μA. Thus, the forward voltage indicated may be a bit lower than what you might expect (eg, by reading the data sheet). For example, silicon diodes measure about 0.5-0.6V. Once determined, the part type and value (or cell voltage) is displayed simply as a number with the appropriate units and multiplier; to differentiate the cell voltage from the diode voltage, a diode symbol is shown with polarity matching the part in relation to the Tweezer probes. After five seconds of no part being detected, the OLED is put into a low-power mode, pin 5 is enabled as an interrupt source, and the Fig.2: this shows the various ways that the Tweezers measure component values. Resistance is measured using the well-known resistance divider formula, while the diode test measures the voltage across the device in both directions. Capacitance measurement is based on the change in voltage over a time interval when discharged via the known resistance. There’s not much to see on the back of the Tweezers, but note that one arm, the OLED header (CON2) and the cell holder (BAT1) are all quite close together. Double-check for short circuits before fitting the coin cell. 18 microcontroller goes into sleep mode. You can wake up the micro by simply touching the tweezer probes together, which changes the pin state. So you can see how such a simple circuit can perform various tests to detect and measure a range of components. Fig.2 shows how these algorithms work in a bit more detail. When the OLED is active, current consumption is around 4mA. This drops to 5μA when the microcontroller is sleeping, and the OLED is shut down. Thus, the cell life will depend mainly on the time the Tweezers are actually used. A typical CR2032 coin cell has a capacity of 220mAh, giving a standby life of around five years, which is good considering a coin cell has a typical ‘shelf life’ of 10 years. Construction If you haven’t already jumped into working with SMD parts, you’re going to start now because we’ve designed the SMD Test Tweezers with SMD components. Use the top and bottom PCB overlay diagrams shown in Fig.3 as a guide during construction. The main part of the SMD Tweezers is built on a PCB coded 04106211, which measures 28 x 26mm and is available from the PE PCB Service. We recommend using solder flux (ideally paste, although a liquid flux pen is better than nothing), a finetipped adjustable iron, solder wicking braid and a magnifier. We also suggest using a pair of tweezers. Since flux can generate smoke when heated, you should work somewhere with good ventilation. Also, check if your flux has a recommended cleaning solution; in a pinch, isopropyl alcohol is a good all-round substitute, with methylated spirits usually doing an acceptable job. Start by securing the PCB to your work surface with the component side facing up. If you don’t have a PCB vice or holder, use some Blu-Tack to stick it to your desk. Apply flux to the pads for the SMD components, then hold IC1 in place. If all the leads are inside their pads, then that is fine. IC1 should have a small dot marking pin 1; ensure that this is at the end closest to the 100nF capacitor as marked on the PCB. Clean the tip of your iron and apply a small amount of fresh solder. Then touch the iron to one corner pin of IC1. This should cause the solder to flow onto the lead. If the part looks to be flat against the PCB and still within all the pads, then solder the remaining leads by touching the iron to them. You can add more solder to the iron if needed, and more flux can help too. Practical Electronics | October | 2022 The only problems with using too much flux are that it will generate more smoke and take a bit longer to clean up. Otherwise, more is generally better. If you find that you have bridged any pins, then it’s easiest to solder the remaining pins before fixing this, as it will help keep the IC in the correct place. Then apply more flux, press the braid against the bridged pins with your soldering iron, and gently slide the braid away once it draws up the excess solder. Inspect the pins with a magnifier before proceeding, and repeat any of the above steps if necessary. You might need to clean up any residual flux if it impedes your view between the pins. The remaining parts can be soldered similarly, with the difference being that none are polarised, and they all have much larger leads and pads. Place the sole capacitor next; it will probably be the only part without markings. Solder one lead, check for correct positioning within the pads and against the PCB, then solder the other lead. Retouch the first lead if necessary. Then fit the resistors; they are both the same value. They aren’t polarised, but it’s good practice to orient the markings to match the text on the PCB to help with troubleshooting. Flip the PCB over to mount the cell holder. A similar soldering technique will work for the cell holder, with the difference being that it is a bit larger, so it will need more heat. Turn your iron up if it is adjustable. Place the cell holder, ensuring that the opening faces towards the curved end of the PCB. If it looks like you might not be able to get the cell in or out, then it is probably the wrong way around. Apply some flux and tack one lead. Check that all is aligned correctly, then solder the other. You can then retouch the first pin if needed. Reproduced by arrangement with SILICON CHIP magazine 2022. www.siliconchip.com.au We’ve left our Tweezers bare to show the construction details, but you might like to cover the main PCB with a short piece of wide heatshrink. This will also serve to hold the coin cell in place. That completes the surface-mounted parts, and this is a good point at which to clean off the residual flux. Because many flux cleaners are flammable solvents, you should allow the PCB to dry thoroughly after this step. If you have a blank microcontroller, now is a good time to program it. Do it before installing the OLED module, as this can interfere with programming when plugged in. Programming IC1 You’ll need a PICkit 3 or PICkit 4 programmer to program this chip, plus the MPLAB X IPE (integrated programming environment) software, a free download from the Microchip website (usually bundled with the MPLAB X IDE). You can also use a Snap programmer if you modify it according to the instructions on p.31 of our June 2022 issue (see the PIC Programming Helper project). This is necessary as the Snap programmer cannot supply power otherwise (or you could figure out another way to temporarily apply power to the micro during programming). While it is possible to solder a programming header to the Tweezers’ PCB, since it will only be used once and would get in the way after that, we prefer to use gentle force to hold the header in place against the pads during programming. Select the PIC12F1572 as the target part in the IPE, then open the 0410621A.HEX file. After that, simply press the Program button to start the process (start to apply pressure to hold the header pins to the PCB just before you do that). If you get the ‘Programming/Verify complete’ message, then programming has completed successfully. Otherwise, try again. Detach the programmer before moving on to the next step. Completion If you want to add metal tips to the arms of your Tweezers (PCBs coded 04106212, measuring 100 x 8mm and available from the PE PCB Service), it is easier to do so before fitting them. Cut brass strip roughly to size – only trim to matching lengths once the Tweezers have been assembled. Fig.3: despite only a handful of components being present, we have used both sides of the PCB. One advantage of SMD components over through-hole parts is that it’s much easier to have parts on both sides without concern over where the leads go. Keep an eye on IC1’s orientation; once it’s fitted, the rest of the assembly is quite straightforward. Practical Electronics | October | 2022 19 Fig.4: there are no components mounted on the arm PCBs; they are basically just flexible conductors that are soldered to the main PCB and clamp the DUT at the other ends. Solder one strip to the end of each arm, letting each overhang by around 5-10mm. Keep in mind that the bars should be on the inside of the arms when assembly is complete (see our photos for details). The surface-mounting copper pads are essentially glued to the PCB, so it doesn’t take much to tear them off. So, try to get some solder into the holes in the PCB, as this will add mechanical strength. If you don’t have brass strip, it will pay to add some small blobs of solder to the Tweezers’ tips. This will provide a larger contact area and also some resistance against the tips wearing down. Place the arms onto the Tweezers PCB at the CON+ and CON− pads and roughly align their positions. Ther ends should be separated about 10-15mm with no pressure applied; this gives a reasonable working force and range. This gap also means that the Tweezers can be used to test throughhole parts like axial-leaded resistors, diodes and capacitors. We found that fitting the arms flush with the edge of the PCB made the soldering easier and kept the CON+ arm clear of the CON2 OLED connection. It also looks tidier; see our photos. Once you’re happy with their positions, apply a generous amount of solder to both sides of the joins to secure them in place. Try out the action, tension and alignment of the arms and adjust if necessary. You can also trim and dress the tips if fitted. Squeezing the arms together and drawing a fine file over the tips will align them if they are slightly different lengths. To make the tips of the arms parallel, place fine sandpaper or a flat file between the tips and work them until the tips are satisfactory. This will also help add some texture to the tips to help them grip components and avoid the possibility of them flying off into the yonder! The OLED screen The OLED module is the last piece to fit. The header supplied with the module has a spacer of just about the right depth to mount the OLED parallel to the main PCB, although the pins probably need trimming. Start by soldering the pin header to the PCB at CON2, preferably with the longer pins facing up. This will make them easier to trim later. Check that there are no bridges between the pins of CON2, the CON− arm and the cell holder. Tack one lead of the OLED to the top of the header and check that it looks right and is not touching anything underneath; adjust it if necessary. Solder the remaining pins and then trim the excess pin length from the top, taking care not to damage the OLED screen. Then remove the protective film on the display. Using it Insert the lithium cell with the negative terminal against the PCB. The OLED should spring to life and show a reading just over 3V for a fresh cell. Squeezing the arms together should show a resistance of a few ohms. If you have no display at all, check the OLED connections. If there is no resistance measurement, you might have a problem with your test circuitry; check the resistors, IC1 and the Tweezer arms. After the Tweezers go into sleep mode, they use low-power digital sensing to wake up. Thus, they might wake up if connected to some but not all parts. Reverse-connected diodes and high-value resistors may not wake the Tweezers, but nearly all capacitors (when discharged) appear to do so. In that case, simply short the Tweezer tips together, then probe the component. Once a part has been detected, the Tweezers will stay awake until no part has been detected for five seconds. Caution Like any project that uses coin cells, the Tweezers should be kept well away from children who may ingest them. The Tweezers also have quite pointy tips, another reason to keep them out of reach of curious young fingers. You can apply a piece of wide, clear heatshrink tubing to the main PCB body to insulate and protect it. This can also be used to secure the coin cell in place; it should not be due for replacement too often, and the heatshrink can be cut and replaced at such times. You might also like to fit some thinner heatshrink to the arms. This will provide more insulation and also add a softer gripping surface to the Tweezers. Parts List – SMD Test Tweezers 1 double-sided PCB coded 04106211, 28 x 26mm (main PCB) available from the PE PCB Service 2 double-sided PCBs coded 04106212, 100 x 8mm (Tweezer arms) available from the PE PCB Service 1 PIC12F1572-I/SN or PIC12F1572-E/SN 8-bit microcontroller programmed with 0410621A.HEX, SOIC-8 (IC1) – available from the PE PCB Service 1 0.49-inch 64x32 I2C OLED module [eBay, AliExpress – for example, at the time of writing eBay item 273942316375] 1 surface-mount coin cell holder (BAT1) [Digi-key BAT-HLD-001-ND, Mouser 712-BAT-HLD-001 or similar] 1 CR2032 or CR2025 lithium button cell 1 5-pin right-angle male pin header (CON1; optional, needed for programming IC1 only) 1 100nF SMD 50V X7R ceramic capacitor, 3216/M1206 size [Altronics R9935] 2 10kW 1% SMD resistor, 3216/M1206 size [Altronics R8188] 2 15 x 2mm short pieces of thin (eg, 1mm) brass sheet for Tweezer tips (optional) 1 40mm length of 30mm diameter clear heatshrink tubing (optional; see text) 2 100mm lengths of 10mm diameter heatshrink tubing (optional; see text) 20 You can get pre-made tweezers with leads designed to be connected to other pieces of equipment like a multimeter. If you prefer these, you can cut off the banana plugs and solder them to our main board instead of our PCB-based arms. If doing this, ensure that the positive lead goes to the CON+ pad on the PCB and CON− to the black lead. Practical Electronics | October | 2022