Schematic & PCB Design for ATTiny85 Microphone Switcher with Active Filter

Project Overview: I need a freelance electronics engineer to design a simple, modular schematic and PCB layout for a custom radio/PA interface using standard, easily sourcable components. Circuit Architecture Requirements: The design must use three distinct, dedicated stages: 1. Logic Control: An Atmel/Microchip ATTiny85 microcontroller that monitors two hardware push-buttons (Volume UP/DOWN) and controls a 10-second timeout loop for an external 12V DPDT audio routing relay. 2. Volume Control: An MCP4131 digital potentiometer controlled via SPI from the ATTiny85 to adjust analog microphone audio levels. 3. Audio Filtering: An LM324 Op-Amp configured as an active bandpass filter optimized for voice frequencies 400Hz to 4000kHz to clean up audio signals before they reach a Public Address (PA) amplifier. Required Deliverables: • Clean, verified circuit schematic (KiCad, Eagle, or Altium format). • Fully commented C++ / Arduino framework source code tailored for the ATTiny85, managing the MCP4131 SPI steps and relay timer. • Production-ready Gerber files for PCB fabrication Upwork. • Complete Bill of Materials (BOM) Excel spreadsheet with component supplier links Upwork.