Experimental Brightness Attenuator / Scan fail

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10-25-2010, 10:32
mixedgas

1 Attachment(s)

Experimental Brightness Attenuator / Scan fail

OK, a while back one William R. Benner wanted to see whom could get the closest to a 69 cent scan fail. His requirement was one opamp, low cost parts, and easy to adjust.

Here is my latest entry.

Note, this is not a absolute scan fail, it is a simple design, and attenuates the laser instead of 100% switchoff. It will pull a 5V analog input to six tenths of a volt, in other words most lasers will not go 100% off. This could be improved by replacing Q2 with a N channel Fet.

Operation, it looks for large voltage swings in the galvo feedback signals, yet tends to ignore small, noisy, signals.

I'm still toying with it in the simulator, but some of you may find the input block "interesting" for conditioning analog signals.

The circuit is not "calibrated" for any given situation, that is up to the builder.
for example, the diode used to set the threshold may be switched to a potentiometer, and altering the resistor (R5) in front of the two coupling caps changes the input frequency response somewhat. You may of course replace the back to back caps with a "NPO" non polarized electrolytic.

One of these days I may try turning Q1 and Q2 into a monostable, but for now, its a nice toy in the simulation software.

Note, corrected attached drawing.

Steve
10-25-2010, 14:27
dar303

Nice, good job!
I don't have the time to try it for real but I hope someone else does it and reports back!
10-26-2010, 13:43
mccarrot

The schematic looks very clean, nice job.

Is it possible to add a horizon control with a potentiometer so you can have the scan fail on below the horizon and still can use static beams above the horizon.
10-26-2010, 14:38
mixedgas

Advancd version for mccarrot.

Quote:

Originally Posted by mccarrot

The schematic looks very clean, nice job.

Is it possible to add a horizon control with a potentiometer so you can have the scan fail on below the horizon and still can use static beams above the horizon.

As that sort of defeats the purpose of "Bills challenge" I'm going to give that to you in a different format.

LT Spice is a free download and a breeze to use, although it has some limitations. You can even convert one axis of a ILDA file to a wav file using Laserboy, and import it as a waveform. Have not tried it that yet, I test things with 2 sine or square waves, plus noise, beat together in a mixer circuit and input into the DUT circuit. If the waves are audio and say 1.00 Khz and 1.020 Khz, you can get nice simulations for test.

LT SPICE DOWNLOAD:
http://www.linear.com/designtools/software/ltspice.jsp

While I could of just added a HC series logic gate, I kept with the M^2L spirit.
Note M^2L is "mickey mouse logic", logic done with crude yet effective, cheap, parts.
Use TL084 for the opamps and LM741 for the compare circuit.

Paste the following text into a text file called scanfail2.asc and copy it to the LT Spice directory.

Steve

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10-28-2010, 14:27
mccarrot

Thanks for sharing!!!

I never used this program, so I'm a little bit in a learning curve at the moment, learning how to do good simulations.

I'm more used to breadboards and a scope.
10-28-2010, 14:43
mixedgas

Quote:

Originally Posted by mccarrot

Thanks for sharing!!!

I never used this program, so I'm a little bit in a learning curve at the moment, learning how to do good simulations.

I'm more used to breadboards and a scope.

Yeah, the sim mode learning curve is steep, and the fact that the default parts are mostly all Linear Tech chips can be a bit of a drag. It has trouble with a few minor things like linear oscillator circuit startup, but its free,has many users, is actually pretty good, and you can reach the guy who writes it if you have issues.

Compared with other spice programs on the market, it is easier to use, in my opinion.

I too do bench models, but when I need to bang out a analog circuit for someone who can already solder, owns a scope, and can decode parts packages and pinouts, it rocks.

So I figured you'd like to do a little learning.

Steve
10-28-2010, 15:11
flecom

see I never understood the point of one of these scan fails... if you have a beam stop to prevent your projection from entering the audience it seems like one extra thing that can go wrong in your show...

plus sometimes a slow, sweeping static dot can be a really cool effect for certain songs... although pangolin usually inserts invisible "points" even if your just projecting a dot so maybe it won't be an issue
10-28-2010, 16:18
norty303

Guidelines for crowd scanning 10xMPE are that you have a suitable scan fail device in operation
10-28-2010, 17:30
flecom

Quote:

Originally Posted by norty303

Guidelines for crowd scanning 10xMPE are that you have a suitable scan fail device in operation

ya for you maybe, over here doing audience scanning legally isn't worth the trouble/resulting effect
10-29-2010, 00:03
mccarrot

1 Attachment(s)

Quote:

Originally Posted by flecom

see I never understood the point of one of these scan fails... if you have a beam stop to prevent your projection from entering the audience it seems like one extra thing that can go wrong in your show...

Now what if your galvo PSU breaks down.

Or one of the galvo's gets a melt down and get stuck (sometimes seen with chinese scanners)

Or some idiot drive over your ILDA cable shorting only the X Y wires

Or one of the Galvo drivers dies

Or one of the galvo cable comes loose due to the 10KW PA system next to the laser.

Or one of the galvo bearings gets stuk

or.....

or...

or....

I hope the scanfail will never have to do its job, but if something goes wrong you will be happy you have it.

Also if there is problem where a audiance member gives you a claim its good to proof you have a scanfail in place, this give you a advantage in court...

finaly the guidelines advise you to use a scanfail.

On only time you do not need to use a scanfail is when you mask of the audiance area with a cover, so in case of a problem there is zero chance a beam hits the audiance.
10-29-2010, 04:08
norty303

Quote:

see I never understood the point of one of these scan fails

I was simply responding to this, by giving you a reason for their existence. I think it is of immediate interest to most people outside of the US.

Although Mccarrott makes some good points about why it might of use in the US, I'd just use a beam mask and be done with it. Far cheaper and likely less prone to error than scanfail.
10-29-2010, 04:24
mccarrot

US scanfail:
http://www.sxc.hu/pic/m/i/iu/iuriata...etal_plate.jpg

EU scanfail:

http://www.pangolin.com/PASS/PASS0002.jpg
10-29-2010, 13:00
flecom

Quote:

Originally Posted by mccarrot

Now what if your galvo PSU breaks down.

Or one of the galvo's gets a melt down and get stuck (sometimes seen with chinese scanners)

Or some idiot drive over your ILDA cable shorting only the X Y wires

Or one of the Galvo drivers dies

Or one of the galvo cable comes loose due to the 10KW PA system next to the laser.

Or one of the galvo bearings gets stuk

or.....

or...

or....

I hope the scanfail will never have to do its job, but if something goes wrong you will be happy you have it.

Also if there is problem where a audiance member gives you a claim its good to proof you have a scanfail in place, this give you a advantage in court...

finaly the guidelines advise you to use a scanfail.

On only time you do not need to use a scanfail is when you mask of the audiance area with a cover, so in case of a problem there is zero chance a beam hits the audiance.

OR, I could put a piece of metal as mccarrot so gracefully illustrated and even if the world explodes inside my projector theres absolutely no way (short of the lasers making a few KW to go through the aluminum) that I will get an unwanted beam in the audience...

audience scanning is a neat effect, but honestly, I personally find it annoying, and to do it legally here really isnt worth it... the resulting effect from a legal audience scanning show that complies with CDRH regs is about as awe-inspiring as shining a .1mW pointer into your face after spending endless amounts of blood sweat and tears trying to get it approved...
11-06-2010, 21:16
aijii

annoying because you can't do it properly over there ?

theres more to the world than the US you know ;)
01-22-2011, 16:09
Badpip

Any updates to this project?

My scope just broke down, so I can't test it myself :mad:

Why use two npn transistors instead of a single pnp?

/Thomas
01-22-2011, 16:46
mixedgas

quote:
Why use two npn transistors instead of a single pnp?
end quote

Simply force of habit, I stock more NPN then I do PNP and it seems to survive better under stressful conditions.

No other updates.

Steve
01-22-2011, 16:59
laserist

Quote:

Originally Posted by aijii

annoying because you can't do it properly over there ?

theres more to the world than the US you know ;)

Oh I have to agree - audience scanning is the height of bad taste. It says in moron speak, "Hey - he he he - look what I can do..." Roughly the equivalent of sticking your thumb in someone’s eye – and then doing it again and again. Dumb in the first place and F’n idiotic without first rate hardware and safeguards...

brian
01-22-2011, 18:17
flecom

Quote:

Originally Posted by aijii

theres more to the world than the US you know ;)

lies! *retardo

15 characters
01-22-2011, 18:43
allthatwhichis

Excuse me; I been drinkin...

Quote:

Originally Posted by aijii

theres more to the world than the US you know ;)

And it is all irrelevant... *retardo
01-23-2011, 01:31
Badpip

Quote:

Originally Posted by mixedgas

Simply force of habit, I stock more NPN then I do PNP and it seems to survive better under stressful conditions.
Steve

I see :p

I still think a PNP/PFET would be better - should the power to the scanfail disappear, the transistor will turn on and attenuate the modulation output.

/Thomas
01-23-2011, 01:36
Badpip

Quote:

Originally Posted by laserist

(...) and F’n idiotic without first rate hardware and safeguards...

That is the whole point in this circuit ;)

/Thomas
01-23-2011, 05:35
mixedgas

Quote:

Originally Posted by Badpip

I see :p

I still think a PNP/PFET would be better - should the power to the scanfail disappear, the transistor will turn on and attenuate the modulation output.

/Thomas

Touche.. Nice Idea, I did not think of that. But I would have a output enable relay across all power bus lines in the system anyways. If the scanner rails, the control system rails, and the ILDA connector is not IN, do not even power up the lasers.

The goal of the project, once upon a time, was a scan fail so cheap, that we could insist that it be included on Galvo amps, and not need a tricky log log squaring circuit to calculate vector velocity like some older 1970s analog scan fails. Absolute Velocity, ie SQR ((Xvelo^2) +( Yvelo^2)) is the transfer function used on scan fails that had been accepted at one time in the US.

There is another thread some place on this, and a LONG one at that.

Steve
03-17-2011, 15:40
logsquared

1 Attachment(s)

just came across this thread. Here is a super simple idea maybe worth some thought/ discussion. Sorry for the crappy schematic i used my eagle cad layout software to draw it. Anyway, It's basically a window comparator that checks the voltage of the galvo feedback against the input. If its Vdiode above or below the output goes low. Obvioulsy needs a gain adjustment on one of the inputs to balance the two levels. Could replace the diodes with voltage sources or pots to adjust the thresholds.
Attachment 24145
03-17-2011, 16:40
DMills

A scan set is (pretty close, at least in small signal terms) to a second order lowpass so that wont really work.

Regards, Dan.
03-17-2011, 23:22
logsquared

Quote:

Originally Posted by DMills

A scan set is (pretty close, at least in small signal terms) to a second order lowpass so that wont really work.

Regards, Dan.

Shouldn't the phase response be fairly flat below the scanner knee frequency? I did consider possibly needing a lowpass filter on "Vin" to compensate for the lag of the galvos. My original thinking was.. the "window" created by the diodes might be enough to compensate for the lag of the mechanical system. If the lag of the galvos is several degrees or more this approach certainly won't work. Its been awhile since I tuned a galvo on a scope... can't really remember what the phase relationship was like??

Had another "awkward" idea as well... Maybe integrate the driver error signal then compare that to set threshold. This way if the scanner is not where its supposed to be for longer than a set amount of time (calculated by integrating the error voltage) we can assume the galvo is not responding and blank the lasers, close shutter etc.

Just typing some random ideas, as I really haven't put too much real thought into it.
03-18-2011, 04:09
DMills

Differentiate the feedback to produce x' and y', square them with a pair of AD633s, gives you |velocity|^2, threshold this and if this value is below threshold and Y is below the horizon line then blank the output (Probably 3 or 4 chips for the core none larger then 14 pins).

It also needs some glue logic to ensure the power rails are valid and that both pots are returning valid voltages and such, but it is not rocket science.

It is just a pity that the AD633 is so expensive.

Regards, Dan.
07-06-2011, 21:44
Qumefox

Sorry to dredge up an old thread but i've been thinking of trying to prototype a scan-fail.

How expensive is 'expensive' here? AD633's seem to run $6-$10 depending on quantity. That seems fairly minimal compared to cost of the rest of a projector.
07-07-2011, 00:06
taggalucci

I could do with at least 4 scanfails if this project progresses.