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Wide-Range hmMeter Features and specifications Resistance measurement range: 1mΩ to 20MΩ Individual ranges: 1mΩ to 30Ω, 30Ω to 3kΩ, 3kΩ to 100kΩ, 100kΩ to 1MΩ, 1MΩ to 20MΩ Resolution: 0.1mΩ in milliohms range (usable resolution closer to 1mΩ) Accuracy: better than ±1%; typically close to ±0.1% Test current: 50mA up to 30Ω, 0.5mA from 30Ω to 3kΩ, <50μA up to 20MΩ Other features: auto-ranging, battery voltage display Power supply: 6 x AA cells; up to 100mA drawn during tests Battery life: around 24 hours of active use This auto-ranging ohmmeter will measure just about any Part 1 resistance – from a handful of milliohms to many megohms! by Phil Prosser T here have been several occasions recently when I have needed to measure low resistances accurately. That includes some speaker projects, where I needed to accurately determine the DC resistance of a voice coil to estimate a driver’s Thiele-Small parameters or find the resistance of an aircored power inductor. Another time it was for the Capacitor Discharge Welder project (Practical Electronics, March and April 2023), where I wanted to check the resistance of the leads. Theory said they should be 8mW (spoiler alert – with the cables and handles, our welder leads measured 10mW). Your garden variety multimeter won’t measure anywhere near that low. Even my fancy, expensive meter was way off the mark. So what do you do when you want an accurate measurement of a really low resistance? You reach for your trusty old lowohms meter. Like many journeys in life, this design started on one path but ended up somewhere else. The initial plan was to update a previous milliohm meter design, adding a digital front end and making it easy to use. But halfway through, somebody said: why not make it measure up to 20MW? This added a bit of a spin on the design, but we think the result is a very handy and versatile device. 18 So here we have a design for a meter that will measure resistances from a couple of milliohms to 20 megohms, with precision significantly better than 1% across that range. Using 0.1% resistors for calibration (which we recommend), we have seen precision in the region of 0.1% across most of its range. this principle. For example, the Wheatstone bridge is a very elegant approach that can be highly accurate. But an automated meter based on one of those would be very complicated. If you are interested in this use for a Wheatstone Bridge, Wikipedia is a good place to find out more. The trouble with multimeters The problem with a standard multimeter is that the lead and banana socket resistance is usually in the 0.2-0.5W range. The variability in these resistances are too high to zero them out. Ohm’s Law is one of the first equations you learn in electronics. It is therefore not surprising that this principal is used in most ohmmeters, with the resistance measured using a constant-current source and a voltmeter. A typical multimeter combines these inside the meter and uses two leads, as shown in Fig.1. When measuring a low resistance this way, the constant current needs to flow through the banana plugs, leads and from your lead tips into the device you are measuring, then back again. The voltage drops created by their inherent resistances all appear to the multimeter to be part of the measured resistance. This results in significant errors in low-resistance measurements. There are other ways to measure resistance accurately that don’t use Kelvin connections A four-wire measurement technique can be used to minimise these errors. Two wires deliver a known current through the device under test, while the second pair measure the voltage across the device under test (DUT), as shown in Fig.2. This neatly avoids the majority of errors above. By using a constant-current source, even if there are lead and connection resistances, the current is always as expected. The voltmeter is chosen to have a high input resistance, so when the voltage measurement leads are connected across the DUT, even if the connection is a bit dodgy, we still read the correct voltage, and the R = V / I calculation avoids the majority of errors. There is a bit more effort involved in making really accurate resistance measurements than just adding two wires, but they are necessary to measure values well under 1W accurately. You might wonder why all ohmmeters don’t work this way if it is so effective. Well, using a four-wire ohmmeter Practical Electronics | August | 2023 Measured resistance R=V÷I Measured resistance R=V÷I Measured resistance Rdut = Rref × (V2 ÷ V1) Current = V1 ÷ Rref V2 = Current × Rdut Fig.1: a standard ohmmeter works by passing a known, fixed current through the device under test (DUT), measuring the voltage across it, then using Ohm’s Law to determine its resistance. The problem is that the test lead resistances are in series with the DUT and included in the result. Fig.2: two pairs of leads are used with Kelvin connections, one to feed the test current to the DUT and one to sense the voltage across it. The voltage drop across the leads supplying current no longer affects the reading, and the voltage drop across the other pair of leads is so tiny that it doesn’t matter. Fig.3: the problem with using the method shown in Fig.2 to measure high resistances is that the test current needs to be really low. So we use this method instead, where the DUT and a fixed resistor form a divider, and we measure the DUT resistance in proportion to the fixed resistor value. is fiddly. There are four wires and most of us only have two hands. Also, the errors are no longer significant above a few hundred ohms. Therefore, all but a few meters (mainly benchtop meters, but some handheld) use the conventional two-wire approach. The four-wire connection is called a ‘Kelvin connection’ after Lord Kelvin, who invented this to measure low resistances in 1861. While working on this meter, we noticed some nice ‘Kelvin clip leads’ available at reasonable prices. These are essentially crocodile clips with two connections, one for the current source and the other for the sense wire. We found that these worked well over the range of our meter, though for really low resistances, four separate wires will give better accuracy. 4mV would introduce errors of up to 80mW on the low-ohms range! The device selected has a worst-case offset of 8µV over its entire operating temperature range, which still could result in an offset error of up to 1.6mW (although we have not seen anything like this sort of error in our testing). This allows our meter to accurately measure a 5mW shunt resistor, which we feel is pretty good. To go beyond this, design approaches that null out these offsets are required – this is usually achieved by switching the current source on and off, allowing subtraction of the nil current offset. By using low-offset parts, we can avoid the need to do this in our design. resistor, the current will be Itest = Vsupply ÷ (Rref + Rdut), about 1.5μA. Keep in mind that Itest = V1 ÷ Rref. This relationship is handy, as we will see in a minute. Ohm’s Law tells us that the resistance of the DUT is defined by Rdut = V2 ÷ Itest, where V2 is the voltage across the DUT. Combining this and the previous equation: Rdut = V2 ÷ (V1 ÷ Rref) = Rref × (V2 ÷ V1). Our ADC does not have two channels, but it does have an independent reference (V1) and measurement input (V2). So by connecting our ADC reference across the reference resistor, we can measure the ratio of V1 and V2 with the ADC, and it simply comes out as the measured value! An added bonus of this approach is that we don’t need to care about the exact supply rail voltage or exact current through the DUT. The catch here is that our measurement of the voltages across the reference and DUT resistors has been assumed to be ideal, ie, our ADC has no impact on the current flowing through the DUT. We already know that the current will be in the region of 0.1μA, so the ADC measuring the reference and DUT voltages needs to have very high input resistances and very low bias currents (the current flowing into or out of the input), or else the above assumption will fail. The ADC we have chosen, the MAX11207, only has a bias current of 30nA. The voltage 30nA will develop across a 10MW resistor is 30 × 10-9 × 10 × 106 = 300 × 10-3V, or 0.3V. This is a massive error, given that we will be measuring about 1.5V. So we had to add a buffer amplifier with a super-low bias current. Our choice, the MCP6V64, has a typical input bias current of 20pA and Other challenges We need to know the exact current through the DUT and the voltage across it. For DUTs with a low resistance, both of these are easily achieved. We use an LT3092 programmable current source and, as the voltmeter, an analogue-to-­digital converter (ADC) with a carefully designed voltage reference. These both provide good long-term stability for the meter and the ability to use 0.5mA and 50mA bias currents, which give measurements accurate into the low-milliohm range. Measuring down to about 1mW is practical with a reasonably simple meter. This is about the lower limit before other factors become problematic. Even with higher currents, low resistances mean we need to measure low voltages. Our design uses special very-low-offset and very-low-drift operational amplifiers. If we had chosen, say, a common TL074, the worst-case input offset of Practical Electronics | August | 2023 Megohms measurements Adding a megohm range would seem to be a simple matter of setting the constant-current source to a very low current and making the exact same measurement. This is true, provided you can generate a stable current source delivering about 0.1μA with an output resistance much greater than 20MW. But that is not easy to achieve. To avoid this, we use a slightly different technique for measuring higher resistances, as shown in Fig.3. We use a high-value precision resistor to establish the test current. Because this is in series with the DUT, the current flowing will depend on the DUT’s resistance. We do not try to control the current; instead, we measure the voltage across the reference resistor to measure the current flowing for every measurement. By also measuring the voltage across the DUT, we have all the information we need to determine its resistance in proportion to the sense resistor. For the 1MW range, we use a 1MW sense resistor. The current through this will vary. If we measure a 1MW 19 a maximum offset current of 200pA (the difference between the bias currents for the + and – inputs). Given the current IC shortages, we have listed a few alternatives that we have tested in the parts list, but the MCP6V64 is our first choice. This reduces the error with a 10MW resistor to 200 × 10-12 × 10 × 106 = 2 × 10-3V or 3mV, a much more manageable error. Circuit description Let’s look at how these decisions come together in our final design. The complete circuit is shown in Fig.4. The heart of this meter is the MAX11207 20-bit ADC. We have also tested this with the similar MAX11210 chip, and the MAX11206 and MAX11200 should also work just fine too. We chose this device as it is very linear, provides great resolution and is available in several pin- and software-compatible forms. It also has fully differential inputs for both the ADC and the reference, which can operate across the entire input range. This means we can pull some tricks and use the reference input in a somewhat unusual manner for high-resistance measurements. This device has a range of settings, the most important ones being internal calibration and internal buffering. The software looks after this, and you should only notice a slight delay at power-on as they are initialised. All the inputs to the ADC are buffered by the MCP6V64 quad operational amplifier. This device provides a very high input impedance, low bias current and low drift buffer for the ADC. All of its inputs and outputs can go close to the supply rails. Its key feature is bias currents in the pA range, and it can operate within 200mV of the rails. When you get to the construction stage, take note that the PCB must be very clean around this surface-mount IC. Flux and residue from soldering can increase the leakage currents on these extremely high impedance inputs, degrading the performance of your meter. Thoroughly cleaning and coating this area with clear protective lacquer is an essential step in construction. We have included 10kW series protection resistors from the sense inputs to the buffers, and a 10nF capacitor across the sense inputs, providing modest protection to the circuit. That said, we strongly suggest that you do not connect the meter to live circuits, as the application of more than a few volts between the terminals could easily cause damage. On the milliohms range, the reference voltage going to the REFP input (pin 5) of IC1 via buffer IC2a comes from an LM336 2.5V shunt regulator, IC5 (lower left). We’re specifying the LM336B type as it has tighter tolerances. The LM336 is set up with series diodes and a trimpot, which allows us to set it to exactly 2.50V, and the diodes minimise its drift with temperature. The reference input is connected across a resistor of either 100kW, 1MW or 20MW resistors on the higher ranges. These can be found near IC5. The stability of these resistors is important for the accuracy of these ranges. Again, we will be calibrating the device, so initial precision is less critical than stability for these parts. The MCP6V64 buffers for the ADC (IC2b and IC2c) can drive to within a few millivolts of the rails, but not quite A preview to part two, showing how the batteries and PCB are mounted. 20 to the rails. To accommodate this, the 2.50V voltage reference and reference resistors connect to ground through D8, a BAT85 schottky diode. Similarly, the DUT connects to the positive rail through D4, a 1N5819 schottky diode. These drop about 0.3V at the currents we operate them. We use a constant-current device (IC3) to pass either 0.5mA or 50mA through the DUT on the milliohms and ohms ranges. The stability of the voltage and current references is essential to the accuracy of these ranges. But because we calibrate this meter against known resistors, absolute precision is less of an issue. With a 3.6V supply rail, the maximum voltage that we can handle across the DUT is 1.7V. This is calculated by subtracting the voltage drops from the supply rail due to diode D4 (0.3V) and IC3 (1.6V). Let’s say we can allow up to 1.5V across the DUT to be safe. This means a maximum reading of 1.5V ÷ 50mA = 30W on the milliohms range, and 1.5V ÷ 0.5mA = 3kW on the ohms range. The maximum readings on the other ranges are limited by the values of the 100kW, 1MW and 20MW reference resistors. The current regulator For the higher current (lower resistance) ranges (milliohms and ohms), we use IC3, an LT3092 constant-current source. We have chosen this for its long-term stability and ease of use. This device sources a constant 10µA from its SET pin, and the OUT pin is maintained at the same voltage as the SET pin. With a 10kW resistor from the SET pin to GND, there will be 0.1V across it (10kW × 10μA). The parallel combination of 205W, 47kW and 1.5MW resistors results in 204.08W between the OUT pin and ground, giving a current of 490μA. Therefore, the IN pin sinks 490μA + 10μA = 500μA for these two currents combined, which is our goal (0.5mA). For the milliohms range, parallel MOSFETs Q2 and Q4 switch on, so the two series 1W resistors are connected in parallel with the 204.08W resistance. But note that the on-resistance of the MOSFETs (40mW || 40mW = 20mW) adds to the 2W from the resistors. With 2.02W in parallel with 204.08W, we get 2.0002W. Thus the current from the OUT pin will be 49.99mA + 0.01mA or 50mA. This way, the software can switch the constant-current source between 0.5mA and 50mA to suit the resistance detected on the meter by controlling the gates of Q2 and Q4. Practical Electronics | August | 2023 Parts List – Wide-Range Ohmmeter 1 double-sided PCB coded 04109221, 90.5 × 117.5mm available from the PE PCB Service. 1 189 × 134 × 55 sloping ABS instrument case [Altronics H0401] 2 3 AA cell battery holders with leads [Altronics S5033 + P0455] 1 backlit 16×2 character alphanumeric LCD screen with HD44780-compatible controller (LCD1) 2 4-pin tactile switches (S1, S2) 1 subminiature DPDT solder tag slide switch with mounting screws (S3) [Altronics S2010 + S2014] 3 Omron G6H-5V or G6S-5V telecom relays or equivalent (RLY1-RLY3) [eg, Altronics S4128B] 1 10kW top-adjust multi-turn trimpot (VR1) 1 10kW top-adjust mini trimpot (VR2) 1 2-pin header with jumper shunt (JP1) (optional; only needed for in-circuit programming) 2 2-way vertical polarised headers with matching plugs (CON1, CON2) [Altronics P5492 + P5472 + 2 x P5470A] 1 16-pin header (CON3; for mounting the LCD) 1 6-pin header (CON4) (optional; only needed for in-circuit programming) 1 2-pin right-angle polarised header with matching plug (CON5) [Altronics P5512 + P5472 + 2 x P5470A] 1 5-pin header (CON6) (optional; for monitoring SPI) 2 red captive head binding/banana posts (CON7, CON8) [Altronics P9252] 2 black captive head binding/banana posts (CON9, CON10) [Altronics P9254] various lengths of light-duty hook-up wire 1 pre-made set of Kelvin clip leads OR 1 DIY set of Kelvin clip leads (see section below) Hardware 4 M3 × 10mm tapped metal spacers 4 M3 × 6mm panhead machine screws 4 M3 × 6mm countersunk head machine screws 8 M3 shakeproof washers 1 small tube of clear neutral-cure silicone sealant 1 can of PCB conformal coating/protective lacquer Kelvin clip leads (if not using pre-made leads) 2 Kelvin alligator clips [Mouser 485-3313 or 510-CTM75K; Digi-Key 1528-2279-ND] 1 2m length of 17AWG (1.0mm2) black figure-8 cable [Altronics W4146] OR 1 2m length of two-core heavy-duty microphone cable [Altronics W3028] 1 1m length of 18AWG (0.75mm2) red silicone hightemperature hook-up wire [Altronics W2400] 1 1m length of 18AWG (0.75mm2) black silicone hightemperature hook-up wire [Altronics W2401] Semiconductors 1 MAX11207EEE+ 20-bit ADC, QSOP-16 (IC1) (alternatives exist – see text) 1 MCP6V64-E/ST quad low-drift rail-to-rail op amp, TSSOP-14 (IC2) ■ We recommend using 0.1% 15ppm resistors for the 10kW and 205W parts, as specified in the parts list. We found 1W 0.1% resistors too expensive, so Practical Electronics | August | 2023 1 LT3092EST or LT3092IST programmable current source, SOT-223 (IC3) 1 PIC24FJ256GA702-I/SS 16-bit microcontroller programmed with 0410922A.HEX, SSOP-28 (IC4) 1 LM336BZ-2.5/NOPB voltage reference, TO-92 (IC5) 1 555 timer, DIP-8 (IC6) 2 AZ1117H-ADJTRG1, AMS1117 or equivalent adjustable 1A LDO regulators, SOT-223 (REG1, REG2) 4 BC547 100mA NPN transistors, TO-92 (Q1, Q3, Q5, Q6) 2 IRLML0030TRPBF N-channel MOSFETs, SOT-23 (Q2, Q4) 7 1N4148 75V 250mA signal diodes (D1, D2, D5-D7, D10, D11) 2 1N5819 40V 1A schottky diodes (D3, D4) 1 BAT85 30V 200mA schottky diode (D8) 1 1N4004 400V 1A diode (D9) Capacitors 7 10μF 50V radial electrolytic 5 10μF 16V X7R SMD M3216/1206-size ceramic 5 100nF 50V X7R through-hole ceramic 5 100nF 50V X7R SMD M2012/0805-size ceramic 2 10nF 100V PPS [Kemet SMR5103J100J01L16.5C] 4 10nF 50V X7R through-hole ceramic Resistors (all axial 1/4W 1% metal film unless noted) 2 10MW 0.1% 25ppm SMD M3216/1206-size 1 1.5MW 1 1MW 0.1% 25ppm SMD M3216/1206-size 2 1MW 1% SMD M2012/0805-size 1 100kW 0.1% 25ppm SMD M3216/1206-size 1 47kW 1 33kW 1 22kW 1 10kW 0.1% 15ppm 7 10kW 4 4.7kW 3 3.3kW 1 2.2kW 2 1.2kW 1 820W 1 205W 0.1% 15ppm 2 100W 1 47W 2 1W 1% 50ppm Calibration resistors (not required if another highprecision ohmmeter is available) 1 27.4W 1/4W 0.1% 15ppm axial [YR1B27R4CC] 1 2.94kW 1/4W 0.1% 15ppm axial [YR1B2K94CC] 1 97.6kW 1/4W 0.1% 15ppm axial [YR1B97K6CC] 1 976kW 1/4W 0.1% 15ppm axial [YR1B976KCC] 1 10MW 1/4W 1% 50ppm axial [MF0204FTE52-10M] ■ compatible op amps need to be rail to rail, unity- gain stable with very low input offset voltages and input bias currents in a TSSOP-14 package. Good alternatives are the MCP6V79, MCP6V34 and OPA4317. we used 1% parts instead. These are MF0207FRE52-1R, which have a 50ppm temperature coefficient, so they should be pretty stable. We have provided the current source with a good heatsink in the form of a large copper fill on the top layer of the PCB. The keen-eyed will also note that 21 Wide-range Digital Ohmmeter Fig.4: all measurements are made by IC1, the ADC, controlled by microcontroller IC4. IC4 switches relays RLY1RLY3 to select the appropriate range and displays readings on the 16x2 LCD module. Voltage reference IC5 is used in the lower (milliohms and ohms) ranges, while IC3 regulates the test current, with MOSFETs Q2 and Q4 switching it between 0.5mA and 50mA. In ratiometric (high-range) mode, IC3 and IC5 are not used, and precision resistors of 100kW, 1MW or 20MW are connected in series with the DUT. we have placed a guard track around the SET pin, which has an extremely low current flowing from it. This reduces leakage currents interfering with our carefully-designed current source. 22 The reference resistors We measure resistances in three ranges above 3kW: 100kW, 1MW and 20MW. Our measurement technique uses reference resistors at each of these values. We have specified parts that should provide a low temperature coefficient and long-term stability. We again recommend 0.1% parts where reasonable. Practical Electronics | August | 2023 20MW tight-tolerance resistors are both expensive and uncommon, so we use two 10MW resistors in series. Stability is probably more important than actual precision, as the meter will be calibrated. Again, cleaning off all flux and residue around these is very important, as is coating it with a protective lacquer to optimise long-term stability. We used 3.2 × 1.6mm SMD parts here (M3216/1206) as our survey of suppliers found that 0.1% parts are more Practical Electronics | August | 2023 available and less expensive in these packages than in through-hole. Switching the ADC inputs Because we have five different ranges and can’t handle any additional bias currents, we need to do some switching, and that’s done with relays. The resulting switching arrangement might initially look complicated but there isn’t too much to it. Regardless, the auto-ranging feature means that the user doesn’t need to know the details. One relay, RLY1, switches the reference input between the fixed 2.50V reference and the three reference resistors. The other two relays, RLY2 and RLY3, connect either the constant-current device (IC3) or one of the three reference resistors to the lower pin on the Force connector, CON1. The PCB has been laid out to handle two of the most common types of signal relays, conforming to the Omron G6H and G6S layouts. These are available from a range of electronic 23 Kelvin leads We used Adafruit 3313 Kelvin clips leads with the prototype, which are amazingly cost-­effective; certainly less expensive than a double espresso! Availability from the usual suppliers is mixed. We also tried Mouser Cat 510-CTM-75K, which is a delight to use but it is rather more expensive. These are simple to wire up, as shown in the adjacent photo. All you need to do is wire the Force+ and Sense+ wires to either side of the ‘+’ Kelvin clip (with the red wire) and the other two terminals to the remaining black wires of the ‘−’ Kelvin clip. Keep in mind that the force and sense wires only contact either side of the DUT lead. Where you measure larger or more fiddly items, separate force and sense test leads might be better. Again, the force current must run through the whole item you wish to measure the resistance of, and the sense lines are connected to measure the part you desire, as shown in Fig.5. We made two sets of leads for our meter. One set had separate sense and force leads, and these are essentially conventional multimeter leads. We made them using 18AWG silicone-coated high-temperature hook-up cable (Altronics W240X), which is very Reproduced by arrangement with SILICON CHIP magazine 2023. www.siliconchip.com.au flexible. We connected these wires to clips for the force and probes for the sense lines. We did not use these much in the end, as the Kelvin clips are excellent right down into the low-milliohm region. We used Altronics Cat W4146 sheathed figure-8 flex for our Kelvin clips, though we feel that a lighter gauge would be easier to use if you can find it. We used coloured heatshrink tubing to clarify which wires are + and – (although this generally isn’t important when making measurements). One Kelvin clip connects to ‘Force −’ and ‘Sense −’ while the other goes to the ‘Force +’ and ’Sense +’ sockets on the meter. The length of leads should not matter as the conductors are close, so any EMI picked up should mostly cancel out. We felt that 600mm was about right, but that is a matter of preference. If you don’t want to make up your own set of Kelvin clip leads, they are availFig.5: when working with Kelvin probes, it doesn’t matter whether you connect able to buy pre-made at reasonably low the ‘sense’ leads closer to the DUT than the ‘force’ leads or not. Regardless, the prices at sites like eBay. Search for ‘LCR section between the two connections on either side is not measured because clip leads’. current is flowing through it or the measurement point is further along. outlets. Just make sure you use 5V non-­latching versions. Microcontroller and display We have kept the display and control circuitry simple. We see this as a utilitarian device, so it should put function over form, and seek to ‘do what it says on the box’ as simply, cheaply and reliably as possible. The LCD screen operates from the VDD rail of about 3.4V, but these displays are almost always powered from 5V. It turns out that the LCD bias between the VDD and VO pins on the LCD module needs to be about 5V, but the actual controller is specified to operate from 2.7V. Therefore, we can generate a negative voltage of about −2V for the VO 24 bias reference and power the LCD from the same VDD rail used for the PIC micro. The reason we need to do this is because some LCDs are incompatible with the 3.3V CMOS outputs from microcontrollers. Annoyingly, it is very difficult to tell which LCDs work with 3.3V logic and which don’t. To avoid this frustration, we have arranged the circuit so that all LCDs should work. The negative VO bias is generated by 555 timer IC6, which oscillates at a couple of kilohertz. This drives a switched-capacitor voltage inverter comprising two 10μF capacitors and two 1N4148 diodes. This runs off the relay 5V rail and generates −2V or so. By using the 5V rail, we avoid running this ‘noisy’ circuit from a rail used for the sensitive current sources and ADC. User interface The goal of simplicity has led us to remove all buttons from the front panel and implement an auto-range function. There are two buttons on the PCB which are only used for calibration; we will discuss them later. Upon initial connection, the Meter will first check the DUT on the 100kW range. Depending on the result, it will increase or decrease the range appropriately until the optimal measurement range is found. We start with the 100kW range as most of the resistors we measure seem to be less than this resistance. The way the Meter does auto-ranging means it Practical Electronics | August | 2023 will generally jump from the 100kW range straight to the final measurement range. The initial test current will be 30μA or less, and this will increase to 500μA for resistances between 30W and 100kW, or 50mA for resistances below 30W. The highest possible power delivered is 75mW for a 30W resistor. This should be safe for all bar the most sensitive devices. The microcontroller used is a PIC24FJ256GA702-I/SS. This is just right for the job in terms of pin count, though we also use four ‘free’ digital I/O pins provided on the ADC, as they were too convenient to ignore! We have used a simple schottky diode to drop the 3.6V rail to something closer to 3.3V for the ADC and the microcontroller, since 3.6V is right at their upper limits. The micro drives a 16-column, two-line alphanumeric LCD with an HD44780-compatible controller. These are bog-standard but, as a result, come in a bewildering variety of layouts. We have included two very common footprints on the PCB, which gives you some options for selecting a display. When you purchase the display, check the pin-out, as the LED backlight, in particular, seems to change around a lot. GET T LATES HE T COP Y OF TEACH OUR -IN SE RIES AVAILA BL NOW! E Two headers need a comment. The first is the ICSP header, CON4. Insert the PIC and program it with a PICkit 3, 4 or 5 (soon to be released). The code is available for download from the August 2023 page of the PE website at: https://bit.ly/pe-downloads There is also a footprint for CON6, SPI_MON. You should definitely not need this unless you want to look at the SPI activity between the microcontroller and ADC. This sort of facility is super helpful when developing a project like this. We also have pads for an external 8MHz crystal and associated 22pF and 100W passives, although these parts are not required in this design because we use the PIC’s internal oscillator instead. The ADC, buffer op amp and microcontroller are all surface-mount parts. They are simply not available in through-hole packages in the first two cases. We also had a desire to fit this project into a handy instrument case. Power supply The circuit operates from six AA cells. We chose this approach to ensure the meter would have a good runtime and that the 5V rail stays up as the batteries discharge. The meter can draw close to 100mA when measuring low resistances. This should provide over 24 hours of Order direct from Electron Publishing PRICE £8.99 (includes P&P to UK if ordered direct from us) runtime on a set of batteries, which will be fine provided you do not forget to switch it off overnight! There are two linear low-dropout regulators. One has a 5V output to power the relay coils, LED backlighting on the LCD screen and the −2V generator (REG3). The other has a 3.6V output (REG2) to power the ADC, buffer op amps and micro. Both regulators are specified as the AZ1117 type, but there are many pin-compatible LDO regulators (usually with 1117 in their part code) that will work fine too. We’ve provided all the components to allow two identical adjustable regulators to be used for REG2 and REG3. Still, you could use a fixed 5.0V output regulator for REG3, omitting the resistor between the OUT and ADJ pin and its series capacitor, and replacing the resistor between ADJ and GND with a 0W resistor (or a short piece of wire across the pads). You could theoretically do that for REG2 as well, but unfortunately, 3.6V is not available as a fixed output option on this type of regulator. So stick with the adjustable type for REG2. Next month We don’t have space in this issue for all the construction, testing and set-up details, so they will be in a follow-up article next month. EE FR -ROM CD ELECTRONICS TEACH-IN 9 £8.99 FROM THE PUBLISHERS OF GET TESTING! Electronic test equipment and measuring techniques, plus eight projects to build FREE CD-ROM TWO TEACH -INs FOR THE PRICE OF ONE • Multimeters and a multimeter checker • Oscilloscopes plus a scope calibrator • AC Millivoltmeters with a range extender • Digital measurements plus a logic probe • Frequency measurements and a signal generator • Component measurements plus a semiconductor junction tester PIC n’ Mix Including Practical Digital Signal Processing PLUS... YOUR GUIDE TO THE BBC MICROBIT Teach-In 9 – Get Testing! Teach-In 9 A LOW-COST ARM-BASED SINGLE-BOARD COMPUTER Get Testing Three Microchip PICkit 4 Debugger Guides Files for: PIC n’ Mix PLUS Teach-In 2 -Using PIC Microcontrollers. In PDF format This series of articles provides a broad-based introduction to choosing and using a wide range of test gear, how to get the best out of each item and the pitfalls to avoid. It provides hints and tips on using, and – just as importantly – interpreting the results that you get. The series deals with familiar test gear as well as equipment designed for more specialised applications. The articles have been designed to have the broadest possible appeal and are applicable to all branches of electronics. The series crosses the boundaries of analogue and digital electronics with applications that span the full range of electronics – from a single-stage transistor amplifier to the most sophisticated microcontroller system. There really is something for everyone! Each part includes a simple but useful practical test gear project that will build into a handy gadget that will either extend the features, ranges and usability of an existing item of test equipment or that will serve as a stand-alone instrument. We’ve kept the cost of these projects as low as possible, and most of them can be built for less than £10 (including components, enclosure and circuit board). © 2018 Wimborne Publishing Ltd. www.epemag.com Teach In 9 Cover.indd 1 01/08/2018 19:56 PLUS! You will receive the software for the PIC n’ Mix series of articles and the full Teach-In 2 book – Using PIC Microcontrollers – A practical introduction – in PDF format. Also included are Microchip’s MPLAB ICD 4 In-Circuit Debugger User’s Guide; MPLAB PICkit 4 In-Circuit Debugger Quick Start Guide; and MPLAB PICkit4 Debugger User’s Guide. ORDER YOUR COPY TODAY: www.electronpublishing.com Practical Electronics | August | 2023 25