Mini version of Scophony - its do-able

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Re: Mini version of Scophony - its do-able

Postby Klaas Robers » Tue Apr 05, 2016 11:36 pm

Gregory, be extremely careful with 50 mW lasers. That light output is really dangerous. I would advise you to do all your experiments with a 5 mW laser diode. Although this is also not absolutely safe, it is 10 times less dangerous.

As long as the system is scanning, there will be no problem. But during building and experimenting you want to have the scanning off for adjusting and measuring. Then a 50 mW focussed beam is far too dangerous and 5 mW is also bright enough to see what is happening.

Then finaly, if everything is tuned up, you might construct a scanning detector, such that when the scanning stops for whatever reason, the laser is switched off automatically. After that precautions you might replace the laser for a 10 x brighter result. And NEVER bridge the built scanning detector safety.
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Re: Mini version of Scophony - its do-able

Postby Gregory » Wed Apr 06, 2016 7:56 pm

Klaas Robers wrote:Gregory, be extremely careful with 50 mW lasers. That light output is really dangerous. I would advise you to do all your experiments with a 5 mW laser diode. Although this is also not absolutely safe, it is 10 times less dangerous.

As long as the system is scanning, there will be no problem. But during building and experimenting you want to have the scanning off for adjusting and measuring. Then a 50 mW focused beam is far too dangerous and 5 mW is also bright enough to see what is happening.

Then finaly, if everything is tuned up, you might construct a scanning detector, such that when the scanning stops for whatever reason, the laser is switched off automatically. After that precautions you might replace the laser for a 10 x brighter result. And NEVER bridge the built scanning detector safety.

Thanks Klaas for the warning, although I have an electronics background, I have not worked with laser diodes before, and can I ask you a couple of questions?

1) To have a reasonable brightness on a 8 cm x 6 cm screen (i.e. not to have to watch it in pitch dark but say shaded indoors), is 50 mw from a green laser diode about right, or can I do this with less. The optical system is as per post, two rotating mirrors deflected onto a diffused screen.

2) I have seen a couple of analogue modulator circuits for the modulation of the laser diode, both rely on current control - Is it feasible to use this for a DPSS green laser diode or must I go for a PWM system. The advantage of the analogue version is simplicity and quite high modulation frequencies, they say.
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Re: Mini version of Scophony - its do-able

Postby Klaas Robers » Wed Apr 06, 2016 9:12 pm

1. All depends on what you call "reasonable". Projection is anyway difficult, because unlit your screen will have the brightness of white in the room. The screen is white, isn't it? So "black" of the picture can never be darker than "white" in the room illumination. This spoils your contrast. All beamers suffer from the same problem.

Better is projection on a ground or etched glass window and viewing from the other side. Then you can make the space behind the window dark and black. The window acts as a diffusing lens and you will get much better contrast, darker blacks.

Anyway I can advise you to start and experiment with a 5 mW laser diode. When the scanning fails and the spot is on a stationary place on the screen, your eyes are not damaged immediately, which they will be using a 50 mW laser.

2. Modulating the current of a laser diode always is VERY unlinear and highly dependant on the diode temperature. Then PWM is the way out. Laser diodes can be switched with very high frequencies, this is the way writing in CD, DVD and BluRay is done. It is difficult to "dim" the laser for reading. To do that an HF signal (more than 1 GHz) is superimposed on the laser current. This is more or less PWM. The IR lasers we used for CD-players oscillate from nature, that was having luck. For the red and blue lasers (DVD, BR) this is not the case. I expect it also not to be the case for the green lasers

I wish you good luck, you will need it.
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Re: Mini version of Scophony - its do-able

Postby Gregory » Thu Apr 07, 2016 7:06 am

Found this circuit, among others for a 1 MHZ laser diode driver with analogue modulation. Needs one resistor to set up the maximum current and adjustment of the pot for the threshold.

modulator.doc
(197 KiB) Downloaded 307 times

Has anyone tried this or anything similar - would this work safely for DPSS green laser diodes around 20 mw to 50 mw, as it seems fairly simple.

Here's another one with shunt modulation

modulator 2.jpg
(84.28 KiB) Not downloaded yet

The pot is used to set up the threshold level and the test point is for checking the current level. The maximum current level is set by the value of R5.
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Re: Mini version of Scophony - its do-able

Postby Gregory » Sun Apr 10, 2016 6:34 am

Some progress made, now the line scan assembly & deflection mirrors have been mounted to the base, and also the laser diode with its mount. Both the laser mount and 2 deflection mirrors are adjustable.

As the only laser diode I have at the moment is a cheap one from a pointer I have quickly hooked up a crude laser driver consisting of a stable 5v supply with a 150R resistor in series to the laser diode. Should hopefully have some better laser diodes, when they come.

deflection mirrors.jpg
deflection mirrors.jpg (58.58 KiB) Viewed 5167 times
The photo shows the 2 deflection mirrors, with the screen removed


laser mount.jpg
laser mount.jpg (63.75 KiB) Viewed 5167 times
A rear view showing the laser mount and diode, as well as the line scan assembly mounted

Time to start testing, and the first test done was to adjust and check if the laser spot can be deflected to the screen and cover an area of 6 cms x 8 cmd by manually spinning the line scans and frame scan mirrors.

After adjustment the results gave a screen scan area of 7.5 cms x 5cms, but the spot quality is poor, probably due to the back surface mirrors and possibly the laser diode is badly focused on the screen as the pointer lens has been optimised for a much larger distance, suitable for a pointer. Also the edges of the screen give a poorer quality spot with reduced brightness compared to the centre of the screen - plenty of room for improvement.

As it is at the moment, it would probably be OK for 32 lines, but would need further improvement for 60 lines
Last edited by Gregory on Mon Apr 11, 2016 11:48 pm, edited 1 time in total.
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Re: Mini version of Scophony - its do-able

Postby Harry Dalek » Sun Apr 10, 2016 8:55 pm

Gregory looks very professional.

Yes any problem with a mirrors angle or such will show up mirrors are not very forgiving .

Again it will not stop any thing working ,you should be right any case love the look of it .
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Re: Mini version of Scophony - its do-able

Postby Gregory » Thu Apr 14, 2016 6:16 am

Started some testing - theres good news and bad news. The bad news is the motor mount on the frame scan does not run true, so I had to strip everything down and remove the machined bolt and replaced it again.

With the replacement it now runs true and after connecting the stepper motor driver driven from a signal generator it can run at the maximum required speed which for an 8 sided mirror at 25 frame per seconds would be (25/8) X 60 rpm = 187.5 rpm

Also manage to improve the spot quality quite a bit on the screen. The two stationary deflection mirrors were replaced with front surface mirror. I cut these from hard disk material, which looks like stainless steel and has a good mirror finish, but a pain to cut.

The second modification, I played about with different lenses in front of the laser diode, and got a good improvement with a smaller spot size using and old reading glass lens, so chopped a small square up and fixed it to the laser diode front.

After connecting the line scan motor I managed at last to get a reasonable but very dim raster - this televisor at the moment is only for viewing in pitch dark with no moonlight.

The second bit of bad news is the raster is distorted in the horizontal direction also which is far worse than the error in the vertical direction and I get the shape below instead of a rectangle for the raster

distorted raster.jpg
distorted raster.jpg (3.8 KiB) Viewed 5114 times


So the curved screen is a must and will need a radius equal to the distance between the screen and line scan mirror. This should correct the horizontal error and help with the vertical line distance error, although not cure the vertical completely.
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Re: Mini version of Scophony - its do-able

Postby Harry Dalek » Thu Apr 14, 2016 1:07 pm

Yes mirrors will be mirrors ! the less of them the better i found ...its a lot easier to just use the line and frame mirror but i know you want a compact design ...it must be the reflecting mirror giving you trouble the line one is made to be correct if you can make it long perhaps you could adjust its shape from behind to get the raster lines to go straight ...may also a laser printer curved lens ?
Well its something to think about and again will not stop it working .
The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.
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Re: Mini version of Scophony - its do-able

Postby Gregory » Sat Apr 16, 2016 9:46 pm

Managed to get the first raster, although as it is very dim, photo shows only with few lines. As soon the green laser diodes 30 mw and 10 mw come I should be able to have more pics, with higher definition.

raster1.jpg
raster1.jpg (48.38 KiB) Viewed 5011 times

The screen has now been changed and I made a temporary one out of cardboard so now the surface is slightly curved, to eliminate the shape error.
Also instead of the tracing paper for diffusion, polyester drafting film has been used, which gives a better image.

The line scan mirror works perfect, but I am still not happy with the frame scan mirror I made, needs something more accurate - have to re-think and maybe have another go at this.
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Re: Mini version of Scophony - its do-able

Postby Panrock » Sat Apr 16, 2016 9:53 pm

Impressive work. Well done so far! I'm watching this with interest.

I never did succeed getting a linear raster with my Mihaly-Traub rig many years ago. This also used a red laser.

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Re: Mini version of Scophony - its do-able

Postby Harry Dalek » Sun Apr 24, 2016 12:53 am

Well Done Gregory not much more to do to get a nice picture,do go with the PWM its really pretty easy and safer for your laser .
Again it looks really good wish i could build something like that so well.
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Re: Mini version of Scophony - its do-able

Postby Gregory » Sun Apr 24, 2016 6:51 pm

Thanks Harry - probably will go for the PWM, but I will build the output stage first, so if linear modulation does not work, I can go for PWM. I will be using shunt modulation, so the laser diode will be safe.

electronics1.jpg
electronics1.jpg (104.91 KiB) Viewed 4925 times

I can't do much on the laser driver until the laser diodes arrive so its time to look at the rest of the electronics. At the moment I have a stepper motor driver for the frame scan, a brushless dc motor driver for the line scan (both ready made modules), and externally I am using a 12v supply, a 5v supply and a signal generator to supply the clock pulse for the stepper motor driver

power supply-frames oscillator.jpg
power supply-frames oscillator.jpg (27.75 KiB) Viewed 4925 times

This is the circuit I am first going to try on the breadboard, which should replace the external 5v supply and signal generator. The circuit uses a standerd 7805 regulator with spike protection on both input and output and a series resistor to provide a soft start.

The frame oscillator uses a 555 timer. The stepper driver has been set to 1/16 steps so for 20 frames per second the clock frequency is
(200 (steps per revolution) x 20 (frames per sec) x 16 (micro steps per full step) /8 (faces of mirror) = 8Khz

The oscillator has two timing circuits, the one sets the frequency of oscillation (8 kHz) and the other sets the ramp rate, so the stepper motor will start with a lower clock rate and ramp up to the 8 kHz.

As soon as this is tested, and assuming it works, I will update the schematic with the correct values.
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Re: Mini version of Scophony - its do-able

Postby gary » Sun Apr 24, 2016 7:23 pm

You may have to go to a xtal controlled oscillator to get the required scan frequency accuracy in the final product, but this type of circuit should be fine for a first iteration.
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Re: Mini version of Scophony - its do-able

Postby Gregory » Mon Apr 25, 2016 7:20 am

gary wrote:You may have to go to a xtal controlled oscillator to get the required scan frequency accuracy in the final product, but this type of circuit should be fine for a first iteration.

Hi Gary, Are there any oscillator circuits, not too complicated that allow the frame scan oscillator to be synchronised to the frame sync, I can have a VCO if needed for the stepper driver.
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Re: Mini version of Scophony - its do-able

Postby gary » Mon Apr 25, 2016 11:23 am

Gregory wrote:
gary wrote:You may have to go to a xtal controlled oscillator to get the required scan frequency accuracy in the final product, but this type of circuit should be fine for a first iteration.

Hi Gary, Are there any oscillator circuits, not too complicated that allow the frame scan oscillator to be synchronised to the frame sync, I can have a VCO if needed for the stepper driver.


Gregory, there are a number of xtal osc circuits for NBTV given in the newsletters but they are for the club NBTV standard.

You really need to design the circuit to suit your application - they are reasonably simple to design and construct - you start with the xtal oscillator itself and divide down to get the frequency you want. If both the frame and line frequencies are generated from the same clock then they will be in sync.

A good place to start ids the MC4060 data sheet, which will show you how to construct a basic xtal osc circuit and indicate the component values and allowable ranges etc.

A sequence of 4518 and 4817 counters should give you the frequencies you want - but the exact arrangement is determined by the frequency of the crystal you start with.

There are many ways to design a xtal osc - have a search on google and choose one that you feel comfortable with (and can get the parts).

This maybe useful: http://www.doctronics.co.uk/4060.htm

To be honest I rarely use xtal oscillators any more - any desktop PC already has one built in and with a sound card I can generate the frequencies I need for most projects - very conveniently. That approach is particularly useful when you need two synced frequencies (frame and line) as they form the left and right channels of an audio signal.

In fact I would recommend using this method to begin with as it is very easy to change things (in software) so you can tweak the system very easily.

That actually brings me to another point. The way that you are driving the stepper motor (i.e. with a square wave) will probably result in "cogging" - that is not a smooth rotation. This may or may not be a problem, but ideally the stepper should be driven by 2 out of phase sine waves for smooth operation.

Here is an example of the principle involved:

http://www.altmann.haan.de/turntable/
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