Design Requirements: This project can use Arduino, Uno or Duemilanove boards, and has three design modes: · Standalone – Measurement data can be observed in the form of character or graphic LCD display modules. · Connected – Can connect to a PC via Arduino IDE Serial Monitor for readings. · Combined – Data can be observed on both devices. The second mode does not require an LCD display module, so the price will be slightly lower. The designed Arduino multimeter should have the following functions: · 3 ranges of voltmeter: 0-10V, 0-30V, 0-100V · 1 range of ammeter: 0-500mA · 2 ranges of ohmmeter: 0-1KΩ, 0-250KΩ · Diode and LED connectivity testing · LED functionality testing · Measurement of the BETA value of NPN bipolar junction transistors
1. Warning: High voltage hazard! I must first issue this warning because some of our operating voltages exceed safe voltage levels, and safety should always be at the forefront of our minds. Additionally, when connecting this multimeter to the computer, ensure that the computer and the device share a common ground.
2. Circuit design, this is the overall circuit diagram. Since the overall circuit diagram looks a bit complex, I will break it down into sub-modules for explanation.
3. Simplified diagram of the voltmeter. The three ranges can be selected using buttons on the Arduino board. When measuring voltage, only one switch is closed.
4. Simplified diagram of the ammeter. The measured current flows through a 1Ω resistor to ground, and its output is amplified and connected to the A1 interface of the Arduino, with an amplifier gain of 10. For overcurrent protection, I configured a 500mA resettable PTC.
5. Simplified diagram of the ohmmeter. The Zener diode generates a reference voltage relative to the positive voltage source, which is applied to the current converter composed of PMOS transistors and operational amplifiers, with the resistor to be measured connected to the source of the transistor. At this time, the voltage across the Zener diode and the voltage across the resistor are equal.
6. Arduino board control, both switches are closed, and both resistors have current flowing through them. Therefore, the current can take two possible values: 10uA and 2.5mA. These currents can be precisely modulated. The generated current then flows through the device to be measured (resistors, diodes, LEDs…), and the voltage drop will be detected at the A2 port of the Arduino.
If the resistance range is 100Ω. At this time, the reference current of 2.5mA flows through the resistor, and the generated voltage is applied to the ADC input of the Atmega chip. We want to measure the resistance varying between 0-1000Ω. The measured voltage also varies between 0-2.5V. The voltage range of Vce (Vds) is 0.5V-3V, and these two voltages will directly affect the collector-emitter/source-drain current, ultimately leading to a decrease in accuracy. This phenomenon can be well understood through this diagram: the transfer characteristics of NPN bipolar junction transistors. This effect can be corrected by software to a certain extent, but if it is nonlinear, correction can be very difficult.
7. How to measure the beta value: The current generated mentioned earlier can flow through different devices: resistors, diodes, LEDs, Schottky diodes…, and the generated voltage drop is related to the corresponding device. This voltage value can provide information about the corresponding device, such as the voltage drop of the device and its corresponding: diode – 0.4V – 0.8V; Schottky diode – 0.1V – 0.5V, LED (different colors vary) – 1.1V – 3.5V. The current used is 10uA. If using 2.5mA, the LED will start to light up, allowing for measurement of the LED’s functionality. The measurement of the beta value also uses 10uA, and the measurement can be read on the screen.
8. Required material list.
9. PCB design files.
10. After the PCB files are completed, they will be sent to the foundry for processing, and I received the finished product after two weeks.
11. Installation and soldering, this is not a big problem; carefulness and attention to detail are key.
12. Some explanations on the PCB.
13. Install components.
14. Configure the Arduino, it is necessary to configure both the mode selection and the multimeter software, and the related configuration files can be found at http://www.instructables.com/id/Digital-multimeter-shield-for-Arduino/. 
15. Calibration is also needed. Find a standard multimeter for relative calibration.
16. Measure battery voltage, you can see the selected range and voltage value information.
17. Calibrate the ohmmeter.
18. Once completed, testing can begin.
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