[URGENT] Help in Green Laser Driver Please

Hi, sorry for URGENT tag, but i really need to do it quickly. It`s a project for college. I am building a simple distance meter with laser and I have a very short period to finish it.

For start i've I bought a 532nm 30mW diode laser with driver. Then I made pulses of laser in frequencies of 20 Hz just using a square wave in voltage supply for the driver. I was able to detect its reflection on the wall with a phototransistor.

The next step would be to raise the frequency. But to do that I thought i should remove its driver and build my own one. Then i made a pulsed current supply with a 555 generating a voltage square wave and using a LM317 as current source. Because 555 has low output current I put also a TIP32 transistor between them to raise current capacity. I do not have the schematic but its pretty simple. The main component is the LM317 working as a current source. I'm currently using a 4.4Ohms resistor so I can have a 0.285A current source.

The current source worked correctly with resistor loads. Then I've tried to connect my source directly in the laser diode. Nothing happens. With or without the 555, nothing happens. I was not sure about the led pinout so I might have damaged it. I bought another 2 green lasers but still nothing works. Both lasers were case positive. I have connected my current source positive wire to its case and the negative wire to the diode pin that was been used within the driver.

The only thing that happens is the appearance of the well known weak red dot. As I read here, it means a strong IR radiation. The current that passes through the diode is something about 100mA and the only light that comes out is the weak red. Any suggestions? I really need this to work and its a big problem to damage lasers here since I cannot buy sh!t in Brazil. Every laser I buy will take months to arrive here and its not so cheap.

Thanks and sorry for my bad english

This isn't going to work with green. The time it takes to "charge" the Nd is way too long to be measuring distances. You need ultrashort pulses and very very fast electronics. As you may know, light travels rather quickly.

I have to be blunt here: if you can't figure out an lm317, a project of this magnitude is way over your head, given your time constraint. Might be time to think about a new project.

The thing is:

If i will measure 2 meters, considering the c=3.10^8:
Time of travel will be something around 133ns.

I understand that I should have a fast circuitry to see this delay. But probably you are thinking in terms of digital conversion. I will not capture this both signals (output and response) to compare inside a uController. Instead, i will XOR both signals and amplify its difference. Something like a PLL does. I can generate 2 different clock signals with 2 VCO's (one based on the laser signal and the other based on the response signal) so I can get clean square waves to compare. I think it will work as I already got some results before.

I can agree that I cannot say that for sure since the scope I used is not too precise. But you really think this idea will not work for sure? Maybe a red laser would?

I do not need a great range of distances and also a high precision. It's an academic project. But still I need it to work and it seems that its possible.

Yeah I know how a LM317 work and this simple current source was tested and it works. I dont know about lasers. My problem is the laser diode that I've never worked with. I have already worked with regular diodes and many other components with no problem.
With all due respect I thought you could help me out here instead of judging me (my knowledge). Thanks for answering anyway.

He isn't judging you. People come everyday asking for help and give as little info as possible, then they get the help they need and never come back to say thanks or even show off their work. This is a hobby forum after all. Not an after school tutor program. And if anybody here is going to be able to help you it is one million percent cyparagon.

Yea i know how it works and i agree with you that is a problem. But as you said, i also think everyone should learn together here. Though the main thing is: one of the main objectives of a Forum is to help each other and answer each other's questions. My question now is the one above. I do not think I gave too few information on that. You may feel free to ask anything too. For me it was a simple thing: current source to drive my laser diode. Also I read here lots of information about that. Unfortunately it did not work and I don't have a clue why is that. Since I dont have a clue, i also cannot give you precisely the information you need because I don't know what are they. But again, anything you ask I will answer and if ppl become interested in anything I will be glad to help.

I study engineering because I like it and I am not doing this to earn money with it. I'm also not doing this just to graduate. So I think I'm not that different from hobbysts.

Well lets address cyparagons first red flag. A dpss laser is not the prime laser you want to use. You want a laser diode. Your green laser is not a diode laser. it uses an ir laser and gets pumped through some crystals, this isn't ideal for high speed pulsing. and your green i don't know if people are using lm317s to drive greens. Preferable a red low powered. It starts instantly and would make your calculations easier and more accurate. As for everything else, way over my head. I'm a tinkerer not a scholar.

Also you do short us on info... What is your input voltage to the 317? If I'm correct the green laser needs around 3v??? But don't hold me to that. What power supply are you using also. Low powered dpss lasers generally need a few hundred ma of current. Also make sure your laser is wired correctly. Use the search function and type in laser diode pin out. That should be useful

Thanks for the answer!

chipdouglas said:

Well lets address cyparagons first red flag. A dpss laser is not the prime laser you want to use. You want a laser diode. Your green laser is not a diode laser. it uses an ir laser and gets pumped through some crystals, this isn't ideal for high speed pulsing. and your green i don't know if people are using lm317s to drive greens.

Click to expand...

I did not know about this difference (diode laser and dpss diode). It makes sense. Though I think one of the green laser pointers that I've bought has a LM317 in its driver. If its a current source I cannot see why it would not work.

Another thing: what you consider to be high speed? I was trying to reach a 10kHz frequency and I dont think this is too high. If still it is not possible I could use lower frequencies. I think the only problem in using low frequency is time of measurement. Then again this is a test project so I dont mind if it takes 1 second to calculate short distance. So I can use even 100Hz or 10Hz pulses to drive it, can't I? The lower the frequency, lower will be my XOR result. But is all a matter of amplifying it. No?

chipdouglas said:

Preferable a red low powered. It starts instantly and would make your calculations easier and more accurate. As for everything else, way over my head. I'm a tinkerer not a scholar.

Click to expand...

So it might be better to use a 5mW laser diode? How can I know if its a diode laser or DPSS laser?

chipdouglas said:

Also you do short us on info... What is your input voltage to the 317? If I'm correct the green laser needs around 3v??? But don't hold me to that. What power supply are you using also. Low powered dpss lasers generally need a few hundred ma of current. Also make sure your laser is wired correctly. Use the search function and type in laser diode pin out. That should be useful

Click to expand...

As I said (maybe its confusing in the first post): i'm using a 5v supply source (DC) to drive LM317 like this:

http://forum.allaboutcircuits.com/picture.php?albumid=3&pictureid=140

I've used a 4.4 Ohms resistor. This would give me something around 285mA. I've tested the circuitry with different loads and input sources (higher than 5v) and it sill gives me 285mA.

As for pinout, I've searched a lot but I saw that there are much divergence in opinions.

Take a look:

https://www.google.com.br/search?q=...29,d.eWU&fp=92853adc6eab3763&biw=1366&bih=600

There are all kinds of connections. So I've tried to wire as the original driver.

You're speaking Greek to me right now. Your going to have to wait for somebody else to help. Sorry.

Due to my bad english?

No lol, due to my bad electronics.... In America when somebody says "you're speaking Greek to me right now" means you explained something to them in an educated way they and they don't know the answer. V

I see. Here it means this too but my english is so terrible that I was afraid you were not understanding a word of it (also could be Greek hehe).

Thanks for you help so far anyway

FelipeRibas said:

If i will measure 2 meters, considering the c=3.10^8:
Time of travel will be something around 133ns.

Click to expand...

My math says 13ns. :undecided:

I'm not sure a laser diode will be enough power. It might work for really short distances, but the return light will be incredibly dim further off. Inverse square law applies here. Doubling the distance cuts light return by 3/4.

This presents a further problem. The return signal will need amplification to more closely match whatever your reference signal is. Amplification takes time. A LOT of time compared with the speed of light.

LM317 isn't the way to go, either. You need incredibly quick rise times, and the LM317 simply doesn't have that.

I've never designed/disassembled a rangefinder, so I can't tell you what works. But I can tell you what won't work.

Your English is fine, by the way.

Thanks!

Sorry about 133ns. It's 13ns for sure.

The distance problem I was trying to cover by using camera lens to focus the laser point. But I came to conclusion that the lens might gives me more problem than solution due to reflection, refraction focus point, etc. So i've started to use only the green laser and one phototransistor in free air right next to the laser. I was able to sense the laser reflection in my hand or in the wall within a distance of something around 50cm with a very very simple amplification circuitry without filters.

This was achieved with a 30mW green DPSS laser and the TEPT5600 phototransistor which has a peak sensitivity in visible light (550 to 650nm).

So lets say that I will use a 10mW RED diode laser now (btw I think I will use it due to low price and easier to buy). The phototransistor is almost as sensitive to red light as to green light. So this will be no extra problem.

An advantage will be the switch time of a red diode laser cmopared to green DPSS laser, as you all said. Of course I will have less light due to less power but I think I can compensate it with amplification.

Let's talk about the circuitry now. I've looked closely to LM317 datahseet and it takes something about 5us to stabilize its current. I dont think this is bad because the stabilization time should be compared with the period of laser pulse and not with the light speed. Since I intend to use a frequency of 10kHz or lower (1kHz maybe) with a square wave (duty cycle=50%) then the laser will be turned on for something about 50us, right? So the laser will stabilize within 10% of the duty cycle at most. Do you think this is critical?

Besides LM317, what do you suggest me to use?

Last but not least, the amplification. The signal output from the phototransistor will go directly to a simple high-pass RC filter just to cut off the DC level from the PT. Then I will amplify the signal with a simple op-amp and apply a high order active band-pass filter (fo=10kHz). What I thought was: I can make the op-amp with ajustable gain controlled by the RMS value of this signal. So when the light starts to become weaker, the op-amp gain will raise to compensate.

Just another note: when we use a remote control from a TV (for example) I've noticed that it covers a great distance. Today I stood beside my television and pointed the remote to the wall to see if the reflection of its IR signal would be detected. The wall was about 2m far from me and the television detected the remote signal. Since the remote controle only uses 1 IR Led and the television uses only 1 photodiode (less sensible than a phototransistor), isn't this enough to bring me some hope?