Green Laser not blanking correctly.

[Jem]

Hi

I've just got my projector up and working, it looks fantastic

However, i've noticed that the green laser (Lasever 200mw) isn't fully blanked. By this I mean that there's a small residual beam when nothing's happening and the projector is in standby waiting for a show to start. It's also sometimes apparent when the show is ongoing, you can see (very slightly) the blanking lines from the greenie.

The question is... where should I start looking to cure this. Is it a laser problem, the analogue modulation unit or is it the DAC, or even the software?

Help.... Please....

Jem

[Laser Ben]

Check that the laser is getting a full 5 volts for modulation. It is possible to amplify the voltage, I forget what IC you need though.

[The_Doctor]

Any cheap op-amp will amplify it ok. Check scale and offest though. Start with mod set to lowest/off. See if you're really getting a zero volt output from the controller. Then set to max, cautiously, watching for 5V at max. 'Cautiously' because if you can get more, don't. Some lasers aren't protected against more, it's up to us to treat them nice. It shouldn't be only up to us, but there it is...

[Jem]

Thanks Doc

I've just measured the voltage, it's measuring 5.09 volts.

At the risk of sounding dumb, or should I say dumber than usual ...

Is full 5 volt = Laser off and a 0 voltage = Laser On full power?

Just so I can have this absolutely straight in my head. For some strange reason I always imagined the opposite.

There are some trim pots inside the analogue module, are any of these worth a twiddle

Jem

P.S. Doc, have you got the screws I sent you yet?, they were posted to you first class on Friday.

[marconi]

Hey Jem

Assuming this is an analog unit,
Does the laser shut down when the modulation voltage is at 0 volts?

If not, that will be the problem.

If it does, that laser may not be loading the circuit enough, you might
try using a resistor to the mod lead to ground , something around 10k.
you may have to experiment with that value some.

While possible, the laser modulation circuit may be mis-aligned, but
I doubt it. usually on those only two pots control the current to the diode..
one for current max limit and the other operating current.
I wouldnt adjust them just yet.

Also, you can swap the modulation with one of the other lasers to determine if it is that channel acting up or something else.

[The_Doctor]

Yes. Sorry, didn't mention them yet cos I hadn't explored fully. Thought a couple might be titanium till I removed them from the strip of tape. Which would be cool but probably too expensive, titanium hardware is expensive. The two brass ones look different in colour which is why I thought of that, titanium can be anodised... The one labed D is the closest match to what I need, except that it has a hex hed like a nut, but a screwdriver slot of some kind would be easier to trim. I'll look more closely at them, gauge TPI and such before saying more. Meant to do that already but I made a SketchUp template for a laser scanner box and that took more time than I thought it would...

Re the voltages, I think the ILDA standard is for 0V to be low and 5V to be high. The 5V is good at 5.09V. This is one reason I advocate a sound card based system, polarity reversals tend to be built in to most stages in such systems making it easy to arrange. If you do need a discrete circuit between output and laser, a single op-amp stage is best, as you can adjust scale, offset and polarity, all with a simple circuit. In this case, you'd feed the control signal into the inverting input of a circuit built as a differential amplifier, and the normal input would have a 5V reference voltage on it. That would make the output track the input inversely around a value of 2.5V. Alternatively, you can see it as an inverting amp with unity gain and a +5V offset, but the circuit is the same.

Carmangary mentioned a TL081 or dual form TL082, and another is the LF411 (or LF412), which are very good general purpose amps. R.M Marston's book of op-amp circuits and a piece of stripboard and some resistors will go a long way. The net's pretty good for op-amp circuits too.

Nothing dumb about getting confused about control conventions. People who specialise in building this stuff seem to like confusing things, usually for no better reason that adhering to a known convention might cost them a few more parts that they don't want to buy. It's better to look at what conventions you need, then design neat ways to meet them. My guess is the ILDA conventions are the way to go. I don't know them all, but those that I've seen make technical sense, and also go with things we find sensible, like 0V means Off, and so forth.



EDIT:
Was slow, Marconi posted during the time i was writing stuff. I agree with him about tweaking. Shouldn't be needed. Threshold of the pump diode will be below anything that can push green out of the business end, so unless the laser is very badly set up (doubtful), it could be that the laser uses a pull-up resistor to maintain 5V, so your control circuit needs to sink as well as source a current. My first test to examine that would be to leave the mod input unconnected, see if the laser puts out full power, then test with a voltmeter to see if there is a 5V signal on any of the mod input connections. There are several ways a mod input can be arranged, so it's easy to get confused. To start with, if there was a mod input plug supplied, as with the one I sold you, does that blank the laser, or change its output in any way if you plug it in, even with nothing fed into its wires? If it does, it is wise to open the mod plug to see what connections it makes. SOme (like that LambdPro) use two of 4 or more pins as a link, forming a switch when the plug is inserted. It's worth establishing whether anything like this is being done before contemplating any adaptations.

[Jem]

O.K., So tonight I swapped the power supply with the one on my blue laser. Problem totally disappeared, so I guess that's where the problem is.

The power supply on the blue Lasever laser is a chassis mounted one, and the one that came with the Green Lasever laser is a switched mode one in a plastic casing. Not quite sure what the problem is with the green laser power supply but i'll ask David at Lasever if he can perhaps sort me out one that's the same as the blue.

Cheers

Jem

[The_Doctor]

Careful, laser heads and PSU's are often tweaked by the makers on a unique basis, you might end up with bad TEC control or diode drive current settings if you mix heads and PSU's.

[Jem]

Thanks Doc, yes, I realised that it probably isn't a particularly good thing to do, but it was the only real means I had to find out where the problem lay. As it turns out it was an easy test for me to do and it kept things really simple.

At least I know where the problem lies. I've emailed David at Lasever to see if he can send me another 'open frame' power supply, the same as on the blue laser. If he's good enough to send me one i'll at least have two supplies the same so the chances are the problem will have gone.

In any case i'm not sure that a different supply will be so much of a problem as the 5v output from this power supply is fed into a separate unit which supplies the laser, it's from this box of tricks that the modulation input from the DAC connects.

Jem

[Jem]

Just thought i'd update this thread...

I've cured the blanking problem (I think). I bought a new chassis mounting power supply off Ebay that's capable of supplying 5v power to both the blue AND the green laser. It's capable of 20 amps at 5 volts, so it will easily run both lasers. I've adjusted the voltage to be EXACTLY 5 volts and the problem seems to have disappeared. I've just finished testing it and all seems to be well. I guess time will tell.

Cheers

Jem