This time instead of using a rotating disc or arm like I've done in the past I've opted to use a wand that oscillates back and forth to write out the image. I got the idea after seeing a commercial clock that uses the same mechanism. But you know it's no fun buying something ........ you have to make it to fully enjoy it!
So I stated off by testing out several methods to generate the oscillations. I took apart several harddrives to harvest the armature coils and magnets only to find that they couldn't bear the load of a long wand and still oscillate quick enough and with large enough magnitude to be useful. So I dug into my junk bin to see if I could find an idea.
I found a stiff yet springy piece of metal, a 'U' shaped plastic cut from a DIP tube, some coils from the brushless spindle motor in an old VCR I took apart ages ago, and two neodymium magnets from an old name tag. These would be the main components that would make up the armature and electromagnetic assembly. The springy metal is secured to the end of the plastic tube and the magnets are fitted on either side of the tube an inch or two above the metal. It looks something like this:
The leds are soldered to a little piece of perfboard and connected down the length of the tube with magnet wire. Here is a few pics of mechanism and leds working:
Warning!!! Physics lecture incoming: so here is the general concept of how the mechanism works. The springy metal is secured to the base, and the coils are mounted on either side of the two magnets on the arm. A microcontroller with an adjustable oscillator drives a transistor which switches the coils which are wired in parallel. This kicks the magnets and the armature every once and a while inserting kinetic energy into the system (it's essentially a forcing function and a mass spring system which can be described by a simple differential equation). By tuning the rate of the driving electromagnetic pulse to the resonant frequency of the spring and mass armature you can get it to sustain a large amplitude oscillation. I knew Physics would come in handy one day!
So, now I have a reliable oscillating mechanism as seen in this video:
And I have managed to get the leds all wired and working with a simple demo going:
So all I need to do now is add a photointurrupter to sense when the arm is at the starting position so that I can sync up the display and then code software for a character set and animations as well as add a real time clock chip. Oh and finish the case and solder the circuit. Don't worry I'll keep you all updated in the next upcoming steps for this project and I'm planning on releasing all of the code and schematics just because you guys are awesome. Stay tuned for the next thrilling installment of POV Clock!
UPDATE!!! I've used the electromagnetic pulse controller's led output as a sync signal to varying degree to display the ceremonious "HELLO" message as a first text demo as seen below.
Note that I am seeing some problems with the low refresh rate manifesting itself as an annoying flickering. I'll have to think something up to fix this problem. Until then!