Wednesday, November 5, 2008
VFD (Vacuum Florescent Display) Clock
After a certain someone nearly threw our 'alarm clock' (mobile phone) out the window the last time Britney Spears woke us up (Blackout has GREAT wake up songs) it was decided that perhaps we should have our own alarm clock. Since a friend inherited 15,000 of said VFD tubes (Russian Made), and had some odds and ends from an electronics factory that were no longer being used; we decided it would be interesting to solve our 'alarm clock problem' with these.
Components are new but have been rejected from the assembly line in the electronics factory, normally due to bent pins and other mild deficiencies. We saved these throw-aways and used them for the clock. The circuit boards used were custom designed and manufactured using a rapid prototyping process (CNC Machining). Three plates (boards) were specifically designed; one for holding the displays and interface button, one for the clock electronics and one for the speaker, and to act as a base for the entire clock.
The processor uses a 16mhz crystal to generate the time signal. The 16mhz signal is first divided by eight and then by two thousand to generate a one second pulse. The program counts the seconds to form the minutes and hours. These are then displayed on the tube.
The interface was designed to give the user a fluid experience when setting time, alarm, and snooze features. The time is set by holding down button one, which stops the dot flashing. Once the dot stops flashing, the time can be programmed. Hours are programmed with the left button (1) and minutes with the right button (2). To set the alarm, the same process occurs but initiated by the right button (2). When the alarm goes off in the morning, there is a snooze feature, which can be 'snoozed' by pressing left (1). To turn off the alarm, hold down the right (2) and press left (1). The best feature? An innovative snooze twist, the clock flashes between time display and the word "Sn" when it is snoozing! Finally you know if your clock is on snooze or if its turned off... We are working on a new feature which may display how many minutes are left between the last snooze and the upcoming buzz. Speaking off, the buzz is hopefully going to be replaced by our great Arduino synth which is capable of outputting some truly fantastic music. We are also considering a dimmer function, the VFD tubes are capable of having their brightness adjusted, and the circuits are prepared for a light sensor, we just have to work it into the interface!
The electronics for driving the VFD tubes is a bit tricky, it needs a slightly higher voltage than the normal 5V signal used for typical LED clocks. The VFD uses 35V as well as a 1.2V filament voltage. Apart from driving the clock and user interface, the processor also has to control the generation of these voltages. Each VFD tube works like a regular 7 segment display, and are actually driven exactly the same way. The processor switches rapidly between the four tubes, so that only one tube is lit at a time (multiplexing). As such you can see eleven little legs, (pins) eight of them control the anodes (segments) within the tube. Two of them have filaments connected between them, and one has a grid which enables or disables the entire tube. All the anodes for the four tubes are connected in parallel and the processor uses the screen to switch rapidly between the tubes. This makes it possible to write individual digits on each tube using only eight common pins.