# Open and rebuild a multimeter firmware

Reconstructs the meter application and bootloader as editable C, then tests it on the meter including Bluetooth.

## Hardware
EEVblog 121GW

A rebuilt meter application runs on the real multimeter, including Bluetooth.

## Shopping list

Experimental open firmware is not a calibration or safety certification. Keep all test leads disconnected while servicing; do not use modified firmware for hazardous-voltage measurements.

Quantities are for one build. Unless marked source-specified, they are planning quantities; confirm kit contents and the chosen configuration before ordering.

### 1 × EEVblog 121GW
- Requirement: Required
- Quantity basis: Planning quantity for one build; check the selected configuration
- Purpose: Exact meter target.
- Destination: https://eevblog.store/products/121gw-multimeter
- Match: Exact product
- Link review: Page checked; stock not assessed
- Evidence: Creator source
- Source: https://circuitdigest.com/news/eevblog-had-claude-code-rebuild-his-121gw-multimeter-firmware-in-c-overnight-and-it-works
- Source: https://github.com/EEVblog/121GW-Multimeter-Firmware-RE

### 1 × microSD card
- Requirement: Required
- Quantity basis: Planning quantity for one build; check the selected configuration
- Purpose: Firmware, operating system or project storage.
- Notes: The README documents the SD bootloader update path; match supported card formatting.
- Sourcing: No exact purchase link verified; confirm the model and requirements above before ordering.
- Evidence: Creator source
- Source: https://circuitdigest.com/news/eevblog-had-claude-code-rebuild-his-121gw-multimeter-firmware-in-c-overnight-and-it-works
- Source: https://github.com/EEVblog/121GW-Multimeter-Firmware-RE

### 1 × microSD card reader
- Requirement: Tool
- Quantity basis: Planning quantity for one build; check the selected configuration
- Purpose: Write the update file.
- Sourcing: Use existing equipment or match the specification above. No exact purchase link verified.
- Evidence: Setup requirement — check against your configuration
- Source: https://circuitdigest.com/news/eevblog-had-claude-code-rebuild-his-121gw-multimeter-firmware-in-c-overnight-and-it-works
- Source: https://github.com/EEVblog/121GW-Multimeter-Firmware-RE

### 1 × Computer for setup
- Requirement: Tool
- Quantity basis: Planning quantity for one build; check the selected configuration
- Purpose: Build, configure or flash the software.
- Sourcing: Use existing equipment or match the specification above. No exact purchase link verified.
- Evidence: Setup requirement — check against your configuration
- Source: https://circuitdigest.com/news/eevblog-had-claude-code-rebuild-his-121gw-multimeter-firmware-in-c-overnight-and-it-works
- Source: https://github.com/EEVblog/121GW-Multimeter-Firmware-RE

### 4 × AA batteries
- Requirement: Required
- Quantity basis: Source-specified quantity
- Purpose: Power the meter.
- Notes: Manufacturer lists four AA cells; use fresh batteries for updating.
- Sourcing: No exact purchase link verified; confirm the model and requirements above before ordering.
- Evidence: Creator source
- Source: https://eevblog.store/products/121gw-multimeter

### 1 × SWD programmer and debug leads
- Requirement: Optional
- Quantity basis: Planning quantity for one build; check the selected configuration
- Purpose: Alternative development/recovery route.
- Notes: Not required for the normal SD update path. Requires opening and correctly connecting the meter.
- Sourcing: No exact purchase link verified; confirm the model and requirements above before ordering.
- Evidence: Creator source
- Source: https://circuitdigest.com/news/eevblog-had-claude-code-rebuild-his-121gw-multimeter-firmware-in-c-overnight-and-it-works
- Source: https://github.com/EEVblog/121GW-Multimeter-Firmware-RE

Source reviewed, not assembled or hands-on tested. A working product page does not confirm stock, firmware compatibility or a complete build.

## Setup
Unofficial firmware

## Coding tool
Claude Code

## How the agent helped
Article describes the overnight Claude Code job, emulator comparison, Ghidra/GCC workflow and Dave Jones testing the meter.

## Before you build
An enabling experiment, not a new everyday feature. Do not rely on modified firmware for safety-critical measurements.

## Sources
- Original build: https://circuitdigest.com/news/eevblog-had-claude-code-rebuild-his-121gw-multimeter-firmware-in-c-overnight-and-it-works
- Supporting source: https://github.com/EEVblog/121GW-Multimeter-Firmware-RE

Source reviewed 2026-11-04. Not hands-on tested.