From Test Equipment to Timepiece: Engineer Crafts HDSP-2000 LED Matrix Into a Wristwatch
Vintage LED alphanumeric character modules in DIP package converted into an ATtiny85-powered wristwatch.
When analog design engineer Vitalli encountered a distinctive LED display, the first thought that came to mind was a wristwatch. The HDSP-2000 Wrist Watch, with its distinctive vintage matrix LED display, stands out as a captivating digital timepiece.
This wristwatch features two HDSP-2000 displays. Avago (HP) introduced these in 1975. Originally, the modules were intended as alphanumeric displays for test equipment and business machines. Each module has 4 characters in a 5x7 LED array with red, yellow, and green options packaged as a 12-pin ceramic DIP. Instead of driving the array directly, each column has registers (28 bits total) that load similarly to the SPI protocol. This application note has more information on driving these unique displays.
The side-by-side modules smoothly scroll the current time, day of the week, month, year, and state of charge. The watch, while having a limited feature set, is fully functional. It can display or program the date/time. Vitalii's decision to focus on the LED matrix ensures a satisfying visual time-telling experience.
A Microchip ATtiny85 microcontroller (MCU) operates the wristwatch's circuits. Despite having only 5 GPIO pins, clever design techniques accommodate driving the HDSP-2000s, accessing the DS1337 Real-time Clock, and reading the state of two user-accessible push buttons. For example, the push buttons share a pin by generating an analog voltage to indicate which button is active.
An LTC4054-based circuit handles a Li-Po battery cell. The power section also includes a DC-DC boost converter to provide the 5 volts the HDSP modules require. A single dedicated LED indicates the battery's charging state.
You can download the AVR code and Altium design files from the HDSP-2000 Wrist Watch GitHub repository. The code includes the firmware for the ATTiny85, while the Altium design files provide a schematic and PCB layout of the watch's circuit. This project log provides an in-depth explanation of the watch's circuit.