The Casio CFX-400 scientific calculator watch was seen during the 80’s and this model was the best and last ever sold. It is said to be the ultimate nerdy engineers’ accessory. This watch can be seen in the image below.

This scientific calculator watch features an open source RPN under the GPL license and Algebraic functions which can be easily built. As shown in the images below, it contains 18-key keypad with a MENU keypad and two rows of 4 soft-function buttons that mapped to the 8 keys directly below the LCD which performs all the extra functions.

The device contains a 16-bit PIC24FJ64GA004 processor, macro-style keystroke programming mode, 8MHz maximum clock speed, calculator modes, 64KB EEPROM & Flash memory, 16×2 line dot matrix display, single/dual lithium CR2032 battery, UART based universal serial port, Microchip ICSP programming interface, and any standard watch band.

Figuring out how to mount the watch band is the hardest part. This has ended with 1mm PCB socket pins being glued or soldered to the board which allows the watch to be usable without a case and worked really well. The first prototype is shown below and the second prototype follows which looks more professional than the first as it uses the band from an old Victorinox Starlight watch. Along with the images is the battery compartment, a view under the hood, the right side view showing the universal I/O port, and a rear view with case & translucent case.

The construction includes supergluing the 4 pin holders in place on the board with the aid of needle point tweezers to hold them in place while gluing. As shown below, the 4 socket head screws are superglued in place on the LCD and the 4 pins were covered with a mixture of small amount of 2-part epoxy as shown below.

During the attachment of the LCD, the ribbon cable must be bent over several times to fit under the LCD while ensuring that the cable does not extend past the bottom edge of the board. Since the LCD connector is the highest component on the board, the cables should be ensured not to bend back over the LCD connector with the metal contacts facing down, the ribbon cable enters the connector. The black tabs on the bottom of the LCD edge are ensured to be bent slightly. Before closing the latching gate, the ribbon should be pushed all the way into the connector.

The battery holders are soldered onto the PCB with the desired type of mounting as shown below. The positive and negative pads are marked on the bottom silkscreen. By pressing the MODE button, a quick test can be performed to ensure the LCD is fully working.

The keypad overlay fits over the keys as shown below. It is placed over the switches and ensure that the switches are centered within the hole cutouts. In gluing the LCD in place, the bottom of the socket head screws should be ideally flushed with the underside of the PCB but the LCD should not be forced down to this level if it does not go.