BLE low-power dynamic current analyzer based on WeChat applet

Brief introduction to the work:

This work uses the WeChat applet as the APP and uses the mobile phone screen as the terminal to display low-power dynamic current waveforms. The APP integrates current range switching, sampling period setting, dynamic trigger mode, dynamic calibration, run/stop mode, zoom control, waveform data saving, Bluetooth automatic reconnection, automatic sleep (AUTO POWER OFF), low battery alarm and other functions. This current analyzer uses TI's CC2640 BLE solution to perform high-precision measurements of dynamic low-power current. The current range covers nA, uA, and mA. The measurement error at mA level is controlled within ±1%, and the error at uA level is within ±1%. Within 1%, the nA level error is within ±45nA, and the sampling period has 5ms, 50ms, 500ms, 5s and other gears. The current meter is equipped with a 1800mah battery. The full-speed operating current is about 3.5ma and the working time is about 500 hours. The automatic sleep operating current is about 170uA and the sleep standby time can reach about 10,000 hours. The shutdown operating current is 0uA. The lithium battery can be charged repeatedly. About 800 times.

1. Details of the work

1. This APP is a low-power Bluetooth application based on the WeChat applet and adopts the WX-CHARTS drawing architecture. The main pages are as follows:      Welcome page: Click on the corresponding applet to enter this page, and automatically jump to the connection page after 5 seconds; Connection page : Connect the BLE device of the current analyzer. After the connection is successful, it can be saved to the paired device. Next time, you can jump directly from the welcome page to the display page; Display page: Power indicator, connection status indicator, zoom control, waveform display area, sampling frequency Settings, sampling range switching, RUN/STOP mode, SIMGLE mode, trigger menu; Trigger page: Set the trigger sampling period, sampling amplitude, trigger mode, trigger variables, dynamic calibration variables (APP can dynamically adjust the ADC value uploaded by the analyzer compensation) and other parameters.

2. Introduction to use

Preparation before use: Insert the red and black test leads into the corresponding copper holes on the surface of the current analyzer. Connect the tip of the test lead to the alligator clip. String the two alligator clips into the wires that need to measure the current. Turn the power button to ON and turn on the phone. Bluetooth, scan the corresponding applet code and enter the connection page, search for the MCURRENT_XXXXXXXXX device, click Connect if the device is found, and enter the display page.     Normal mode: After entering the connection page, click the button to select the corresponding sampling period and current range. The current waveform will be displayed directly on the mobile phone display. The current size (average value) is displayed at the bottom of the waveform display area. Press RUN/STOP. key and when the key turns red, the waveform temporarily stops updating the currently displayed current waveform. You can save the current waveform according to the OS's save screenshot method for later reference and comparison. Press RUN/STOP again and the key turns green to continue updating the new current waveform. , press the + and - keys at the top of the waveform display area to achieve zoom adjustment.     Trigger mode: Click the trigger menu button to enter the trigger page, select the corresponding trigger sampling period, sampling amplitude, trigger mode, trigger value and other parameters, return to the connection page, press the SIMGLE key, if data that meets the trigger conditions appears, the waveform will It is directly displayed in the waveform display area. The calibration mode is shown in the following figure       : When the two input test leads are short-circuited, the ADC value uploaded by the analyzer can be zero-compensated. If the waveform displayed in the waveform display area is small, the calibration variable can be increased. , if the waveform in the display area is large, the compensation variable can be reduced. This variable only compensates the ADC value, and the compensated ADC is converted to form a current value, as shown in the figure below

3. Introduction to low power consumption  

This current meter uses Texas Instruments' CC2640, which has the lowest power consumption in the industry, as the main control chip. When the broadcast time interval of BLE is 100MS, the power consumption is about 170ua. If the broadcast transmission interval is increased, lower power consumption can be achieved . In broadcast mode, the automatic sleep function can be realized, which is the AUTO POWER OFF function. Users often worry about forgetting to turn off the power. With this AUTO POWER OFF function, when the user moves away from a certain distance, it will automatically enter the broadcast mode, and the operating current will automatically drop to 170uA. The system will only enter when the APP is connected. In full-speed operation mode, the operating current is about 3.8mA. At this time, the ADC chip also uses pulse conversion. When the collection is completed, the ADC immediately enters the power-off mode. When entering the BLE broadcast mode, the OS turns off the power supply of peripherals, amplification op amps, bias op amps, ADC collectors, and relays. In order to achieve the best energy consumption effect, magnetic latching relays are used to reduce power consumption. When the system is running It will automatically enter sleep mode in 10 minutes to prevent false connections. If the non-false connection APP has an automatic reconnection function, it can be connected and work again immediately. ** 2. Describe the challenges faced by the work and the problems solved Battery life issues: Handheld devices often have to worry about power consumption, so the devices we choose are all low-power devices, and we use MOS tube switching circuits to control peripherals and power supply to the device for optimal performance. When BLE is disconnected, only the BLE broadcast is working, and the power of other peripherals and devices is automatically turned off. For BLE, we also choose the CC2640 chip from Texas Instruments, which has the lowest power consumption in the industry. This chip has won the Low Power Award in the industry competition. Consumption award, this work uses 1800MAH lithium battery, you can use an external 5V to continue charging after use, the full speed operation can be at least about 450H, the standby time can reach 10000H, this work uses a magnetic latching relay to make full use of the relay, as long as the action is completed, it does not consume Electrical characteristics in order to obtain low power consumption; Consistency: There are ADC quantization errors, amplification circuit errors, device errors, operational amplifier offset voltages, and various errors in the system. In order to eliminate errors, the APP of this work can zero ADC (two input terminals short-circuited) performs corresponding compensation to eliminate the impact of various error superpositions on the measured values, ensuring the accuracy of the measurement of the work and the consistency of the batches; Cost-effectiveness: This work uses devices from some well-known manufacturers , although the unit price is high, the quantity is small, the system is compact and compact, and the performance is good; Stability: The measurement of this work is a weak signal at the mV level, so the stability of the power supply and the software filtering algorithm determine the final performance; Calibration: This work is due to funding issues There is a lack of professional calibration instruments. The current calibration method is to use a simple lithium battery and an adjustable resistor to get the corresponding test current, and then the results obtained by a universal multimeter. If there is a professional instrument for measurement, it can be calibrated directly through The calibration variables in the trigger page of the APP are directly calibrated and compensated.

3. Describe the key points involved in the hardware and software parts of the work

1. Hardware part main control CPU: This work uses Texas Instruments' CC2640 as the main control CPU, which is responsible for tasks such as ADC data timing collection, data filtering, data upload, parameter setting and storage, and relay action control; ADC collector: This work uses Texas Instruments 24-bit ADC converter ADS1247 as the analog-to-digital collector. The conversion rate of this ADC can reach up to 2KBPS, which just meets our fastest 5MS sampling cycle, differential input, differential reference source, and input RC filter circuit. , Use the bias voltage of the signal acquisition circuit as the reference source of the ADC to eliminate the common mode signal; Signal amplification circuit: This work uses a two-stage non-inverting amplification circuit + bias voltage circuit + magnetic latching relay; uses non-inverting amplification Because the input internal resistance of the non-inverting amplifier circuit is extremely high, it will not affect the tiny current. The total amplification factor of the two stages is 37 times. The magnetic latching relay only needs to energize the corresponding action coil or release coil. , it can be released after charging, thereby achieving the purpose of low power consumption; the APP sends the corresponding parameters, the magnetic latching relay hits the corresponding gear, and different resistances correspond to different current gears; Power circuit: This work uses 1800MAH 3.7V lithium battery is the main power supply. The external 5V can charge the lithium battery through MICRO-USB. The lithium battery is stepped down to 3.3V by MICROCHIP's MCP1702 for use by the CPU. The CPU controls the switch of a PMOS tube to control the collector, Power supplies for amplification op amps, bias op amps, and relays. When BLE is disconnected, the power supply to peripherals and devices is cut off to achieve the goal of low power consumption.

2. BLE part low-power design: When BLE is connected, the corresponding callback is called, the power switch of the collector op amp is turned on, the timing switch, the LED indicator light flashes, and the relay is activated; when BLE is disconnected, the corresponding callback is called , directly perform a hard reset operation on the CPU, let the CPU directly return to the BLE broadcast state, and turn off the power of peripherals and external devices; Filtering algorithm: This work collects extremely weak signals, so the ADC collection results are easily affected by power supply fluctuations. Therefore, the collected ADC values ​​must be filtered. The filtering algorithm used is the difference comparison method. The difference between the current sampled value and the previous one, two, and three is saved to the buffer within a certain range, otherwise the current value is discarded. .

3. APP part connection logic: When BLE goes offline, the corresponding callback function will be triggered, and the APP will call the connection function again to implement the automatic connection function. After the current connection is successful, the corresponding device mac and device name will be saved for use the next time you connect after being disconnected or logged out; Trigger logic: Compare the difference between the before and after values ​​in the buffer sent by BLE, and when When the corresponding conditions in the trigger parameters are met, save the current data, trigger conditions, and collect the next few frames of data, and then store them in the data warehouse for display; Calibration logic: When the input two test leads are short-circuited, the ADC value transmitted from BLE can be calculated Compensation is performed. The current compensation value range is between -10 and 10 (can be adjusted according to the mass production value). The compensation formula is the measured value directly plus the compensation value. The compensated current waveform is displayed on the APP and fluctuates around the zero position. That’s fine.

4. List of materials for the work 1. APP 2. One black and red test pen each 3. 1800mah lithium battery 4. 3D printed shell 5. Two alligator clips 6. Spiral copper holes with nuts 7. Several nuts and screws 8. PCBA

5. Appearance pictures of works:

    Performance Testing: