Adjust your laser's output on the fly(digital pot)

[erdabyz]

NOTE: if you don't wanna read technical details, go to the end of this post.

As some of you may know, i've said some times at this forum that i was going to make anithing like electronic switches or similar... and then i ended doing nothing. But this time it'll be different. I had a nice idea, how about having an addon for your laser driver which would allow you to change your laser's power SAFELY on the fly? This can be done, just changing the driver's  original potentiometer for a digital potentiometer.

Well, some of you may say: that is very complicated, just add a external bigger potentiometer and you could adjust the output twisting it, without more electronics...

Well, true, but laser drivers's current reponse is not directly proportional to the pot's resistance, and as you turn the pot, smaller resistance changes means bigger current changes.
So, having a potentiometer with a mechanical way to make it stop turning at a desired point is not a good idea. Multi turning potentiometers would be a solution, but the mechanical part would be difficult to make, and it should be adjusted for every laser.

How about electronic potentimeters?
First, we need 100ohm potentiometers for a daedal driver, and i have to test the value of a rkcstr driver's pot. (or lavadrives too)
There are NO commercially available 100 ohm digital potentiometers... and there are NO low resistance potentiometers with enough taps (steps).

The best i've found is a 1024 taps, 10Kohm potentiometer...

So, ¿how can we make a 100 ohms potentiometer out of 10k ones? Well, just putting two of them in parallel and making calculations, i can get a range of resistances from 30 to 100 ohms, with 0.5 ohms resolution (each step increases 0.5 ohms the resistance), and if neccesary, i can also get 0.1-0.2 ohms resolution.

But this calculations require a microcontroller to store the wiper's  position of the two potentiometers and set it. Also the potentiometer has a serial interface.

The microcontroller will also allow calibration for every laser's max current to avoid going over that value and frying the poor diode. And will also allow a threshold calibration, so your diode will lase in the whole range.

Two push buttons will be the interface, one to increase power, the other to decrease, and some combinations to enter calibration mode.

The addon will be smaller than a rkcstr driver, and will work from 2.5v to 5.5v.
I'll have to make them by hand, expect quality, but not a lot.  I won't be able to make ig amounts of them unless it sells very good. the firmware of the microcontroller won't be open source because it's gonna be a HARD program and i don't want it copied.

What i've done until now is:
-programming a part of the software
-Requesting some samples of the potentiometer
-thought about some issues and how to solve them

And now, the most important part, it'll be a bit... expensive.
Digital potentiometers are $5 a piece (expensive hardware... but required)
Microcontrollers (PIC12F683) are $1.10
I'll make them by hand, so the final price will be about $10 plus shipping.

I'll also sell only preprogrammed microcontrollers for $2+shipping.

I will post more details of the project if you want. But i have a... bad english and some times i can't express as i want.

So, FOR THOSE THAT DON'T WANT TO READ THE ENTIRE POST,

WOULD YOU PAY $10+Shipping FOR A SAFE WAY TO CHANGE YOUR LASER POINTER' S OUTPUT POWER ON THE FLY?

 

[lasersbee]

Re: Adjust your laser's output on the fly(digital

I've used digital pots controlled by a Micro Controller  on a different project in
the past... I don't want to sound negative... just throwing out some past experience...

The problem that I can see... using a Digital Potentiometer like the AD5231 is that
the max current for the device is 2ma...

The LM317 constant current driver has it's output current go through the sensing
resistor and therefore the output of the driver will be limited to the 2ma max
to not damage the digital pot...  (absolute Max Ratings page #7 of the datasheet)
I believe the "rkcstr" driver is also used in the constant current mode... and it's
resistor or pot performs the same function...

I'm not sure about the "lavadrives"... :-?


Jerry

 

[Kage]

Re: Adjust your laser's output on the fly(digital

What provides the regulated current to the LD? Do you have a block diagram? :-?

 

[erdabyz]

Re: Adjust your laser's output on the fly(digital

I've used digital pots controlled by a Micro Controller  on a different project in
the past... I don't want to sound negative... just throwing out some past experience...

The problem that I can see... using a Digital Potentiometer like the AD5231 is that
the max current for the device is 2ma...

The LM317 constant current driver has it's output current go through the sensing
resistor and therefore the output of the driver will be limited to the 2ma max
to not damage the digital pot...  (absolute Max Ratings page #7 of the datasheet)
I believe the "rkcstr" driver is also used in the constant current mode... and it's
resistor or pot performs the same function...

I'm not sure about the "lavadrives"... :-?


Jerry

rkcstr drivers use 1/10w potentiometers, so i think that only a bit of current goes through them.

I have to check this.
you are right with daedal drivers... this circuit wouldn't work for them.




Kage: this would be a device to replace the potentiometer in the laser drivers.
I mean, you'll have the same driver, but with an electronic potentiometer to adjust current.

 

[691175002]

Re: Adjust your laser's output on the fly(digital

Is there a reason why a cheap pic microcontroller wouldnt work?
You would just need an A/D converter, PWM output and perhaps an optoisolator, sounds like it might be simpler plus you could take advantage of the microcontroller to do all kinds of cool things. I'm not certain how apparent brightness relates to current for a laser diode, so PWM might look nicer too.

 

[positron]

Re: Adjust your laser's output on the fly(digital

Is there a reason why a cheap pic microcontroller wouldnt work?
You would just need an A/D converter, PWM output and perhaps an optoisolator, sounds like it might be simpler plus you could take advantage of the microcontroller to do all kinds of cool things.  I'm not certain how apparent brightness relates to current for a laser diode, so PWM might look nicer too.

I reckon you are right, if you just use an output pin with variable PWM connected to a logic level fet you could adjust the brightness that way by directly switching power on and off to the LD. It would be more efficient than using a linear regulator to turn down the current as you aren't wasting power by turning it in to heat, the FET will have very low switching losses.

 

[danq]

Re: Adjust your laser's output on the fly(digital

I've used an existing pwm control circuit to modulate a driver, and it worked very well.

The control circuit was a chip from a Photon Micro-light. The signal from that was fed to the shutdown pin on a buck/boost regulator.

btw, You don't need to run any kind of power through a digital pot to use it as a control... just depends on where it is. In a proper circuit, you don't need the variable element in the power stage ;-)

I'm currently working on a pic controlled driver with many of the features desired... but anybody starting now will undoubtedly get one done before me!

DanQ

 

[laserfreak3d]

Re: Adjust your laser's output on the fly(digital

i dont see a real need to do it the digital way if the analog way also works fine and is safe aslong as there is a current limit.
Also u can just use a laser driver with analog blanking to do the same in a safe way
u can just modify the lm317 driver to accept analog blanking

 

[positron]

Re: Adjust your laser's output on the fly(digital

I've used an existing pwm control circuit to modulate a driver, and it worked very well.

The control circuit was a chip from a Photon Micro-light. The signal from that was fed to the shutdown pin on a buck/boost regulator.

btw, You don't need to run any kind of power through a digital pot to use it as a control... just depends on where it is. In a proper circuit, you don't need the variable element in the power stage ;-)

I'm currently working on a pic controlled driver with many of the features desired... but anybody starting now will undoubtedly get one done before me!

DanQ

Sounds like a cool project

With the digital pot, you could probably do away with it entirely if you use PWM into a RC network to give a variable output voltage that you could feed into your circuit instead of feedback directly from a current sense resistor. You would still need a current sense resistor though so you can read the voltage with a/d converter and adjust the PWM duty to maintain the current that you want.

 

[danq]

Re: Adjust your laser's output on the fly(digital

You would still need a current sense resistor though so you can read the voltage with a/d converter and adjust the PWM duty to maintain the current that you want.

Yes, the current-regulating portion needs a sense resistor. But in the circuit that I made, the pwm changes the duty cycle, not the current. I guess you could say the average current is being varied, but the actual current is at any given moment either zero or maximum.

What I would like to do with my ideal circuit is to have pwm dimming plus variable current, or at least high/low current settings. So you could turn it to "low" and the max output would be 5mw; then switch it to "high" then the output would be, say, 200mW - and in both modes the output duty cycle would be variable from 0 to 100%.
And hopefully, an access code to lock the unit or to enable the high range :

DanQ

 

[Dark_Horse]

Re: Adjust your laser's output on the fly(digital

I think this is a long overdue-idea. I think there is a market for these. Who would not want to run a PHR at 70mW most of the time, but soft-click the tail-cap and go to 120 or 150mW for a little burning?

If you could make it small enough and controll it by clicking the power-switch, like the variable flashlights do, I would take many of them for $10 each. They would have to fit in small "Lava-Drive builds".

DH

 

[potatorage]

Re: Adjust your laser's output on the fly(digital

If it's easy to use and works efficiently, I'd be interested.

 

[erdabyz]

Re: Adjust your laser's output on the fly(digital

Guys!!!
First time i thought about PWM... but charlie, we have a problem!!!

And that problem is called output capacitor!!

If you control the input by PWM, you won't get a great variation because the capacitor filters the output. Not adding it would mean... spikes in buck-boost regulatrs (lavadrives) and less protection for the diode in linear drivers.

Switching the output would mean killing the diode... again because of the capacitor.

So, we have to dessign a new driver without output capacitor... or find another way to do it.

That's why I thought about of this. I can do them witha SOIC150mil PIC and a 5x5mm TQFP dual potentiometer... really small. And if they work with currently available drivers so everyone could upgrade them without a lot of effort... better.

It would be a very hard program to do, and very hard to think about everything that could make it go wrong, but if i can get them working perfectly it wouldn't matter how complex was it's design if i don't make them more expensive because of that. Because, as i'm a student, all the work i'd do would mean learning for me, so i wouldn't have the need to get payed for it because i already had my reward (knowledge)


PS. reducing the current through the diode doesn't mean wasting power.

 

[danq]

Re: Adjust your laser's output on the fly(digital

I think this is a long overdue-idea.

Prototype - Feb 12, 2008 - check it out

EDIT: 10uf output cap works fine.

EDIT2: I know, I know... that's a Very primitive current-reg... I'll be doing it differently now ;-)
the link above is just the first one I did

EDIT3: erdabyz, I'm not trying to steal your thread... you'll have yours done before I finish figuring out how to turn on a led with a picc... I just want to show the doubters that it is possible!

DanQ

 

[erdabyz]

Re: Adjust your laser's output on the fly(digital

I think this is a long overdue-idea. I think there is a market for these. Who would not want to run a PHR at 70mW most of the time, but soft-click the tail-cap and go to 120 or 150mW for a little burning?

If you could make it small enough and controll it by clicking the power-switch, like the variable flashlights do, I would take many of them for $10 each. They would have to fit in small "Lava-Drive builds".

DH

If you just want to have 2 or 3 values (not 100 or 150), the design is more simple. No digital pot's to worry about, just 2-3 resistors (or small pots to make it suitable for every laser, not a fixed value) and 2-3 mosfets, and if you want to change them by clicking the cap, a simple IC or a more simple microcontroller would do the job perfectly. Using 1206 SMT resistors, SOT23 mosfets and a PIC10F200 in the smallest package available (sot223), i think it could be as small as about 1 square centimeter

 

[lasersbee]

Re: Adjust your laser's output on the fly(digital

[quote author=Dark_Horse link=1231947558/0#10 date=1232150961]I think this is a long overdue-idea.

Prototype - Feb 12, 2008 - check it out

EDIT: 10uf output cap works fine.

EDIT2: I know, I know... that's a Very primitive current-reg... I'll be doing it differently now ;-)
the link above is just the first one I did

EDIT3: erdabyz, I'm not trying to steal your thread... you'll have yours done before I finish figuring out how to turn on a led with a picc... I just want to show the doubters that it is possible!

DanQ[/quote]

Hey DanQ...
I just looked at your link using the "chip" out of a Photon Micro-Light....
That unknown chip is an 8 pin PIC micro controller... you can tell by the pin usage...

Jerry