Low cost - long-ish distance laser?

Why not a simple RF link? ...Jim Thompson

I encourage you to forget the laser and pursue a wireless solution. For example, my wireless mailbox (300 feet away) alarm triggers when the front door is opened and transmits a signal to my indoor receiver. The transmitter sleeps most of the time and the battery has yet to be replaced after two years. I manually reset the indoor receiver with a PB.

Good luck.

Note pilots have been complaining about folks on the ground using those little "presentation pointer" lasers to "blind them" in flight.

This should give you two pieces of information:

- range is probably easily addressed with a cheap, disposable, battery unit

- do you want to risk damaging the eyes of anyone unfortunate enough to pass through the beam (changes in elevations between "gate frame" and "vent pipe"; miscreants seeing intriguing visible line-of-light and wanting to "see" where it goes, etc.)

I seriously doubt that is possible. More likely lazy and ill-educated reporting. It is the norm these days.

I don't care about the eyes of 17' tall people. Patio is 4' off the ground.

F*ck em. They are not called "miscreants" for no good reason.

Thanks, I had tried using a router and wifi last year in the pump-shed between the gate and the house. Very unreliable at 300-ft.

My electronic skills are not such that I can design my own system. Capable of building kits with soldering iron etc. I guess I could buy an RC controller and try messing with that but it gets expensive fast.

Dave

Several, not so young, "kids" have been arrested in the Phoenix area by the FBI for using "pointer" lasers aimed at commercial aircraft.

A laser seems the hard way... but I guess you must have your fun ;-) ...Jim Thompson

Oops. Sorry for the empty post.

I'd keep the sheet for daytime, and rig a battery-powered LED light for nighttime. Just have the same sort of trip-wire that drops the sheet turn on the light. You'll still need to look, but your eyeballs are going to be a much better detector than anything mechanical, and they're already paid for.

The super-bright LEDs are obnoxiously efficient these days, and if you set up a flasher circuit that turns the light on for 1/10 of a second once a second you'll stretch battery life even longer, and probably make the light even more noticeable.

For your WiFi - you needed a high-gain antenna at the pump-shed. Remember that for next time.

Half a milliwatt of red is way more than you need for that job, and is entirely legal to use unshielded. Keeping it clean and dry will be the issue. But if the OP isn't comfortable designing his own gizmo, then a wifi Yagi antenna is probably the ticket.

Cheers

Phil Hobbs

Pointer lasers are a few milliwatts and are badly collimated. And hard to aim.

"Clean and dry" is always a problem. I've been wrestling with how to use the nearby street light (nice high-up light sensor) to control my yard lights without having the maintenance of lens cleaning. ...Jim Thompson

A modulated laser pointer wouldn't be hard to make. Modulate at some nice audio frequency, so the receiver can be a photodiode+audio amp+speaker. You could maybe hear other interesting stuff, lightning and modern car headlights and such.

I wonder how big a spot a typical pointer would make 900 feet away. I don't have one handy to try.

The miscreants are typically using something a little more powerful than that - now far too easily available on the internet.

Beam divergence of a cheap and nasty laser pointer is around 0.002 rad so at 1km range it is roughly a 2m spot size. Dark adapted pilots can be flash blinded by people pointing them at the cockpit.

Green ones (frequency doubled NdYAG) are available very cheaply. You might want to deliberately spread the beam a bit with a lens at the source to avoid issues with idiots staring into it.

At night my green laser pointer appears to go to the vanishing point on backscatter so that I can quite literally point at stars for astronomy lectures. I think you may need to use a Mk1 eyeball to detect it though in your application or some other technology altogether like radio.

Hi Dave,

Retro-reflective tape on the blue flag with detection by shining a Maglite on it from your house might work. 3M makes good quality retro reflective tape and it is carried at Amazon.com.

The really cheap 2.4GHz Yagi antennas found one eBay and Amazon are junk. I've tested a few and found them seriously lacking in everything except cost. If you tear apart the feed, it's a simple dipole, no balun, and plenty of exposed wires from the coax, which doesn't work well: When I tested one of these, it had more gain in the reverse direction than forward.

Others are simply designed badly, such as this MFJ-1800: which lacks a balun and where analysis shows a characteristic feed impedance of 200 ohms instead 50 ohms.

There are decent Yagi's available, but they're not cheap:

If the OP needs more gain to cover the 900 ft range, a panel, patch, biquad, coffee can, dish, or whatever antenna will probably do better. Or, he can go cheap and just add a corner reflector:

On the other foot, the OP can buy a "mailbox alarm" or "wireless remote control switch" on eBay. It might be a better solution than Wi-Fi.

This one claims 2000 meters and looks like it just might work: This one claims 1000ft range, which is probably inflated: Would you believe 3000 meters? I don't but it's cheaper: For USA, make sure it's on 433 MHz, not 315 MHz.

I've also built remote controls, alarms, and remote triggers using common GMRS/FRS walkie-talkies which will certain have the range.

A decent quality one about 2' a bad one 6'. Beam divergence is typically around 2 mrad.

Main worry is that cheap and nasty ones might be leaking significant quantities of the near IR NdYAG pump laser as well as green light.

The suggestion of a flashing high power LED made elsewhere in the thread with a simple collimating lens is probably better.

Good idea, thanks. We have some LED spotlights that can light up road signs almost a mile away and facing off at a weird angle. That'll do it I have some reflective tape that came with a digital tachometer. Might be all I need.

Dave

Thanks Tim, I was going for that idea until I saw the reflective tape suggestion below. :)

Tape means less work, so lazy wins over cool.

Dave