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The Altronics Arduino LC Meter Shield Kit Altronics have just released a complete shield kit based on Jim Rowe’s Arduino LC Meter from the June 2017 issue (siliconchip. com.au/Article/10676). It includes all the parts needed to build it on a custom shield for Arduino, which makes building it that much easier. It even has the ability to auto-calibrate and detect if you have an inductor or capacitor connected. T his Altronics kit (Cat K9705) comes with everything you need to build a standard-sized Arduino shield (70 x 54mm) which incorporates all the functions of the Arduino LC Meter. The kit is sold for $26.95 and the only parts that aren’t included are the Arduino itself and an enclosure to put it in. The new feature of this kit, mentioned in the introduction, is automatic detection of the type of component being tested. Jim’s design for the LC Meter included a toggle switch to select between inductance and capacitance measurement modes. The Altronics shield uses a relay instead, under control of the Arduino, and it automatically detects when it needs to switch modes to suit the component you have connected across the test terminals. To make construction easier and the final result a bit more streamlined, the Altronics shield also uses a different approach to calibration. Rather than providing a switch and link to make fine tuning adjustments, you can do By Bao Smith this over the USB serial console, if necessary. Or you can skip that step and just use it with the default calibration which is normally pretty accurate. You will want to put it in some kind of enclosure to make it handy to use (as well as making it look more professional). You could build it into a jiffy box like Jim did in the June issue. Or you could put it into the spiffy instrument case that’s supplied with the Altronics Mega Box kit that was described last month, with pre-cut holes for the LCD, USB/power supply and test terminals. Circuit changes Shown above are all the parts that come with the LC Meter Shield. The resistor values are not marked on the PCB, so refer to the overlay diagram (Fig.2) for clarification. Newer versions of the board will have the resistor values printed. 44 Silicon Chip Celebrating 30 Years The shield circuit diagram is shown in Fig.1. This also shows how it interfaces with the Arduino. If you compare this to our original circuit on page 30 of the June 2017 issue, you will see that there are two main differences. Firstly, this shield does not include momentary toggle switch S3 or calibration link LK1 from the original design. As mentioned above, calibration is performed via the serial interface from a PC instead, saving on the cost and the space required for those components. The other difference is that DPDT toggle switch S1, which was used to switch between inductor and capacitor mode, has been replaced by DPDT siliconchip.com.au Fig.1: complete circuit diagram for Altronics’ LC Meter Shield. The LCD module is hooked directly to the shield, compared to using the I2C serial module shown in the original June 2017 article. relay RLY2, as mentioned earlier. RLY2 is driven by NPN transistor Q1 and has its coil back-EMF quenched at switchoff by diode D2. Because the switch is now activated by the Arduino, there’s no need for the Arduino to sense the position of this switch. In fact, input pin D2, which was used previously to sense the position of the switch, is now an output which drives transistor Q1 to energise the relay when measuring inductance. The basic operation of the circuit is still the same; the resonant LC network formed by L1 and C1 is driven by an inverter built around high-speed comparator IC1 and oscillates at a frequency dependent on the values of those components. The DUT is connected either in parallel with C1 (if it’s a capacitor) or in series with L1 (if it’s an inductor) and the shift in oscillator frequency is used to calculate and display the component value. The final difference you will notice is that the Altronics design does not require the alphanumeric LCD to have an I2C interface module attached. It instead drives the LCD module using the standard old 4-bit parallel interface. Again, this saves you a little monsiliconchip.com.au ey and time and it’s possible because the Arduino has plenty of free pins to drive the display in parallel mode. It does require a few more wires to be run but it isn’t hard, as you will see. Only a small change to the program was necessary to allow this and you could change it back if you really wanted to use a serial LCD instead. Construction The biggest advantage of using the Altronics shield kit, besides not having to collect all the parts yourself, is that you don’t have to do as much wiring since the PCB connects up all the components for you. You just need to solder the supplied components onto the PCB, plug it into your Arduino, wire up the LCD, program it and away you go. While all the supplied components are through-hole, a fine tip solder iron will help as some of the pins are a bit close together. Use the overlay diagram, Fig.2, as a guide to mounting the components. Start by fitting the low-profile components first (ie, the resistors and diodes). Be careful with the orientation of the diodes; they face in opposite directions, so pay attention to Fig.2 and the PCB silkscreen. Celebrating 30 Years We also recommend that you check the resistor values with a multimeter before fitting each one. Solder the two 1nF MKT capacitors (C1 & C2) next. We found they were a little too wide to fit flush to the board but you can bend the pins slightly to help them fit. We have been told by Altronics that the next batch of PCBs will fix this, but it’s not a big problem. Follow by mounting the single 100nF multi-layer ceramic capacitor (C5). The MKT and ceramic capacitors are not polarised. Next, solder the two relays and the IC socket. All three must go in the right way around, as shown in Fig.2. BC337 transistor Q1 should be fitted next; note that it is mounted quite close to the adjacent relay but it will fit. It’s then time to solder in the four long-pin headers, with the long pins poking out through the underside of the shield board. This is a little fiddly since you need to solder around the bases of the pins but it isn’t too hard if you use decent solder. You can also solder the 2x3 dualrow pin header at this point; it’s the only component that’s mounted on the underside of the board, with the pins soldered on the top side. January 2018  45 Fig.2: PCB overlay for the LC Meter Shield from Altronics. Take care to note orientation of the components when applicable, and the values of the resistors as they aren’t marked on the board. Make sure to not confuse the 47kW and 4.7kW resistors as their colour band codes are quite similar. Finally, fit the two tantalum electrolytic capacitors (C3 & C4) and inductor L1. Take care with the orientation of the capacitors since it is critical; the printed label on the capacitor body will have a + sign indicating the positive lead and this must be soldered to the positive pad as indicated in Fig.2. In other words, the capacitors should be soldered with their positive leads facing in towards each other. Check your soldering carefully, then plug IC1 into its socket (being careful not to bend any of its leads underneath the IC) and you are ready to plug the shield into your Arduino board. Before you can program it, though, you will need to attach the LCD panel. Unlike Jim Rowe’s version of this project, this one does not use an I2C adaptor for the LCD. So rather than having four wires, two for the power supply and two for the I2C bus (SDA/SCL), this one requires all sixteen pins of the LCD module to be wired up. However, because it’s being driven in 4-bit mode, about half of them are connected to ground. The required connections are shown in the circuit diagram; the final software may change some of these pins, so double check that your pin connections agree with the software. Fitting it in the Mega Box One thing to keep in mind is that if you are building this unit using the Altronics Mega Box described last month, a 10kW contrast adjustment trimpot is provided on the board. The Mega Box also has pins 1 (GND), 2 (Vcc), 5 (R/W), 15 (BL+) and 16 (BL-) already connected. If you’re not using the Mega Box, these spare pins will need to be connected before the display will work properly; similar to what is shown in Fig.1. Note though that pin 16 on the Mega Box is wired up to transistor Q3 and you will need to connect its base drive to +5V to enable the backlight. Wiring up the LCD screen may seem daunting but all the other connections are taken care of by the shield, so once you have done this, you’ve almost finished. The easiest way to wire the screen up is to use male/female jumper leads; the female end can plug into the header on the LCD and the male plug goes into the relevant pin on top of the shield or Mega Box header. Note that you can’t easily run connections to the top of the shield in the Mega Box or the lid won’t fit, as there just isn’t enough clearance. So wire up to the headers provided on either side of the Arduino board instead. The array of extra ground pins in the Mega Box will come in handy for connecting the unused LCD pins to ground. Software The software for this shield is a modified version of our LC meter code from the June 2017 issue. For further details on its operation, refer to that article (see siliconchip. com.au/Article/10676). Like the original firmware, you need to install two libraries before you can compile the software: FreqCount and LiquidCrystal_I2C. FreqCount is available from www.pjrc.com/teensy/td_ libs_FreqCount.html You also need the LCD and LiquidCrystal Arduino libraries if you don’t already have them. Having loaded the libraries and opened the sketch file in the Arduino IDE, plug your Arduino/Mega Box into your PC using a USB cable and upload the code. Once loaded, the program should go through the initial calibration, the relays should click over and the LCD should start showing a reading. You can then connect a capacitor or inductor between the test terminals and wait a couple of seconds and you should get a reading showing its value. Using it Here is an overview of the Mega Box PCB shown in last month’s issue. Note the repeated pin number 5 on the board (for any readers who didn’t spot it last month) will be fixed in newer versions of this board. 46 Silicon Chip Celebrating 30 Years There are a couple of things you need to note when using this device and this applies to any L/C meter. Firstly, the banana sockets make it convenient to plug in a pair of alligator clip leads and these are then easy siliconchip.com.au to clip to the leads of the component you want to test. But keep in mind that such leads will have some capacitance (a few tens of pF, depending on how close together they are) and some inductance (maybe as much as 1µH). So to accurately measure a small capacitance, make a note of the reading before and after connecting the clip leads to the test capacitor and then subtract the stray capacitance from the reading. To accurately measure inductance, connect the alligator clips together, read off the inductance, then connect them to either end of the test inductor and subtract the earlier (stray inductance) reading. If making a direct connection to the test socket, simply touching the test component leads to the contacts on the sockets may not be good enough. This could introduce enough resistance to throw the reading off. You need to make sure the component leads are pressed firmly into the test socket surface to get the best result. Calibration As stated above, the LC Meter Shield automatically calibrates itself the first time it is powered up. But if you need to make adjustments to the readings (eg, because you have a more accurate reference meter), you will need to do this using the serial console instead. Parts List 1 double-sided PCB, coded K9705, 68.5 x 53.5mm 1 set of four Arduino stackable headers (1 x 10-pin, 2 x 8-pin, 1 x 6-pin) 1 2x3-pin dual-row female header (ICSP connector) 1 EDR201A0550 reed relay (RLY1) 1 2A 5V mini DIL relay (RLY2) 1 8-pin DIL IC socket (for IC1) 1 black PCB-mount banana socket (CON1) 1 red PCB-mount banana socket (CON2) Semiconductors 1 LM311P high-speed comparator (IC1) 1 100µH inductor (L1) 1 BC337 transistor (Q1) 2 1N4148 diodes (D1,D2) Capacitors 2 10µF 25V tantalum electrolytics (C3, C4) 1 100nF multilayer ceramic (C5) 2 1nF±1% MKT/MKP (C1,C2) Resistors (all 0.25W, 1% metal film) 3 100kW (R1,R2,R4) 1 47kW (R5) 1 4.7kW (R3) 1 1kW (R7) Once you’ve uploaded the code to the Arduino from the IDE, you can open the serial console by using the CTRL+SHIFT+M key combination. You can perform calibration with either an inductor or capacitor but you must accurately know its value. Before connecting it up, measure the stray inductance or capacitance of your test set-up, as described above, and compensate for it. 1 6.8kW (R6) This means adding the stray capacitance/inductance measured before connecting the component to its known value. Now connect it up and wait for the reading to stabilise. If it’s exactly right, you don’t need to do anything. Otherwise, in the serial console, enter: calibrate xxx.xxpF/nH Here we used the Altronics LC Meter with alligator leads to measure a 150nF±10% capacitor, our Agilent LCR meter recorded exactly 150nF for the capacitor. The leads by themselves measured roughly 30pF. siliconchip.com.au Celebrating 30 Years January 2018  47 Here is the Altronics LC Meter reading a 200µH toroidal inductor. In comparison, our Agilent LCR meter read an inductance value of approximately 204µH. Overall, not too bad considering the difference in price of the two pieces of equipment. in the place of xxx.xxpF/nH, enter the value you computed above. For example, if your component is 1.01nF and you measured 23pF of stray capacitance, you would use “calibrate 1033pF” while if you have a 10.7µH inductor and measured 300nH of stray inductance, enter “calibrate 11000nH”. You should get a confirmation on the console and the reading on the display should then update to be the correct (computed) value. That completes calibration. Accuracy and drift We found our uncalibrated test unit to be within a few percent of the error value for numerous components that we tested, compared to the readings on an Agilent LCR meter. We believe some of this discrepancy is due to the fact that component values can vary depending on test frequency and the Agilent meter uses a lower test frequency than the Arduino LC Meter. Varying the test frequency on the Agilent LCR meter would often cause the result to change. As some readers have pointed out, LC meters based on this design will drift as they warm up. The June 2017 article suggested rebooting the unit prior to taking subsequent measurements, which does help as it gives it a chance to re-read the “no test component” oscillator frequency. Drift is almost entirely due to changes in the behaviour of the LM311 comparator as it heats up from its own dissipation (power consumption). The other solution is to leave the meter running for some time before using it so that its temperature has stabilised. What could be improved? We have some ideas as to how to compensate for this temperature drift but they require a more complex circuit. We may present an update at some point in the future, should we come up with a meter design that eliminates (or mostly eliminates) drift in the readings. An example could involve using a thermistor or similar to monitor temperatures and then adjust the relay. Alternatively, we could repeatedly switch the device under test in and out of the circuit and measure the oscillator frequency shift, although that would require more complex circuitry. On the functionality side, if you’re using the Mega Box with the LC Meter there is some direct functionality that isn’t easily accessible. As it stands, you can only calibrate via the serial console, or let the software handle it automatically. However, with the Mega Box the rotary encoder could be used to handle nudging the calibration value similar to how the SPDT momentary switch was used in the June 2017 project. Then one of the other pushbuttons could be used to zero out the calibration value, which can be helpful in dealing with any drift. This requires software changes, but SC they shouldn’t be too difficult. Resistor Colour Codes No.  3  1  1  1  1 48 Value 100kΩ 47kΩ 6.8kΩ 4.7kΩ 1kΩ Silicon Chip 4-Band Code (1%) brown black yellow brown yellow violet orange brown blue grey red brown yellow violet red brown brown black red brown 5-Band Code (1%) brown black black orange brown yellow violet black red brown blue grey black brown brown yellow violet black brown brown brown black black brown brown Celebrating 30 Years siliconchip.com.au