ESP32 development board used to implement WiFi multi-meter with following functions :
- Capture and display load bus voltage and load current, in meter mode or chart recorder mode. Uses INA226 sensor.
- Frequency counter with range 1Hz to 40MHz and 1Hz resolution, and independent oscillator (signal generator).
- Uses websockets for control and data communication between browser and ESP32.
Tested with Chrome and Opera browsers on Ubuntu 20.04 LTS.
If not configured, the meter boots up as a stand-alone WiFi Access Point (SSID ESP32_METER, no password required) and web server.
Connect to this WiFi Access Point, then open the home page http://192.168.4.1 in a browser.
If your OS has mDNS support, use the url http://meter.local.
MacOS has built-in support for mDNS. On Windows, install Bonjour. On Ubuntu, install Avahi.
Full meter functionality is available in this stand-alone mode.
However, it is more convenient for the meter to connect as a station to an existing WiFi Access Point.
On the home page you can specify an external WiFi Access Point SSID and password.
Submit the WiFi credentials and restart the ESP32.
Now it will connect as a station to the configured Access Point and then start the web server.
The web page is at http://meter.local as before.
If you do not have OS mDNS support, and you still want to use the meter in station mode, you will need a serial debug connection to the ESP32. Check the serial monitor log on reboot to get the station IP address assigned to the meter by the external WiFi Access Point.
Full-scale bus voltage = ~40V.
There are two ranges for current measurement :
- HIGH : Full-scale 1638mA, with a resolution of 50uA.
- LOW : Full-scale 78mA, with a resolution of 2.4uA.
Meter configuration, capture and display functions are available via a web page.
There are two display modes :
- Meter display with current and voltage readings updated every second
- Scrollable chart display with manually triggered or gated capture
DUT current and voltage readings are updated every second.
The meter uses a fixed sampling configuration :
- Vbus ADC conversion time 332uS
- Shunt ADC conversion time 332uS
- Averages 0.4 seconds of samples for every meter reading
Select the current scale from one of the following options :
- HI : Full-scale 1638.35mA, resolution 50uA
- LO : Full-scale 78.017mA, resolution 2.4uA
- Auto
If you select Auto, samples are first captured with the LO range setting, as this has higher resolution.
If any sample is off-scale, the averaging process is restarted with HI range configured.
Do not select the LO or Auto option if the expected current is high and the DUT cannot tolerate the voltage drop across the 1ohm LO current range shunt resistor.
For any selected option, if the final averaging process used off-scale samples, the current display background will flash orange as a warning.
There are two capture options : manually triggered with a selected capture interval, and gated capture.
Sample rate options :
- 2000Hz : no sample averaging, vbus ADC conversion time 204uS, shunt ADC conversion time 204uS.
- 1000Hz : no sample averaging, vbus ADC conversion time 332uS, shunt ADC conversion time 332uS.
- 400Hz : averaging 4 samples, vbus ADC conversion time 140uS, shunt ADC conversion time 140uS.
Current scale options :
- HI : Full-scale 1638.35mA, 50uA resolution
- LO : Full-scale 78.017mA, 2.4uA resolution
In Chart Display mode, current auto-ranging is not possible due to the high sampling rate.
Click on the legends ma and V above the plot to toggle the display of the corresponding data.
Use the scroll bar sliders to display a subset of the captured data.
Up to 16000 samples can be captured with a single trigger. So the maximum capture interval depends on the selected sampling rate.
- max 8 seconds capture @ 2000Hz
- max 16 seconds capture @ 1000Hz
- max 40 seconds capture @ 400Hz
The external gate signal is input via an opto-isolator. This gate signal can come from the Device Under Test (DUT) or other external trigger.
Once triggered, capture will start when the gate input goes high.
Capture will end when the gate input goes low, or 16000 samples have been captured.
The DUT is an ESP32 development board running an Internet connectivity test.
The power supply is a fully-charged 18650 Li-Ion battery.
On reset, the DUT ESP32 executes the following sequence :
- Connects to a WiFi Internet Access Point as a station
- Connects to a Network Time Protocol (NTP) server to get local time
- Enters deep sleep for 5 seconds
- Restarts
The DUT ESP32 is periodically active for approximately 2.4 seconds, and in deep-sleep for 5 seconds.
The DUT ESP32 uses a GPIO pin to set the meter gate signal high on restart, and resets the gate signal just before going to sleep.
This is an example of an 8-second manually triggered capture. The capture was triggered approximately half-way during the DUT ESP32 deep-sleep interval.
We can see that the DUT load current is ~10mA when the ESP32 is in deep-sleep. This residual current is due to the USB-UART IC and LDO regulator on the DUT ESP32 development board.
When the DUT ESP32 is active, we can see high current pulses (> 450mA) corresponding to WiFi radio transmission bursts.
This is an example of gated capture. It records the load current & voltage only while the DUT ESP32 is active.
Sampling at 400Hz will result in less noise due to sample averaging. It also allows you to capture longer intervals.
However, it may not capture brief current pulses or record accurate maximum/minimum values.
The measurements are noisier, but it captures all the current pulses due to WiFi transmission bursts.
Maximum and minimum current values are more accurately captured.
- 3.3V TTL level frequency counter input on GPIO 34.
- 1Hz to 40MHz range with 1Hz resolution.
- Independent oscillator output on GPIO 33.
- For the screen snapshot above, the oscillator output pin was connected to the frequency counter pin, and the oscillator frequency set to 13429Hz.
- Ubuntu 20.04 LTS amdx64
- Visual Studio Code with PlatformIO plugin using Arduino framework targeting
esp32devboard. - The file
platformio.inispecifies the framework packages, ESP32 configuration, and external libraries. - External libraries used :
- EspAsyncWebServer
- AsyncTCP
- ArduinoJson
- Chart.js v3.7.0
- Circuit Schematic (PDF)
- Any ESP32 development board with on-board USB-UART.
- INA226 current sensor
- A pi-filter network + low-noise MIC5205 LDO voltage regulator provides the power supply for the INA226.
- High-side current metering. Si4925 dual PMOSFET, 2N7002 NMOSFETs are used to switch shunt resistors for low and high scale current measurements.
- PC817 opto-coupler for gated measurement.
- 0.05ohm 1% shunt resistor for HIGH current scale (0 - 1638mA, 50uA resolution)
- 1.0 ohm 1% additional shunt resistor for LOW current scale (0 - 78mA, 2.4uA resolution).
- SS56 schottky diode, protects the 1.0 ohm shunt resistor when the LOW current scale is selected.
- Frequency counter input signal (3.3V TTL level) on GPIO 34.
- Independent Oscillator output signal on GPIO 33.