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USB Power Monitor Kit (SC7683, $50): includes the PCB and all onboard parts U Simple S Power B Monitor By Richard Palmer This circuit uses just a handful of parts but it can measure USB voltage, current and power over a wide range (up to 36V, 3A & 108W). It displays the readings on an OLED screen. Displays USB bus voltage, current, power and energy delivered Simple and detailed display formats Supports USB 2.0 and 3.0 power delivery up to 3A/36V Display settings remembered between sessions ±0.3% voltage and ±1% current accuracy Display rotation at the touch of a button Resolution: 100μA, 10mV Serial data logging W hile USB-C is slowly taking over, there are still many devices using USB-A plugs and sockets. While our recent USB-C Power Monitor (August 2025; siliconchip.au/ Series/445) is also capable of measuring USB-A devices with appropriate adaptors, this project is substantially simpler – it is basically an update on the December 2012 USB Power Monitor (siliconchip.au/Article/460) with higher resolution, modern components and the ability to measure the wider range of voltages and currents when the device uses a USB power delivery (PD) mode. The monitor has two display modes. The default shows the USB bus voltage and current flow in large characters. A second mode, with smaller text, adds the power and the energy that has been delivered while the unit has been powered on. The display can also be flipped upside-down, if required for more convenient reading. The unit also provides a TTL-­ compatible serial logging output, shown in Screen 1, which avoids the need for copying down a long series of readings when monitoring over an extended time period. the many variations and how they evolved. In summary, a standard 4-pin USB 1.0 port can supply up to 500mA at 5V. USB 2.0 upgraded the 4-pin standard to 1A. USB 3.0 introduced power delivery and a 9-pin USB-A connector, which is backwards-compatible (ie, four of the nine pins are in the same locations and have the same functions). USB-C PD can deliver up to 5A at 20V after negotiation between the source and sink over the CC (Configuration Channel) pin using BMC signalling. When no negotiation occurs, Component selection USB connectors and power delivery There are a range of USB port types, each with its own power capabilities. The Wikipedia article on USB hardware (https://w.wiki/3oc8) details 60 Silicon Chip 5V is supplied. With USB 3.1, which is exclusively delivered via USB-C connectors, up to 5A at 48V can be delivered. In suspend mode, when the PC or laptop is asleep, the host controller stops sending keep-alive (Start of Frame) signals to devices. This tells the connected device to go into idle mode, with a reduced allowable current draw. A USB device may adopt very different power profiles, depending on which kind of power source and cable are used, and whether the host is active or asleep. Screen 1: example output of the serial logging feature in the Arduino Serial Monitor. Australia's electronics magazine The monitor uses just three ICs, an OLED display and a handful of passive components. The SSD1306 128×64-pixel 0.96inch (24.4mm) I2C OLED screen’s footprint determines the size of the PCB, with additional strips at the edges to accommodate the tactile switch (S1) and USB connectors. The USB connectors, CON1 and CON2, are soldered directly to the PCB. The 9-pin USB-A plug and socket are compatible with USB 1.0, 2.0 and 3.0 devices. SMD connectors were selected because very few 9-pin through-hole versions are available, making reliable component sourcing difficult. Voltage and current measurements are made by an INA237 power monitor siliconchip.com.au USB 3.1 PD Warning The unit may be damaged with USB 3.1 power delivery modes sourcing more than 36V. This can occur when connected to a USB-C power source designed to deliver more than 180W, which permits up to 48V to be supplied. chip, which has an internal 16-bit analog-­ to-digital converter (ADC), offering better than ±0.3% accuracy and providing up to 85V high-side voltage and current measurements. The higher-priced INA238 chip can be substituted, as it is pin- and code-equivalent. It has better (±0.1%) current accuracy, which will not increase the overall accuracy of the unit, as the shunt resistor’s 1% tolerance is substantially greater. Through-hole power resistors with resistances of less than 1W and tolerances better than ±5% are rare and expensive, so a 1W ±1% M6332 (2512 imperial) low-resistance metal alloy (LRMA) SMD resistor is used for the current shunt (R1). LRMA resistors use a cupro-nickel alloy and have a very low temperature coefficient, in the range of ±75ppm (±0.0075%) per °C, which translates to less than 0.4% variation across a working range of 10-40°C. As the stated temperature coefficient allows for either a rise or fall in resistance when the temperature rises, we have not included temperature compensation in the current calculations. The shunt resistor value is determined by the 3A maximum current requirement and the 163.84mV full scale of the INA237’s ADC. A value of 0.025W results in 75mV across the shunt and ¼W dissipation at 3A. While 0.05W could have been used, the lower value resistor means reduced power dissipation and a lower V+ voltage drop without significantly affecting the overall accuracy. To minimise the effect of any temperature rise on current readings, we have connected the resistor’s terminals to as substantial a PCB copper area as the USB signal traces allow. As USB 3.0 power delivery mode can negotiate anything from 5V to 20V on the V+ USB pin, a 3.3V regulator with a wide input range is required to reduce the voltage to drive the circuitry. While both the microcontroller siliconchip.com.au The INA237 DC power monitor IC The INA23x series of DC power monitors offer current, voltage, power and die temperature measurement (see the diagram below). It has a 16-bit ADC that is shared by voltage, current and temperature measurements. The chip can operate at 3.3V or 5V, is controlled over an I2C serial bus and consumes less than 1mA. Two address pins allow up to four chips to coexist on a single I2C bus. Other products in the INA2xx series come with higher-accuracy 20-bit ADCs or SPI control interfaces rather than I2C. The current shunt resistor can be located on the high or low side of the load, as the current input’s common voltage range is -0.3V to +85V. The chip has two ranges for measuring the shunt voltage, 42mV and 164mV full-scale, providing flexibility in choosing the shunt resistor’s value to best balance measurement accuracy and heat generation. As the chip has a very low input bias current, accurate current measurement from microamperes to kiloamperes is possible anywhere in the permitted input voltage range. A digital filter rolls off the ADC response at half the sampling frequency to avoid aliasing measurement errors. The sampling time is individually adjustable for the voltage, current and temperature measurements, ranging from 50μs to 4ms. The chip can average up to 1024 samples, further reducing noise. Any necessary calculations are undertaken in the background to minimise measurement lag. The INA237 also has an alert pin, which changes state when any desired combination of current, bus voltage, power or die temperature goes outside set limits. The INA237 chip includes a shared ADC that measures voltage, current and temperature. The embedded processor can average up to 1024 readings and calculate the power figure. and power monitor chip can operate at either 3.3V or 5V, 3.3V was selected to provide headroom for this regulator when operating from a 5V supply. The MIC5233-3.3 regulator was selected as it has a small SOT-23-5 footprint, can handle the required input voltage range and is readily available. It is connected to V+ on the upstream USB connector to avoid the unit’s current consumption being registered by the INA237. Like with the December 2012 design, no case is required. Instead, the unit is protected by enclosing it in a length of clear heat-shrink tubing. Circuit description The USB Power Monitor circuit is shown in Fig.1. Regulator REG1 reduces the input USB input voltage, Australia's electronics magazine V+ on CON2, to 3.3V for the ICs and display. Its heatsinking requirements are not substantial, as the circuit only draws a few milliamps. It can be disconnected at JP1 while the microcontroller is being programmed. Power and data signals travel between the two USB connectors, CON1 and CON2, with the V+ line interrupted by shunt resistor R1 (25mW). This resistor translates the current consumed by the device under test to a voltage, which is captured by the power monitor chip’s Vin+ and Vin− terminals. The USB V+ voltage is measured on the CON2 (output) side, so that the voltage available to the device under test is correctly displayed. An ATtiny85 microcontroller operating at 16MHz drives the power June 2026  61 A TTL serial-to-USB adaptor (with the black PCB) can be connected to the Power Monitor for logging power-on time, bus voltage, current and more. monitor chip and display. It has a Universal Serial Interface (USI) that can be configured as an I2C or SPI port. During device programming, the SPI mode is used, while under normal operation, the USI is in I2C mode to communicate with the OLED and INA237 ADC chip. The ATtiny85’s I2C interface has SCL on pin 7 and SDA on pin 5. Pull-up resistors for these pins are provided by the OLED display. The I2C bus runs at 400kHz, at which speed all the required traffic is completed well within the two-second display update cycle. The I2C pins are shared with the SPI interface used for in-circuit programming (via CON3), which is initiated when pin 1, RESET, is pulled low by the programmer. When the unit is booted normally, the code sets the pins to I2C mode. Once programming is complete, the MISO SPI signal (PB1 at ATtiny85 pin 6) is no longer needed in I2C mode. It is re-assigned as the unused RxD (receive) serial pin for serial log data, with the TxD (transmit) logging data coming from pin 2 (PB3). Software When power is applied, the microcontroller initialises the OLED display, which is expected at I2C address 0x3C. The power monitor chip, IC2, has an address of 0x40 with its A0 and A1 pins tied to ground. The resistance of the shunt and the maximum current to be measured are provided to the power monitor chip’s driver software, which calculates the calibration value for the chip’s SHUNT_CAL register using the value of the shunt resistor. Voltage calibration is inbuilt. The chip is set to sample the current and voltage every 280μs and average them over 1024 readings, which provides a final set of readings at approximately 300ms intervals. It provides measurements in signed 16-bit integer format, which are converted into floating-point measurements in software. The display updates once every two seconds. Fitting the display driver and font into the available memory proved challenging. The microcontroller only has 8kiB bytes of flash memory program space and 512 bytes of RAM. A full font would consume more than the total flash, but for this project, we don’t need the full ASCII character set. So large and small fonts were created that contain only the characters 0-9 plus the decimal point, space, “V”, “A”, “W” and “m” characters. The usual practice of creating a bitmap in memory and then copying it to the display would have taken 1024 bytes of RAM. Instead, each line of characters is converted to bitmap format and written directly, in rows of eight pixels at a time, to the display. These 8 × 128 pixel ‘pages’ create a restriction that a new line can only start at the top of a new page if the previously written data isn’t to be overwritten. In practice, this means eight rows of tiny characters, four rows of medium-sized characters, or two rows of large characters. Only the two- and four-row options are used. The tactile switch (S1) is sensed by the ATtiny85 on pin 3 with the GPIO port’s pull-up current enabled. It is checked once per display cycle. A press lasting one display cycle changes between the two display modes, while a two-cycle press rotates the display by 180°. Fig.1: the device uses three integrated circuits, one OLED display module, a shunt resistor and not much else. The circuit simplicity is mainly due to the features of the INA237/238 being a perfect fit for our needs in this application. 62 Silicon Chip Australia's electronics magazine siliconchip.com.au Serial logging is accomplished by driving the serial port in software (bit-banging) as the hardware USI is occupied with I2C communication for the OLED screen. The signal is at 4800 baud with 3.3V TTL-compatible levels. A TTL serialto-USB adaptor and a suitable terminal program can be used to view and save the data. Only the RxD and GND pins on the serial adaptor need to be connected to the USB monitor. The data is comma-delimited, including the time since power-on, USB bus voltage and current written in the same format displayed on the screen. PCB design To ensure minimal voltage drop in the USB ground return path, the USB GND pins are connected to the PCB ground plane as well as having pointto-point copper traces. The ground plane has been removed under the USB signal traces to minimise parasitic capacitance, which can degrade high-speed signals. All the signal traces are of equal length to minimise relative phase shifts. Parts List – Simple USB Power Monitor 1 double-sided PCB coded 04104261, 44 × 29mm 1 128×64-pixel monochrome 0.96-inch I2C OLED module [SC6176/SC6936] 1 Würth 692112030100 9-pin USB 3.0 SMD plug (CON1) 1 Switchcraft RAHUA30E 9-pin USB 3.0 SMD socket (CON2) 1 2×3 pin header (CON3; optional, for ICSP) 1 3-pin header (CON4; optional, for serial logging) 1 4.5 × 4.5mm, 5mm tall SMD tactile pushbutton switch (S1) [Altronics S1112A] 1 6-pin vertical 2.54mm-pitch pin header (for mounting OLED module) 1 50mm length of 35-50mm wide (measured flat) clear heatshrink tubing 1 USBasp programmer with 6-pin adaptor and IDC cable (optional; for ICSP) [Jaycar XC4627 + XC4613] Semiconductors 1 INA237 or INA238 power measurement IC, VSSOP-10 (IC1) [Mouser 595-INA237AQDGSRQ1, DigiKey 296-INA237AQDGSRQ1CT-ND] 1 ATtiny85V-20PU microcontroller programmed with 0410426A.HEX, DIP-8 (IC2) [Altronics Z5105, Jaycar ZZ8721 (both supplied blank)] 1 MIC5233-3.3YM5 or MIC1793-330OT LDO 3.3V linear regulator, SOT-23-5 (REG1) [Mouser MIC5233-3.3YM5-TR] Capacitors/resistors 2 4.7μF 25V SMD M2012/0805 X7R multi-layer ceramic capacitors 1 100nF 50V SMD M2012/0805 X7R multi-layer ceramic capacitor 1 0.025W ±1% 1W+ M6332/2512 LRMA SMD current-sense resistor [Mouser LRMAP2512-R025FT4] Construction All components mount on the 44 × 29mm PCB, which is coded 04104261 – see Fig.2. The OLED screen, tactile switch and USB connectors fit on one side, with the remaining components on the other. The ATtiny85 chip comes in an 8-pin DIL package. While it could be socketed, that is not recommended as it will produce a bump in the heatshrink cover on the bottom of the unit. Leave mounting the OLED until last, as the USB connectors and in-circuit programming pins can’t be soldered in once it is in place. If you are programming your own ATtiny85, solder it in and follow the instructions below. Programming may be done at this point or after other components have been mounted. If the regulator is in place when programming, JP1 must be broken to prevent the regulator being reverse-powered and possibly damaged. Next, install the surface-mount parts. Begin with the three ICs and then follow with the passive components. Most of the SMD parts are big enough not to present too many difficulties. We’ve covered SMD soldering on many occasions in the past, so we won’t go into detail here. siliconchip.com.au Fig.2: like the circuit, the PCB is simple and assembly is straightforward. Make sure to mount the screen last, and carefully check all the SMD solder joints, especially on IC1, before moving on to the through-hole parts. The INA237/8 power monitor chip is in a VSSOP package with 0.5mm pin spacing. If you accidentally bridge any of the pins, simply use solder wick to clean it up. A dab of ‘no-clean’ flux paste applied to the bridge beforehand makes clean-up easier. Solder in USB connectors CON1 and CON2 next. I had to lever up the shield on CON2, as the pins were unreachable with a soldering iron. Don’t worry if the shield breaks off. Once testing is complete, it can be clipped back on and a couple of dabs of solder on the top edge will hold it firmly in place. At this stage, plugging the unit into a USB power source should produce 3.3V between JP1 and either of the grounded USB connector cases. Mount the OLED display using a pin header for the connections and two single pin pieces of header strip Australia's electronics magazine soldered through the mounting holes at the other end of the display to anchor it. Before soldering, make sure that the pins on CON3 don’t foul any components on the OLED module. Cut all the OLED pins off flush on both sides. If logging is required, trim off any protruding leads from pins 2 and 4 of the ATtiny85. Remove the middle pin from the three-pin header and mount it on the underside of the PCB, parallel to CON2. Extra solder pads have been provided to make the connections more robust. Programming the ATtiny85 If you haven’t purchased a pre-­ programmed ATtiny85, you will need programming hardware and software. While there are many options available, I have found the following to be straightforward and reliable on June 2026  63 Screens 2 & 3: the main Zadig screen with the USBasp device selected and libusbK as the target driver (shown at left). Device Manager showing that the USBasp driver has been successfully changed to libusbK (shown at right). Windows. For Mac and Linux users, there are several good online tutorials for ATtiny85 USBasp programming. First, purchase the USBasp programmer (see www.fischl.de/usbasp) from your favourite source. Make sure it has a 6-pin socket and IDC cable or includes a 10-pin to 6-pin adaptor. For Arduino users, complete code is also included in the download pack. Board and device settings are listed at the top of the main program. The programmer to select is the “USBasp (ATTiny Core)”. Otherwise, download and install AVRDUDESS (siliconchip.au/link/ acb4), which includes the AVRDUDE command-line programming software. Next, download and install Zadig (https://zadig.akeo.ie). Plug in the USBasp programmer and run Zadig (Screen 2). If USBasp doesn’t show in the device field, click on Options → List All Devices and select it from the list. Select libusbK from the dropdown list that the green arrow points to, and click on the Install (or Reinstall) Driver button. Wait for the process to complete. Now if you open Windows Device Manager, you should now see an entry for libusbK USB devices, similar to the one in Screen 3. Programming is undertaken with the ATtiny85 mounted on the PCB and the programmer connected to the ICSP header. No USB cables should be connected to the USB Power Monitor while programming. Everything is now ready to program the ATtiny85. If your USBasp programmer has a voltage selector jumper, choose 5V. Open link JP1 on the Power Monitor board to prevent reverse-­ powering REG1 during programming. Connect the USBasp and Monitor boards via the 6-pin connector. Pin 1 (red stripe on the cable) is marked with a white dot on the PCB. Plug the USBasp into a USB port on your computer and run AVRDUDESS (Screen 4). Select “usbasp-clone” from the list of programmers and select “ATtiny85” from the microcontrollers list. Locate the HEX file from the download package (siliconchip.au/ Shop/6/3621) using the “…” button to the right of the Flash field. Leave the EEPROM field blank but change the “Fuses & lock bits” settings to L = 0xF1, H = 0xD7, E = 0xFF, LB = 0xFF. The AVRDUDESS window should look similar to Screen 4. Click on the Write button next to the Fuses & lock bits settings, then click Program! The console panel should show progress, ending with a message indicating that the flash memory or fuse bytes have been verified. Disconnect the programmer and re-­solder JP1 to restore the power supply from REG1. The Power Monitor is ready for use. Testing The unit can now be fully tested. Plug it into a power source. The display should light up after a second or so, displaying close to 5V and 0.0A. Screen 4: this AVRDUDESS window shows the programmer type, target microcontroller and HEX file selected. Note the values in the “Fuse” and “lock bits” settings. These need to be written to set the clock speed correctly. The larger PCB is one version of the USBasp-based AVR programmer. Since this one has a 10-pin socket, you’ll need a 10-pin to 6-pin adaptor (shown adjacent), or to purchase one with a 6-pin socket. Australia's electronics magazine siliconchip.com.au 64 Silicon Chip Press and hold the tactile switch; the display should change format after a display cycle. Plug any USB device into the measurement port. The displayed voltage should drop marginally, and the current reading should show a non-zero value. Calibration is not required. If all is well, the heatshrink sleeve can be added and shrunk on the outside edges. If header pins were fitted to CON3, the pins should be trimmed down to the plastic retainer before fitting the heatshrink tubing. While large-bore clear heat shrink tubing isn’t readily available from the main Australian suppliers, eBay and AliExpress both have suitable products. Be careful not to shrink the tubing too tightly, as the screen rotation switch can become permanently depressed. If this happens, cut a circle in the heat-shrink tubing around the switch’s plunger. Operation Operation is straightforward. Simply connect the monitor to a USB power source and the device to test into the USB socket on the monitor. Initially, the unit will display the USB bus voltage and the load current in large characters. If the button is pressed for a display cycle (approximately two seconds), the display will change to also show the instantaneous power flow and the total energy that has been delivered since the unit was powered on. The display automatically switches between amps, watts and amp-hours and their milli- equivalents when the current/power/energy reading is low. If the text on the display is upside down, hold the switch down for two display cycles and it will flip. When logging, always connect the Power Monitor to its USB power source before connecting the TTL serial adaptor. Otherwise, the monitor may not operate correctly. The USBasp programmer connected to the underside of the Power Monitor (with a red colour PCB for the 6-pin adaptor this time. While an IC socket was used for IC2 here, we do not recommend using one, as it will produce a bump in the heatshrink. The Monitor attached to my vintage decade resistance box during testing. Conclusion The INA237 power monitor chip enabled this project to be developed with a high level of accuracy using only a few components. The ability to calculate the energy consumed and log readings on a computer extends its usefulness for devices such as power banks and battery chargers where the load varies SC over time. Here is the Power Monitor connected to my mobile phone from a power delivery capable charger. siliconchip.com.au Australia's electronics magazine June 2026  65