Electronics project opinions...

I’m thinking about embarking upon a new electronic project; a scrolling message display, but with a twist.

Now that laser pointers are down to a quid each, I thought I could strip the diode out of, say, eight of them, align them so that they are projecting onto a rotating mirror-faceted cylinder and project the scanned beams across a wall; it ought to be fairly simple to drive them with from logic inputs using fast-switching transistors so that the thing projects dot-matrix letters instead of just straight lines.
If I slightly adjust the angle of each mirror facet, I can even get an interlaced scan and have more than eight vertical pixels.
I’ll be fixing an opto sensor on the shaft so that I can detect the start of a scan cycle

So… How to modulate the beams; I originally thought to do it with shift registers, but this would entail rather a lot of chips for no real reason; access is only required to the currently projected vertical pixel line, so I thought maybe just a bit of RAM; drive the address bus from a counter (in turn driven by a timer), reset the counter by the signal from the shaft encoder. All that is required then is to tune the timer to the right frequency that the right amount of addresses are cycled in one scan.

I haven’t yet worked out how to easily update the contents of the RAM between cycles.

Or should I use a microcontroller? Or something else?


I thought you were talking about telepathic machines.


Have you tried working with BASIC Stamp microcontrollers? http://www.parallax.com They are much preferred to doing everything with shift registers, timers, etc. They are relatively easy to program.

You will want to multiplex the signals to the LEDs. Here is one method you might adapt: Jon Williams Nuts & Volts #70 There’s a yahoo group concerning the BASIC stamp, and I recall seeing a project a guy had done where he mounted a line of LEDs to a ceiling fan blade, and used the Stamp to make it display text. There was a video. It was really cool. I think it was sometime in 2001–you can search the archives.

There’s probably no need to take apart laser pointers: you can buy the laser diode alone from various electronics supply vendors, for example, Digikey, Mouser, Newark, perhaps others in the Kingdom of Butter.

There should be no need to multiplex the LEDs(actually laser diodes) because there are only 8 of them and their beams are scanned horizontally across the projections surface, I only need to modulate them as the scan progresses (although many of the principles are similar to that of a multiplexed array).

I can get the laser pointers for a pound; the cheapest laser diode I’ve seen sold as a component was nearly a fiver.

I was wondering whether I could actually drive it straight from the PC’s parallel port (with optoisolators and a few other bits) - the machine that runs it won’t need to be running any other software and could even be a DOS machine.

I’ve done some preliminary tests and unless there’s actually a hardware latency problem or something, I can easily push the data out to the parallel port more than fast enough to drive the thing in real time.

The advantage of this is that the PC driving the device will also be the PC on which the text entry interface resides, so there are no interfacing and data transfer complexities. I’ve hauled an old but serviceable PII 300 off the junk heap at work, wiped it down and installed DOS only (actually moved the modified contents of a Win98 boot disk onto it).

I’m going for a mirror drum consisting of eight mirrors which will be finely adjustable, allowing me to set them with a progressive tilt - during a complete revolution, each of the eight laser beams will describe eight closely-spaced lines on the projection surface, giving me a total of 64 vertical scan lines (or pixels when modulated), the horizontal resolution will be somewhere around 800 pixels - enough for some really fancy-looking text and graphics.