One big change since I did the first version of the clock is my access to professionally made PCBs. At the time, I was only able to produce PCBs via hand etching or using my home-made PCB mill. A board like this requires at least a double sided design which is not easy using the above methods and so I used veroboard. This is painfully slow and messy.
For the new version I will use a professionally made two layer PCB.
The old version was “dumb” in that it contained no programmable logic to keep time. Discrete logic chips were used to count cycles of the mains supply and decode this number to drive the Nixie tubes. It also used the mains supply voltage to light the Nixie tubes which is not very safe (as discussed in a previous post). The new version is quite different.
The new design can be broken into five parts.
High Voltage Supply
This section uses an MC34063 boost converter to step-up the +12V to +180V for the Nixie tubes. This way, the high voltage is isolated from earth and is much safer. Greater detail here.
A DS32kHZ chip from Maxim which is a 32.768kHz Temperature-Compensated
Crystal Oscillator, accurate to +/-7.5ppm or four minutes per year (1 minute if you pay a bit more), is used to generate a steady clock signal.
The design features 29 high voltage bipolar transistors to switch the current in each digit of each Nixie tube.
There are four 74HC595 8-bit shift registers. The outputs of these are connected to the transistors. This way all 29 characters can be controlled using only 3 wires (more detail in an upcoming post).
This section uses an ATTiny87 to bring it all together. Time is counted from the DS32kHZ chip and data is shifted to the shift-registers to control the Nixie tubes and display the time. LEDs are included for debugging. Two debounced switch inputs are used to set the time.
The board has been designed, fabbed and soldered.
All sub sections have been tested and proven to work individually. Now it all just needs to be brought together!