Project Introduction

The Hobby-Scale Digital Multimeter project represents a comprehensive exploration of embedded systems design within the EEL 3923C Electrical Engineering Design 1 course at the University of Florida. This project integrates fundamental concepts of analog and digital electronics, focusing on creating a functional measurement tool that demonstrates practical engineering skills. Rather than aiming for commercial viability, the project emphasizes learning through hands-on implementation of various electronic concepts.

Design Journey

The project began with a clear set of requirements: create a multimeter capable of measuring voltage, current, resistance, and providing continuity testing. The design process prioritized educational value, incorporating multiple communication protocols and analog-digital interfaces. Key design considerations included selecting appropriate components that would balance accuracy with educational value, leading to the choice of the Raspberry Pi Pico as the main microcontroller for its accessibility and robust feature set.

The system architecture was carefully planned to integrate several key components: a 16-bit ADS1115 ADC for precise measurements, an LCD display for user feedback, and an audio system using the LTC1661 DAC chip coupled with an LM386 amplifier. The power system was designed around the LM2940 voltage regulator, converting 9V input to a stable 5V supply for the Pi Pico.

Prototype 1 Implementation

The first prototype successfully integrated all core functionalities through a combination of hardware and software solutions. The system uses differential pairs on the ADS1115 for various measurements, with dedicated channels for voltage, current, and resistance readings. A button interface allows users to switch between measurement modes, with LED indicators providing clear visual feedback of the current operating mode.

Technical Specifications

The implementation utilizes the Pi Pico's versatile I2C and SPI capabilities. The ADS1115 and LCD display share an I2C bus with different addresses, allowing efficient communication. The DAC system, implemented through SPI communication to the LTC1661 chip, generates distinct waveforms for each measurement mode, providing audio feedback through an 8-ohm speaker.

Hardware Implementation

The analog front-end employs careful consideration of voltage dividers and current shunts for accurate measurements. The power regulation system, built around the LM2940, ensures stable operation from a 9V source. The audio feedback system combines the LTC1661 DAC with an LM386 amplifier configured for 20x gain, providing clear audible feedback for different measurement modes.

PCB Design

The PCB layout was created in Altium Designer, incorporating best practices for mixed-signal design. Special attention was paid to component placement and routing to minimize interference between analog and digital sections. The design includes proper bypass capacitors for stable operation and considers the separation of analog and digital grounds.

Results and Testing

Initial testing demonstrated successful implementation of all core functionalities. Voltage measurements showed good accuracy within the design parameters, while current and resistance measurements performed reliably across their intended ranges. The continuity testing function, complete with audio feedback, proved effective for basic circuit testing.

Challenges and Lessons

Several key challenges emerged during implementation. The DAC-SPI communication required careful timing considerations, and the audio amplifier circuit needed optimization to reduce noise. These challenges provided valuable learning opportunities in debugging mixed-signal systems and understanding the importance of proper PCB layout techniques. The project also highlighted the critical nature of proper power regulation and the importance of systematic testing procedures.

The first prototype successfully met its educational objectives while providing a foundation for future improvements. The implementation process revealed both the strengths of the chosen architecture and areas where enhanced performance could be achieved through design refinements.