Lasers and LDR's

[Jak]

Hello all.

I have a technology question, and this forum was the first to come to mind.

On the subject of light dependent resistors: is there a type that only reacts to laser light? or light of a specific frequency?
If not, is there a lens that only lets a specific frequency through?


Also, do they even make lasers that you can just plug in? as in no batteries?


It's for a Laser trip-wire project.


-------------------

tl;dr

Do Laser Dependent Resistors exist? for sale?
Do plug and play lasers exist? for sale?
My Thanks \[T]/

[factotum]

Well, there's no realistic way to tell the difference between a laser and any other kind of light with a simple diode. A laser will obviously be highly polarised and at a specific fixed frequency, but you'd need a lot of extra hardware to detect either--unless you could absolutely guarantee what polarisation the incoming laser light is at, in which case a simple polarising filter may be enough to do the job. That would still reduce the intensity of non-laser light passing through, though, and it's unclear whether that's a problem for your application or not?

[DavidSh]

If not, is there a lens that only lets a specific frequency through?

There are narrow-band filters called Dichroic Filters that can be bought. It looks like the narrower the band you need, the higher the price. For example, a one-inch diameter filter that passes green light of wavelength between 505 nm and 575 nm goes for about $20. (That's nanometers, not nautical miles.)

But I thought the topic was going to be a new RPG, "Lasers and Long-Distance-Relationships".

[Xyril]

If you use a filter and a laser frequency that doesn't occur (or occurs at very low intensity) in your ambient light, you can probably achieve the effect you're looking for. Off the top of my head, infrared would work if you're setting up somewhere with a lot of random black body emitters lying around, and infrared lasers aren't too expensive (and they plug in.) One challenge is that infrared lasers are regulated by many governments. Depending on where you live, this could mean anything: They might completely ignore lasers below a certain power level, which shouldn't affect you much, or there might be certain licensing considerations to consider for even low power lasers, which could make it difficult for you to buy anything. Infrared (or anything outside the visible spectrum) does present a particular danger--since we don't instinctively look away, if it catches someone in the eye they're more likely to linger long enough to cause serious harm, and infrared is particular good at transferring heat and energy, which is why they're used in many industry applications.

If you have control over where you set up your trip-wire, you might also be able to do a bit of a hack: White LED lights simulate natural white light by combining three monochromatic LEDs. The frequency spectra don't have perfectly sharp peaks, but if you look at the combined frequency spectrum you should find that certain wavelengths only occur at particularly low intensities. If you can find a filter that lets in only those wavelengths, and a corresponding laser, and you make sure nobody ever opens a window, you can probably cobble together a setup that does what you want.

I don't know what your goals are for this project, but if finesse isn't one of them, your best bet might just be to use brute force: Use any laser with an intensity that's well beyond that of the ambient lighting, and design your circuit to trigger only at a very high threshold.

[Gnoman]

Do plug and play lasers exist? for sale?
My Thanks \[T]/

These would do what you want, but they're pricey.
https://www.thorlabs.com/newgrouppag...group_id=12994

[gomipile]

I couldn't find precisely what I was looking for to recommend. These are a lot cheaper than what I was thinking, but they might be in the ballpark of working for the OP. They should at least give an idea of what to search for to find wall-powered lasers.

.
https://www.amazon.com/650nm-Module-...dp/B0757L997D/
.
https://www.rnathalie.com/index.php?...ucts_id=516264
.

[Thomas Cardew]

Is there a particular reason you need a resistor? Typically, we use photo-diodes which generate current in response to light. It would be relatively straightforward to set up an alarm circuit that depends on the current from the photo-diode when the laser beam is broken, the current would stop and the circuit would be broken. If you picked a photo-diode with the appropriate thresholds, it would only trigger when the light hits intensely enough i.e. from the laser as opposed to ambient light. Photo-diodes are also cheap. Alternatively, filters do exist which block certain frequency. You can look at the Thorlabs optics page to get a feel for the ranges but everything on Thorlabs is probably higher grade than you need.

EDIT: Oh and yes plug and play lasers exist. I'd suggest searching for diode lasers. They're cheap. You can find fully assembled ones or you can get ones that you wire into your electronic set up directly.

[Jak]

Okay, these are some good options. I like the idea of using a photodiode; the resistor I used for the prototype seemed to not trigger unless you broke the beam for at least a quarter to a half second. I feel like the diode will be more strict.

Are simple diode lasers able to remain on indefinitely?

[gomipile]

Are simple diode lasers able to remain on indefinitely?

Low power ones in the 5mw range are designed to be operated continuously, yes. They typically have a lifetime measured in the tens of thousands of hours.

Just make sure the diode and housing you use doesn't get too hot. That'll reduce operating lifetime. If it does get hot, add a heat sink with enough room around it to get good airflow.