Laser Tripwire Project

So I am trying to work on a laser tripwire that will sound off some sort of alarm or trigger system.

I am trying to decide what sort of laser diode to use, I am most likely going to go with a 650 nm or somewhere in that spectrum. I have built a burning laser using the PHR803T and it came in an Aixiz module.

For this project i plan on using a low power (5 mW for safety reasons) red laser for the 'wire'. I am having trouble finding a focusable module (like my Aixiz module) that will suit a 5mW red laser and I also am having trouble finding a reasonable NOT BULK 5 mW diode.

I plan on making the driver myself using an LM317 current regulator, some resistors, a pot, etc.

If anyone has any suggestions of any of the parts i mentioned, please feel free to reply.

Perhaps you should use a low powered IR diode instead of the red.It would be little bit more dangerous to put together if you dont have a lot of experience, but would make for a more "stealthy" trip wire.

Just go to your local Dollar Store an get a red pointer there...
And you can use a small solar cell and one transistor
amplifier as the detector...
I'd stay with red so you can align it easily.... IR is really difficult
to see and align at <5mW...

Jerry

Solar sell? what happens when he opens the door? You would have to enclose that solar sell pretty well. That could work though.

However, the most common thing to use in this case would be something called a photo resistor. CdS Photoresistors (5-Pack) - RadioShack.com .
Then again, i'm not sure what spectrum of light this works well with. Still, its resistance is proportionally to the light emitted to it.

Here would be my design for it. (yes i'm sure theres better out there so shush =P )
Output would be active high. So once the wire is tripped, the output would be 5V. Would have to press the reset switch/ button in order for it to reset. You would want this since trip wires are only tripped for a split second.



Would require and external powersource. To use laserbees idea of a solar panel and power your circuit with it would be kinda cool but super hard to do. This would require an external battery.

I wanted to make the driver for a low power Laser Diode using the LM317 instead of being a cop-out and purchasing a pointer. I wanted to hook it up to a toggle switch as well. Greater than 5mW is acceptable but I don't want to be accidently damaging peoples eyes in 0.01 seconds with a 100mW laser. I can't imagine getting the ... what I assume to be 20-30 mA out of an LM317 to be difficult to configure and there is probably a little wiggle room, I am not a super expert though so correct me if I am wrong on that.

I still don't know where to find a focus-able lens w/ a low power diode installed (driver not needed)

I was already thinking of using the cadmium-sulfide photocell resistor for the triggering system, possibly in conjunction with a MOSFET. Getting a trigger system to work probably would be easy, I am still working on a reset system....

My Primary focus right now is making sure I can make the driver using the LM317 and making sure i can get my hands on a laser diode...which has been the biggest difficulty lol

Well... you can't just use a mosfet. You could use a mosfet as a current driver, but that still would only be active high for a split second unless the person decides to stand in front of your laser for awhile. If you want a alarm or something to go off, your going to need logic of some sort which would require a decent amount of mosfets or just 1 lm324 which is the equivalent 30+mosfets. However,... I guess you would want 1 mosfet to drive your output (a op amp would not have the power to drive a big alarm or something)



1 battery, 1 photo sensitive resistor, 1 lm324, a pack of 1ks and a pack of 10ks, 1 cap+1diode and your good to go for the sensor. (+ whatever you want to activate such as an alarm..ect)

as for the laser, any .5mW store bought laser would work. Or just about any laser you make. Eitherway wattage does not matter!!!!! lumens/brightness does for a photoresistor. Unless your goal is to fry it... A .5mW laser can be plenty bright. +its a focused beam. Technically for a security system though you would want a IR 800nm something laser. However, a IR laser can be detected by some cameras so... lasers are not always the best way to go for security.

As for the output of a lm317...
Ideally it can supply 32V at 1.5A. Thus 48W? practically speaking, and as far as Powering LEDs, should be able to pump 1.5~ amps through a diode which can correspond to almost 2W of optical output if using a 445. Could be wrong on that myself.

random person said:

Solar sell? what happens when he opens the door? You would have to enclose that solar sell pretty well. That could work though.

However, the most common thing to use in this case would be something called a photo resistor. CdS Photoresistors (5-Pack) - RadioShack.com .
Then again, i'm not sure what spectrum of light this works well with. Still, its resistance is proportionally to the light emitted to it.

Here would be my design for it. (yes i'm sure theres better out there so shush =P )
Output would be active high. So once the wire is tripped, the output would be 5V. Would have to press the reset switch/ button in order for it to reset. You would want this since trip wires are only tripped for a split second.

Would require and external powersource. To use laserbees idea of a solar panel and power your circuit with it would be kinda cool but super hard to do. This would require an external battery.

Click to expand...

What solar are you selling.....:thinking:

What door....:thinking:

It is a known common fact that if you are detecting a
specific light source that all other light sources need
to be removed (as much as possible)

The post was to give him a simple idea that he could elaborate
on... I really don't like spoon feeding members that could easily
assemble a small part count detector. I will give them the basics...
It is the best way to learn... IMO

A Photo resistive cell (selenium cell) is old technology and has
a slow response time compared to a solar cell or a photo diode
or photo transistor...

BTW..... where do you find a variable photoresistor....:thinking:

Oh really...... Let me check that out in the shop tomorrow..
I'll get back to you on that one...



Jerry

lasersbee said:

What solar are you selling.....:thinking:

What door....:thinking:

It is a known common fact that if you are detecting a
specific light source that all other light sources need
to be removed (as much as possible)

Click to expand...

right, hence the encasement, I guess that should have been a no brainer... Door was in reference to a garage door, was thinking that was what he was designing at the start of my post. My bad

The post was to give him a simple idea that he could elaborate
on... I really don't like spoon feeding members that could easily
assemble a small part count detector. I will give them the basics...
It is the best way to learn... IMO

Click to expand...

Very few people ever really truly "learn" something on their own. Only a few people in the history of electronics ever actually learned something on there own. (aka postulated the theories and concepts of resistance, voltage, ..ect). So if one doesn't "spoon feed", they are merely providing inadequate information(assuming they don't already know how) In which case they will simply have to go elsewhere to "learn" from someone else. A book, friend, internet, IE. That takes along time... Ahh well, different styles. Neither style of post is inherently wrong. I just prefer one as you the other.

A Photo resistive cell (selenium cell) is old technology and has
a slow response time compared to a solar cell or a photo diode
or photo transistor...

Click to expand...

You may be right? Never looked at the datasheet of the response times of photosensitive resistors. It may not detect a bullet, but it should detect a human walking/running through a beam of light.

Edit: I love my scope =) Decided to try it out.



20ms delay time till double vf.

Soo.... The person would have to move past it in less than .02 seconds for it not to detect. I would think thats enough unless this person is mighty fast? Theres probably faster response photoresistors, this one cost less than a dollar.

Edit2: http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=350-00009-ND

5ms fall time. Person would have to move past the sensor faster than .005 seconds in order to not trip it. I'd say thats realistic enough.

BTW..... where do you find a variable photoresistor....:thinking:

Click to expand...

BTW..... where do you find a non variable photoresistor....:thinking: ? All photo resistors are proportional to light emitted on them. Not sure what your getting at. I used a pot in my schematic because I lacked a better symbol and it acts as a pot in a way. :yabbem:

The reason i want a low wattage from the laser is because I don't want it to be a potential hazard to some doofus who decides to stick their eye up against the laser. I am aware that intensity isn't an issue, 1mW from a red laser can cause a resistance increase from 0.4 up to 250 ohms, etc. etc.
-Should i just buy an Aixiz module and outfit it with a low power diode (WHICH I STILL CANNOT FIND)

But i don't want to cop-out and just buy a cheap laser pointers (i already have one).
I'd rather just get a low power Laser Diode and use my own driver with the LM317 + a SPST switch. So, i am still unsure where to find lower powered laser diodes that are accompanied by focusing modules, the 100mW and greater are really abundant on the internet. (I don't know why i find it really difficult to find one)

EDIT: The trip-laser system is more of a hobby thing people entering rooms, or whatever, It doesn't have to serve some insane practical purpose (ie. being hooked up to a bunch of alarm systems or operating cohesively with other trip-lasers, The reason I am doing it is because I'm a Senior majoring in Electrical Engineering and I didn't get an internship for the summer so i am trying to find better things to do with my time than play video-games...
[I took a trip to the store, all my components for the driver...except the LM317 (forgot it) Radioshack didn't have a 100 Ohm micro-pot and it made me angry.

Question: Is the voltage drop across a laser generally universal for the different types of laser diodes, IE. red approx 3V drop, etc.

Maybe try get one of these: $3.95 - Red Laser Module - Focusable Dot (3.5V~4.5V 16mm 5mW) - Red Lasers

I saw that, the depressing part is that it comes with the driver already so all you need is a voltage source of about 3-5 V, which takes the fun out of making the LM317 Driver...(I already bought all the stuff :'( ) It is still an option though.

random person said:

Very few people ever really truly "learn" something on their own. Only a few people in the history of electronics ever actually learned something on there own. (aka postulated the theories and concepts of resistance, voltage, ..ect). So if one doesn't "spoon feed", they are merely providing inadequate information(assuming they don't already know how) In which case they will simply have to go elsewhere to "learn" from someone else. A book, friend, internet, IE. That takes along time... Ahh well, different styles. Neither style of post is inherently wrong. I just prefer one as you the other.

Click to expand...

Man... you have a dismal outlook of the LPF Community....
I'm really going to need to disagree with you on that one...
Again sorry.... but I'm not writing a book... What I was doing
is helping a member by pointing him to a solution to his
questions...
I'm also sorry that that is not your method.... but that is mine.

You may be right? Never looked at the datasheet of the response times of photosensitive resistors. It may not detect a bullet, but it should detect a human walking/running through a beam of light.

Click to expand...

Agreed.... but the OP was not specific in what the use of the tripwire
was... perhaps he needed to trigger a camera when a bullet tripped
the sensor...

BTW..... where do you find a non variable photoresistor....:thinking: ? All photo resistors are proportional to light emitted on them. Not sure what your getting at.

Click to expand...

I was referring to the Photoresistor with the 3rd wiper pin...(like a Pot).
Without an explanation it looked very strange... perhaps you had found
a new component that I had never seen before...

Well... you can't just use a mosfet. You could use a mosfet as a current driver, but that still would only be active high for a split second unless the person decides to stand in front of your laser for awhile.

Click to expand...

I can tell that you have not built the circuit that you proposed...
Do you even understand its functioning...:thinking:

That is a latching circuit.... once it is triggered it stays in that
state until it is "reset" by the switch identified as "Reset" on your
schematic... It is a charge pumped latching circuit...:beer:


Jerry

Sorry for the double post but the last post was long enough...IMO..

Here is a circuit that I put together and tested this morning...
It works well with a 5mW red pointer at a distance of up to 25 feet
in the shop light environment...

I first used a Solar cell from a $1.00 calculator with the identical
circuit... but it required some small amount of ambient light
shading..

I then tried an old trick of using a large T1 Red LED as the detector...
and that provided enough ambient light elimination that it worked
under the shop lighting as well as movable local work light without
extra shading...

I added a CD4584 Schmitt trigger buffer to make sure the output
signal transitions cleanly without chatter while the Laser beam was
at the level switching threshold..

To interface that circuit to the outside world you can easily drive
the LED in an optoisolator from the Schmitt buffer's output.


Jerry

Your trigger circuit looks pretty similar to the one I started to come up with in my head,
except I am confused because where is the detector, what I would think is the detector is labeled as a red LED, and i can't tell where the "photons" are ponted towards

I appreciate everyone's help with the trigger system,
My primary problem right now still being getting my hands on a laser diode that wont hurt peoples eyes after they stare into the laser for less than 4 seconds
1-5mW is about 10 seconds (or so i've read)
[Do not want to use a premade laser pointer, prefered if i can just use an LD and a focusable lens]
I need to determine the parameters of the photo-cell and/or solar-cell

Yeah... it's a basic 1 trasistor amplifier circuit.... with component
values chosen for the parts that I used... I tried to keep it simple
and cheap... The parts should cost ~$1-@$4 including the 5V PS..
More on the lower side if you scrounge around or have some
of the parts in your electronic junk box...

As I explained in the last post.... The RED LED is the detector...
The little sguiggly lines show that the LED is not used as an
Emitter but as a Detector...

Like I've said before and a member even linked to... Get a Cheap
Dollar Store Laser with PS already incorporated and use your LM317
as a 5Volt regulator to power your circuit..
The LPM317 Data Sheets give examples...


Jerry

OK i ordered the LD the previous post suggested, going to start getting to work on the Trigger since the laser wont come for a while. =)

Completely forgot you can use LEDs as photo-detectors