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SolderTime Desk Clock

Solder:Time Desk Clock

We had three workshops on building the Spinkenzie Labs Solder:Time Desk Clock at my office and made a timelapse of the construction process.

Kit notes

Soldering order

My suggested ordering of soldering:

  • 100 Ohm resistor (brown, black, brown)
  • Four 10K Ohm resistors (brown, black, orange)
  • Eight 0.1uF capacitors
  • Three diodes (be sure to check polarity)
  • Crystal (be sure to not short pins)
  • Five sockets (be sure to line up notch with U-shape on PCB)
  • Voltage regulator (solder one pin to hold it in place and lay it flat against heat sink)
  • Battery
  • Buzzer
  • Programming pins (12 normal headers, short side through the PCB. solder one pin then adjust for straightness)
  • Two buttons and power jack -- done together they make a nice triangle to support the PCB
  • Cut the Swiss pins into eight 7-pin segments and install on each of the LED modles.
  • Install the LED modules on the side opposite the components-- be sure to note the orientation of the "bumps"
  • Install the chips -- carefully bend the legs straight and pay attention to the U-shaped cutout on top of the chip.
  • Smoke test it!


  • If you want to hack new hardware onto the board, do not assemble the acrylic case. It is a press fit and once assembled very difficult to get to the prototyping area.
  • Be sure to sand or file the tabs on the case; otherwise it will break the acrylic when you force it.
  • RTC not advancing or running very slow? You might have bridged the crystal legs. They are very close together.
  • Part of the display flickers, flashes or doesn't appear? Check your solder joints on the 74138 chip for that region. Each handles 1 ⅓ of a display.



  • Schematic
  • Four 5x7 red LED panels, with normal column/row address lines.
  • Three 74138 octal decoders connected to the columns (4 unused on the last decoder)
  • PORTB0-6 directly source current to drives the 7 rows inputs
  • PORTD5-7 selects the column
  • PORTC2-3 selects the decoder (01, 10 and 00. 11 = all off)
  • PIND2-3 are pulled low for the two buttons.
  • Because the currently selected decoder sinks current from all the LEDs turned on in the current column, turning on a different number of LEDs causes varying brightness.


  • Program as Arduino Dsomething with ATmega328P, programmer Arduino as ISP
  • When compiled with Arduino 1.0.5, there is lots of flickering.
    • Change the constant to Timer1.initialize(100) to Timer1.initialize(50) in ST2_Setup.ino
    • Or use my improved firmware.

Improved firmware

My updated Arduino source files are The major changes:

  • Reworks the matrix code to use fewer magic numbers in the main loop. Hopefully easier to understand what is going on.

  • Removes (dead?) code in the startup and main loop. Perhaps left from debugging or other clock platforms.

  • Converts the font to use an ascii art bitmap for each character so that it is easier to edit. Now rather than defining characters like "127,9,25,41,70, // R", the font.c file has:
        {  // R
  • Converts from a 1-bit monochrome to 8-bit grayscale and adds per-pixel PWM support. Pulsing dots and dimming are now possible.

Clocks Classes 2014

Last update: November 8, 2020