Build a 10$ X Y laser scanner !

First version

The goal is to build a very low cost X Y scanner for a laser pointer, this X Y scanner must be able, from a PC, to display fixed or animated simple pictures or some characters.

Problem is, of course, to build very fast actuators. Ultra fast galvo are very expensive. Here are the solutions I tested:

CD player actuator :

I take an old CD player: the lens coil actuator is used to set the distance between focal lens and CD. I remove the lens, glue it again but with the axis of the lens perpendicular to actuator motion. Then I dispose a second lens : the same as the first one, on the same axis, at a few mm from the first but fixed. When the actuator move in X or Y direction, laser beam is deflected.

Then I built 2 low cost 4 bits DAC with 8 resistors : 40 k, 20 k, 10 k, 5 kohms, with an operational amplifier connected to the actuator coil. The 8 bits are connected to the PC LPT1 // output port.

It works but frequency limit is about 10 Hz and distance between lens must be adjusted depending from distance between scanner and target.(focus adjust)

Piezo buzzer :

I took 2 low cost standard piezo (about 20 mm diameter, 0.3 mm thin metalled piezo disk) and I sold the piezo 2 on the piezo 1. Then on the piezo 1, I glue a CD player lens in front of another fixed CD player lens.

It works : at 3 m, spot can be moved on a few cm but a lot of inertia.

I improved the system attaching the first lens on the first X piezo and the second lens on the 2nd Y piezo : better but maximum frequency is still a few Hz.

8 Mirrors drum :

I didn't realise it but we can perhaps install 8 mirrors on a cylinder (not very far from laser printer): every 360/8 °. First mirror is perpendicular 90° from axis, 2nd is 91°....8th is 98°. So when the cylinder rotate : drum scan 8 // lines and is able to write characters. It works like a TV receiver, but with only 8 lines in vertical resolution.

Problem : it's a dot system (compared with previous vector systems) and laser spend a lot of time "OFF" : for the same display, total light will be less than for a vector system... But drum can rotate as fast as we want...

Hard disk head actuators :

Hard disk are very well made components very easy to get free. Hard disk head must rotate very quickly with angle like +-20°. Exactly like scanners! Using the hard disk electronic is really very difficult. We power the hard disk coil with a little audio power amplifier IC. We remove hard disk heads and corresponding piece of metal (to decrease inertia) and we glue a about 10 x 10 x 5 mm mirror (from an old photocopier) on the axis. In fact we glue it upper on the axis in such a way, mirror surface where Laser beam is reflected is aligned with axis : we can use very little mirror. On an hard disk, there is no medium position we must add a spring. First we add 2 coil springs but coils vibrate so we replace coil spring with "flat" spring.

Well, it works well but again it's hard to be better than a few Hz maximum frequency

Loudspeakers :

Here is my today best solution :

I use 2 very common little loudspeakers (diameter is 80mm). On each one, on the radius of the loudspeaker, I dispose a first little piece of wood (a match ;-) (50 mm long perpendicular to loudspeaker axis), glued on the diameter and glued on the centre of loudspeaker (through another little piece of wood 10 mm long on the loudspeaker axis). Then I glued a little mirror on the 1st piece of wood, near the diameter of loudspeaker. When the loudspeaker vibrate, the first piece of wood rotate and the mirror too.

I then install the 2 mirrors in front together but loudspeaker 90° decay.

I install my laser (red 30 mW) (simulated beam):

I build 2 x 4 bits DAC with 100 k, 200 k, 400 k, 820 k resistors with 4 x LM386 little power audio amplifiers (bridged)

Then, a little C software: biosprint(0,17,0); or outport(0x378,17); and I am able to display simple characters !

By night, I use it to display a phone number, 2 figures at a time, on a stone wall 100 m far away!

Last Christmas, I also displayed a red animated running Santa Clauss...(Father Christmas)

4 bits wasn't enough: I modify electronic to have 6 bits and to be able to light on and off the laser.

In the program, care should be taken with delays for each point to prevent overshoot : When a line must be displayed, you can't simply send Datas to scanner : go from 0,0 to 20,0. You must go to 0,0, then wait 1 ms, then go to 1,0, wait 1 ms...A good trick : when delays are correct and overshoot minimum, loudspeaker produces minimum noise, you can hardly hear it, making a sort of "buzz".

Vu-meter :

Vu-meter or galvo used to measure mA or uA technology looks like real fast Laser galvo. A little copper coil around a big magnet. They are cheap. We use different models without a lot of success. We add a strong spring to make it faster. Again, it works but not as fast as a loudspeaker.

DC motor :

I used also fast DC motors: those where there is no iron in the moving part: just elliptical copper coils. I also suppress the collector and add a spring : still not as fast as loudspeaker.

Comments :

We speak about loudspeaker but comment is the same for all other DC motors.

If 1 V in the loudspeaker move it 1 mm, 2 V move it about 2 mm. Moving is proportional to the tension. Is that right? not exactly. When you send a 20 Hz 10 V signal in a big boomer you can see it moving: a few mm. If you send a 20 kHz 10 V signal to a big tweeter, you can't see it moving, even if you apply the same power: moving is far less than 1 mm. To simplify, power required is about the product of the tension by the frequency. For a galvo scanner application, we are not interested in the power apply to the loudspeaker or other DC motor but only to its displacement. That means that, if we want to be very fast:

- We need powerful loudspeaker and powerful amplifiers (and corresponding noise for loudspeaker)

- We need to correct the signal (tension) apply to the loudspeaker: we must remote it with electronic current control (is it enough?)

Links :

Amelink laser page with bigger loudspeakers, bigger amplifiers...

http://spt06.chez-alice.fr/00/lasers.htm

E-mail :

03/06/05.