The clock uses the 50Hz mains signal to keep time. Therefore, we need some kind of circuitry to count the pulses, and convert them into minutes and hours.
For this I am using a series of CMOS logic 4017 Decade Counters. These chips basically count from 0 to 9, incrementing at each pulse on the Clock pin, and output a pulse once per cycle. However, they can be forced to have a shorter cycle, e.g. 0 to 5.
Therefore, to convert a 50Hz pulse into a 1Hz pulse, I pass it first through a decade counter from 0 to 9, producing a 5Hz pulse, and then pass that through a decade counter from 0 to 5, producing a 1Hz pulse.
This process can be repeated until I have a pulses with period 1 minute, 10 minutes, 1 hour and 10 hours. The counters generating each of these pulses can then drive the Nixie tubes.
This could all be done on a micro-controller very easily, and this would reduce the chip count from about 9 chips to 2 chips, however that is a bit boring and this way I learn more about digital logic. The result of this is some pretty complex wiring.
My original plan was to use my CNC to produce a single PCB for the logic board, however I found that the schematic was so complex that the board would either have to be double sided, or the traces be very thin. Both of these options are not feasible on my CNC, so the possibility of having a PCB was out the window.
Making the circuit on stripboard is much more fiddly and slow, with every connection having to be wired individually, however it actually worked out relatively compact and I’m happy with the results. Of course, if I wanted to produce multiple boards, it would make much more sense to design a PCB and get it made professionally.
Because this project operates largely at mains voltages, it is important that the circuit is not exposed when powered on. I also want to make sure that the board doesn’t slide around on my desk, potentially touching me. So I quickly put together a stand for the board to give the board a bit of weight and hold the board above the desk to prevent shorts (e.g. due to touching a screwdriver). So far this has been effective!
The last two months have consisted mainly of the stress of exams. Meanwhile I have been working on my next project… A Nixie Clock!
A Nixie Clock is a clock which uses Nixie Tubes as the display (mine were made in 1984 in the USSR). Mike’s Electric Stuff, has a good page on Nixie Clocks, if you want to read more. In fact, I am basing my design on Mike’s design, with some modifications, e.g. making it a 24 hour clock.
The circuit counts mains cycles (50Hz in the UK) and uses that to keep time. While this may not sound like the most accurate method, it is. The grid frequency is controlled such that, over the course of a day, the average frequency is exactly 50Hz to a very high accuracy.
The tubes are powered from a rectified mains signal and are switched with transistors.
Having tested the logic circuit on a breadboard, I am now working on building the full logic and power circuit on strip board. The Nixie Tube control circuits will be on separate PCBs.
Because the circuit is build using mainly mains voltages, it is imperative that I take caution when powering and testing the device. To ensure safety I have:
Installed a 3A fuse in the plug (the smallest I had)
Installed a 1W fusible resistor in series with the circuit
Mounted the board on a block of wood with the traces facing downwards. (This ensures that the board doesn’t slip and short/electrocute me)
Been very careful to ensure that the power is off when touching the board.