Reverse Engineering Cheap Wrist Watch LCD

When I was a child, I once attempted to reverse engineer my wristwatch with the hope of integrating it with LED seven-segment displays. At the time, my knowledge of electronics was limited, especially when it came to working with LCDs. In my enthusiasm, I ended up peeling off everything from the display, including the reflective surface and the polarized film on both sides. The tiny indium oxide traces, which connect segments of the display to various pins, were barely visible. Unfortunately, I couldn’t map out which segment corresponded to which pin, and the project stalled.

Fast forward about 10 years, and I came across 1-inch LED seven-segment digits. Instantly, I knew I wanted to build a clock using these components. Rather than use a dedicated clock IC, I had the idea to repurpose the chip-on-board (COB) module from a cheap wristwatch as the brains of my clock. My first step was to search online for similar projects. To my surprise, I couldn’t find a single guide or project that aligned with my idea.

Undeterred, I purchased the cheapest wristwatch I could find and set out to reverse engineer it. Upon disassembling the watch, I found three key components: the PCB with the COB, the LCD display, and a ribbon conductor. My initial plan was to connect the PCB traces to a logic analyzer to capture the data sent to the LCD and figure out the segment layout. However, I didn’t own a logic analyzer at the time. So, I turned to my trusty Arduino UNO, planning to use it as a makeshift logic analyzer.

During my research, I learned that the ATmega328 microcontroller in the Arduino UNO interprets voltages between 1-1.5V as logic low (0) and anything above 3V as logic high (1). The problem was that my watch operated on a single 1.5V coin cell battery, which made direct interfacing with the Arduino tricky. To resolve the voltage mismatch, I considered using a level shifter. However, the electronic components available in my area were limited, and I couldn’t find a level-shifting IC. The alternative would involve building a level shifter with resistors and transistors, but that felt overly complicated for this particular task. I prefer straightforward approaches.

Then, as often happens during moments of clarity, an idea struck me while I was in the bathroom. Instead of capturing the signals with a logic analyzer, I could reverse engineer the LCD directly by applying voltage to its pins and manually tracing which pins controlled which segments. Since applying direct current (DC) to an LCD can damage it, I needed to use alternating current (AC) at around 1.5V.

To generate this low AC voltage, I modified an old 9V transformer by rewinding it to output 1.5V-0-1.5V (with 3V AC intended for future use with larger LCDs). I created makeshift probes from an empty ballpoint pen and a small stick, and began testing each pin of the LCD to determine its corresponding segment. The image above shows the pin-to-segment mapping I discovered.

While I successfully reverse-engineered the LCD and identified which pins on the COB control which segments, I encountered a major obstacle: my LED seven-segment displays couldn’t be used with the COB. The reason lies in the fact that my LED displays have a single common pin, while the LCD relies on two backplanes. Each backplane drives different digits, making it impossible to interface my LED displays with the COB.

Despite this, I wanted to share my findings here on Instructables, in case someone else is interested in reverse-engineering an LCD display or finding a creative solution to a similar challenge. While I didn’t achieve my original goal, the journey was a valuable learning experience, and I hope this information will be helpful to others.