“This is a vintage VFD tube clock that uses Ethernet for both power and data. The power is provided using 802.3at PoE+ and a Molex PD Jack that contains both integrated magnetics and a PoE Type 2 PD controller. The IP stack runs on a Microchip PIC18F67J60 microcontroller that has an integrated Ethernet MAC and PHY. The IP stack includes DHCP, DNS, NTP, and LLDP functionality.
The clock powers up and receives its IP address and a DNS server address via DHCP. LLDP is used to negotiate 7.5 Watts of power. The clock performs a DNS lookup to get an NTP server address from pool.ntp.org. Once the clock has the address of an NTP server, it uses NTP to set the date and time. With the date and time set, the clock displays the time using the 6 VFD tubes and three MIcrochip HV5812 high-voltage display drivers. Once an hour, the clock corrects any drift using NTP.
Vacuum fluorescent displays occupy a niche in history between their high-voltage Nixie predecessors and their expensive-at-the-time LED successors. They can be identified by their relatively fast response times, wide viewing angles, and characteristic blue-green glow.
The dual-axis inclinometer photo above shows a relatively modern vacuum fluorescent display. This display is a drop-in replacement for a character LCD module. It’s visible at night, in bright sunlight, and from any angle unlike the LCD it replaced. This display module is circa 2008. Thisinclinometer was built to show the pitch and roll of my Jeep long before vehicles included this features in their instrument clusters.
This clock project uses even older VFD tubes. The tubes are available in a variety of formats. In the photo above are two styles of VFD tubes. The tube on the left is an IV-11 display tube from 1989. The tube on the right is an IV-12 display tube from 1991. The only real difference between these tubes is the IV-11 on the left has a right-hand decimal point and wire leads while the IV-12 tube on the right lacks a decimal point altogether and has pins to mount the tube in a socket.
A Nixie tube requires 170 V DC to drive the tube’s anode. Vacuum fluorescent display tubes only require 25 V DC. As a result, they’re much easier to drive. Both tubes have a grid, a heater, and an anode per display segment. The electrical requirements for each are displayed in the table below:
Volts Minimum Current Nominal Current Maximum Current Heater 1.5 V 90 mA 100 mA 110 mA Grid 25 V 12 mA — 17 mA Anode / Display Segment 25 V 3 mA — 5 mA
With six tubes, the worst case heater current consumption is 6 tubes * 1.5 V * 0.110 A = 0.99 W. With six tubes, the worst case grid power consumption is 6 tubes * 25 V * 0.017 A = 2.55 W. With six tubes, the worst case segment power consumption is 6 tubes * 7 segments * 25 V * 0.005 A = 5.25 W. A six digit clock is then going to need a 1.5 V power supply capable of supplying 0.99 Watts and a 25 V power supply capable of supplying 7.8 Watts.
I decided to use the IV-12 tubes with the pins for sockets on their base. In the event of a mistake on one of my PCBs, I could unplug the tubes and re-use them on a corrected board. In the event of a failed tube, I could swap tubes on the project without significant PCB rework.”