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Old 12-28-2007, 02:33 AM #1
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Default Controlling multiple lasers simultaneously?

Hello everyone. I think it would be cool to control the output of multiple lasers simultaneously using a single control. But being a n00b, I am short on ideas on how to implement this, if it is even possible (well, anything is possible I guess).

In Daedal's driver design, the current output of the LM317 is set by a potentiometer (optionally). So the brightness of the laser can be adjusted by turning the pot. If the power supply voltage was high enough, would it be appropriate to add additional laser diodes in parallel (within limits), and set the output current of the LM317 to ~0.15 A using a single pot (for example)?

Or if I'm completely wrong, are there any other ways that I can accomplish this?

Thanks very much!


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Old 12-28-2007, 08:19 AM #2
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Default Re: Controlling multiple lasers simultaneously?

My guess would be that as LD's have individual characteristics they would draw different currents, the one that gets brightest gets warmest and draws more current so gets brighter, so gets warmer and draws....well you see where this is going

You could try ganged pots...

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Old 12-28-2007, 11:20 AM #3
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Default Re: Controlling multiple lasers simultaneously?

Thanks for your reply! I can see how that could be a problem.

I'm afraid my project is a bit more ambitious than I originally stated.

I ultimately want to control the brightness of many identical laser diodes (50+) using a single control. I am at a loss on how to even begin to think about doing this, however, lol.

If anyone has any ideas on how this can be accomplished, I'm all ears.

Operating all lasers at a fixed output would be cool for now, but given the cost of so many components I am hoping to avoid rebuilding 50+ circuits in the future when I inevitably want to experiment with the brightness.

If something like this is well beyond the understanding of a n00b, I would also appreciate book or website recommendations where I could learn more.

Thanks a lot!

Edit: I just wanted to add that I wasn't expecting the LM317 to handle 50 diodes. But I was kind of hoping it could handle around 8 because this would be a n00b-friendly, though not ideal solution (only 7 to 8 controls to adjust).
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Old 12-29-2007, 01:04 AM #4
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Default Re: Controlling multiple lasers simultaneously?

It's a bad idea. *Sure the LM317 circuit could possibly be adjusted to output the current to service multiple diodes, but consider what would happen if one of the diodes, for some reason, failed. *If it shorted out, the worst that would happen would be a blown lm317. *If it died and left it's part of the circuit open, then suddenly the other diodes get all the current it was using, and then they blow.

You're really better off using 1 driver per laser. *

If you want to control the brightness of the laser, that driver wouldn't work anyway. *You need something more like what a guy named Norm made (google "Norm's Laser Show", or look on ebay for seller "ekitforall&quot which was an analog driver. *Brightness adjusted by sending 0-5v to a control line.


edit: oops, misspelled Norms ebay handle (had an "s" after the word "kit&quot
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Old 12-29-2007, 01:35 AM #5
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Default Re: Controlling multiple lasers simultaneously?

Yeah, like ajohnson said. Its definitely possible, but its a bad idea. First off, 50 diodes on a single circuit may be too ambitious for an LM317, but its possible to string quote a few in parallel with the right heatsink on your 317. Problem with this though is if just one thing goes wrong with your driver... POOF, all diodes go to heaven.

Having a driver for each diode with analog modulation input would be better. Input signal from 0-5V controls driver output, then you just get a single 0-5V singnal and you're set.

If you don't mind me asking though, what type of project are you working on here that requires 50 diodes being run at once?
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Old 12-31-2007, 01:24 AM #6
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Default Re: Controlling multiple lasers simultaneously?

Thanks very much for your input.

ajohnson:
I agree that each diode will need it's own driver. Norm's laser diode driver with modulation looks very good, and it's something I will investigate further (although it's too expensive to buy 50!). I have been studying modulation and op-amps over the last couple of days, and when I understand how they can be used to alter the laser intensity (if I understand, lol) I will consider buying one of Norm's kits to play with. Hopefully I could eventually use the principles of Norm's circuit to build a simpler but less flexible driver circuit for this application.

pseudonomen137:
I would cry if I lost multiple lasers simultaneously, so I will definitely heed your advice . Regarding analogue modulation, would you be able to suggest a combination of chips that could be used in a driver circuit for this purpose? I would like to search for some example circuits to help me understand how they can be used.

It is confusing to me how modulating a signal can affect the current output of a regulating circuit. I think a voltage comparator or op-amp can be used for this purpose, but I haven't reached this "chapter" just yet...! If someone could offer a basic explanation, I would be very appreciative.

Regarding the purpose of such a circuit, I think there would be many applications. LLLT could theoretically be assisted by such a circuit, but of course I would never use it for that! If I did though, as far as the technology is concerned, the wavelength and intensity are most relevant. Which is why having a convenient control of the intensity would assist researchers in their role. Adjusting 50 pots, even occasionally would be pretty tedious. But this is all just speculation, of course .
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Old 12-31-2007, 01:55 AM #7
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Default Re: Controlling multiple lasers simultaneously?

Just my quick commentary, since I am building a multi-diode project.

1. Use 1 driver per LD. They're small, and like ajohnson said, if you drive multiple LDs with one current source, a single failed diode will let current dump to the other diodes, causing a cascade failure. And crying and moaning and gnashing of teeth!

2. If you're running 50+ diodes- and I assume you're combining the beams - then just use digital control for brightness - turn on/off individual modules to control output intensity. This will give you 50 "brightness" levels!

Have fun!

Dave

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Old 01-01-2008, 07:41 PM #8
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Default Re: Controlling multiple lasers simultaneously?

If you need 50 drivers you should look into just buying 50 commercial driver chips, they only cost like a few dollars each and they have analog inputs so you can mod all 50 with a single controller.

How could a adjustable voltage be used to control the current output was a question. Basically all you have to do is look at how a transistor works and you will understand it right away.
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Old 01-02-2008, 08:04 AM #9
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Default Re: Controlling multiple lasers simultaneously?

If you want to go all out and use a single adjustment, here's one option: Individual driver sub-circuits, each one tuned for a safe level max output. Each of these sub-circuits would be further reduced by a transistor. These transistors would be bias adjusted individually, and then capacitively driven with all the others by a pulsed circuit. This pulsed circuit would have its duty cycle controlled by your single potentiometer. The result would be a perceived brightness control from a single potentiometer. Overall work involved would be excessive, and a royal pain in the backside, but it's one option.
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Old 01-02-2008, 11:29 AM #10
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Default Re: Controlling multiple lasers simultaneously?

xanatos:
Unfortunately I need all lasers to be on and at approximately the same intensity simultaneously. I agree that I will need a driver circuit for each one. The thought of killing many at once is truly horrifying, lol. Which I why I want to learn as much as possible about the theory behind the circuits I will need.


likewhat:
I had a brief look at the commercial drivers, but most of them are based around a photodiode (or do not allow analog modulation). I don't understand the circuit diagrams well enough (yet...!) to determine whether they are still effective without the photodiode. There are a couple of kits that are suitable (Norm and die4driver), but these are too large and/or expensive unfortunately. If you could recommend a driver that allows analog modulation and does not require a photodiode, I would appreciate it. I have been reading about transistors on your suggestion, and it seems there is much to know. Thanks for your ideas!


GreyFox:
That's a very clever idea, thank you! I didn't think of altering the signal after the driver circuit, but it seems less dangerous than controlling the output of the LM317 directly (OT, would a JFET suit this purpose?). I can see how it would be tedious to work out the bias for the transistors on top of the output current of the driver circuit, and correlate that with the laser's output at certain currents. But this will only need to be done once, so I will definitely investigate further.

Are there any possible complications of using the output of a transistor to power a laser diode (spikes, etc)? Assuming I interpreted your suggestion correctly...!

Also, would you be able to suggest a type of transistor (IGFET, BJT, JFET, etc) that would be appropriate for this application?

Thanks again!
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Old 01-02-2008, 01:10 PM #11
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Default Re: Controlling multiple lasers simultaneously?

What is the problem with using the photodiode?
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Old 01-02-2008, 06:51 PM #12
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Default Re: Controlling multiple lasers simultaneously?

I didn't have one in mind, but the general design could be made around whatever you have available. The big problem you'll run into is in designing a good, reliable driver for the pulsed mode operation. If you intend to use LM317s, you'll probably need to connect the transistors as a resistance bypass circuit so you can directly drive the LM317 output without resulting in dangerous spikes. For instance, a 5ohm resistance in series with a 20ohm resistance. Connect the transistor across the 20ohm resistance, and use it to vary the value from 0-20ohms. This would theoretically shift the current to the LD from 250mA all the way down to 50mA without producing spikes of a damaging level.

The other options would involve either a precision voltage and current regulator driver or a current-soak configuration (much like that used in linear power supplies), which is horribly wasteful of energy and still may not completely eliminate spikes.
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Old 01-02-2008, 11:52 PM #13
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Default Re: Controlling multiple lasers simultaneously?

likewhat:
My laser diodes don't have photodiode monitors (only anode and cathode pins). They will be run below max power at a low duty cycle however.


GreyFox:
Thank you again! Setting the range of the LM317 with static resistors is very important if the LM317 is under "remote control". I've also considered trying to find a positive voltage regulator with a max output current more friendly to laser diodes, since the device won't be used for burning or anything requiring max output. Since this is most likely redundant with your configuration (resistance bypass circuit), I will probably be driven on price more than anything in choosing a regulator. But not completely driven, depending on how much. Redundancy is good...

A reliable pulsed mode driver is another problem. More so because I have not yet decided what will power this thing. I will spend the next few days reading up on this area and no doubt return with more questions .

Thanks again
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Old 01-05-2008, 05:41 AM #14
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Default Re: Controlling multiple lasers simultaneously?

I've been messing around with a circuit simulator, and I have some more (very n00b) questions if you have the patience

1. I'm confused how the resistance bypass circuit is supposed to work (h***://laserpointerforums.com/forums/YaBB.pl?num=1198812822/11#11). Placing the 20 ohm resistor across the collector/emitter or drain/source pins seems to exclude the transistor from the circuit. Placing it across the collector/base or source/gate doesn't limit the current to 0.25 A. Placing it between the emitter/base or drain/gate terminals prevents current from flowing at all. This is using NPN transistors in depletion mode. What am I doing wrong?

2. If using PWM to control the positive voltage at the base/gate, would these individual transistors be connected in parallel with each other?

3. If the transistor base voltage is capacitively driven (effectively demodulated, in this application?), is a pulsed signal required? I am under the impression that it would be more stable?

4. I think this circuit (h***://allaboutcircuits.com/vol_6/chpt_6/9.html) looks like a pretty good base for PWM. Any thoughts?

Thanks!
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Old 01-06-2008, 03:35 AM #15
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Default Re: Controlling multiple lasers simultaneously?

1: You were correct the first time. You would connect the transistor across the resistor from collector to emitter (drain-source).
The transistor, when saturated, should turn the resistor into a near short. By doing this, it would take the resistance out of the sense segment of the LM317's circuit. With less resistance, the LM317 will increase the current output.

2: Yes. You would also use a high impedance bias circuit on the base/gate of each transistor to keep the base-emitter current down.

3: That depends on your configuration. Capacitive coupling may work, but you would need to run some tests to make sure the transistor wouldn't simply see a short impulse rather than a square wave.

4: Definitely a good start for the basics of a PWM source. As always, run simulations with it to make sure it'll provide the exact results you want.

Overall, an excellent start to your project! Just for reference, which circuit simulation software are you using?
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