Laser Security System: Keep Alarm Sounding

Thread Starter

baykojt

Joined Jun 19, 2019
7
Hey everyone,

I'm a complete novice when it comes to electronics so hopefully this question isn't too insulting to anyone's intelligence. Essentially I'm trying to make a portable laser trip wire security system. Let's say I go camping for example. I'd like to be able to set up a few mirrors and my sensor and have a 360 degree perimeter around my campsite that will sound an alarm if anything crosses it.

I have everything working, but the alarm only sounds when the beam is broken; when whatever caused the disruption moves out of the beam, the alarm stops. What do I need to add to my circuit to make it so that the alarm stays on?

Bonus question: My alarm is very small and is powered by two AA batteries. If I wanted to use a louder alarm (I'm thinking a smoke detector), will swapping out AAs for a 9V damage anything? Again, I'm a total novice so I appreciate your patience.

-John
 
Changing the voltage/batteries won't change the behavior. You will need to add components, either a latching circuit that will require manual reset or a timer that will sound for a specified interval. If you want a loud alarm to sound for several minutes then you will need a bigger battery with more capacity.
 
You have a low power requirement problem which makes things tougher. A standard DPDT relay can be made to latch, but it consumes power. It would have a pull-in voltage to worry about. An SPDT normally closed push button will unlatch it.

There is a dual coil magnetic latching relay that requires a pulse on one coil to latch and a pulse on another to unlatch. They are available in 5 and 12V (possibly appropriate). They to have a pull in voltage specification. they don't consume power when latched. You might need a switching power supply to boost up or down your power supply.

An SCR is inherently latching, but requires a pulse on DC, but with a minimum voltage on the order of fresh AA batteries in series.

Your going to have to re-think your power source. You could have a separate source for the alarm system and another for the horn.

Long duration low power timers (4 min or more) are harder to come by, Is alarming for 5-10 minutes a possibility?

Lithium-ION has a high power density but is difficult to charge, A small SLA Sealed Lead Acid deep discharge battery might be able to be charged and re-used. Solar could be an optional charger.
 

Thread Starter

baykojt

Joined Jun 19, 2019
7
Changing the voltage/batteries won't change the behavior. You will need to add components, either a latching circuit that will require manual reset or a timer that will sound for a specified interval. If you want a loud alarm to sound for several minutes then you will need a bigger battery with more capacity.
Latching circuit. Got it. Follow up question: Where in the circuit do I install it? I can show you a picture of what I have so far if that helps.
 

Thread Starter

baykojt

Joined Jun 19, 2019
7
You have a low power requirement problem which makes things tougher. A standard DPDT relay can be made to latch, but it consumes power. It would have a pull-in voltage to worry about. An SPDT normally closed push button will unlatch it.

There is a dual coil magnetic latching relay that requires a pulse on one coil to latch and a pulse on another to unlatch. They are available in 5 and 12V (possibly appropriate). They to have a pull in voltage specification. they don't consume power when latched. You might need a switching power supply to boost up or down your power supply.

An SCR is inherently latching, but requires a pulse on DC, but with a minimum voltage on the order of fresh AA batteries in series.

Your going to have to re-think your power source. You could have a separate source for the alarm system and another for the horn.

Long duration low power timers (4 min or more) are harder to come by, Is alarming for 5-10 minutes a possibility?

Lithium-ION has a high power density but is difficult to charge, A small SLA Sealed Lead Acid deep discharge battery might be able to be charged and re-used. Solar could be an optional charger.
Even a few seconds would be sufficient. I just need it to wake me up/alert me. 20 seconds maybe?
 

Bernard

Joined Aug 7, 2008
5,394
A schematic of your laser detector would be helpful in mating it with a timer. For SW reset a 2N5060 or other SCR would work as a latch. My choice would be 2 555's or a 556, first as a timer, second as an oscillator driving a speaker or sub. a powered piezeo buzzer for 2nd 555.
 

BobaMosfet

Joined Jul 1, 2009
1,002
Hey everyone,

I'm a complete novice when it comes to electronics so hopefully this question isn't too insulting to anyone's intelligence. Essentially I'm trying to make a portable laser trip wire security system. Let's say I go camping for example. I'd like to be able to set up a few mirrors and my sensor and have a 360 degree perimeter around my campsite that will sound an alarm if anything crosses it.

I have everything working, but the alarm only sounds when the beam is broken; when whatever caused the disruption moves out of the beam, the alarm stops. What do I need to add to my circuit to make it so that the alarm stays on?

Bonus question: My alarm is very small and is powered by two AA batteries. If I wanted to use a louder alarm (I'm thinking a smoke detector), will swapping out AAs for a 9V damage anything? Again, I'm a total novice so I appreciate your patience.

-John
You need an SCR (silicon controlled rectifier). Once it trips, it stays tripped until you remove power from A, K, and G. As for battery, use a car battery. A 9V doesn't output enough current. Or use a motorcycle battery.
 

Thread Starter

baykojt

Joined Jun 19, 2019
7
A schematic of your laser detector would be helpful in mating it with a timer. For SW reset a 2N5060 or other SCR would work as a latch. My choice would be 2 555's or a 556, first as a timer, second as an oscillator driving a speaker or sub. a powered piezeo buzzer for 2nd 555.
Best I could do with a picture!

Just need to know which of those components I need to put the latch in between.
 

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crutschow

Joined Mar 14, 2008
24,298
If I wanted to use a louder alarm (I'm thinking a smoke detector), will swapping out AAs for a 9V damage anything?
Only if the alarm can tolerate 9V.

I go along with using an SCR as a simple latch, which has only three connections.
You just turn off the power to reset the SCR.
The laser circuit would trigger the SCR, which would turn on the alarm sounder.
The SCR has a volt or so drop when on, so would likely work better with a 9V power source.

But we need to see a schematic/wiring diagram to know how to connect it in your system.
That picture doesn't tell us much.
 

Bernard

Joined Aug 7, 2008
5,394
Looks like a light sensitive resistor, LDR, in picture so I set up a simple circuit using a 200 uA gate current TL4003SG2 SCR with a 3V buzzer paralleled with a 22 ohm resistor to raise the keep alive current. Power
is 3 AA Alk. batteries. Ground is batt. -, cathode, & one side of LDR. + V supply to SW, 470 ohm, other end to gate, + buzzer, & 22 ohm resistor. Buzzer off in subdued room light & laser, Beam break = buzzer on until power is cut with SW. Circuit is sensitive to SCR & LDR type.
 

Halfpint786

Joined Feb 19, 2018
12
Why not just run the alarm trigger signal through a transistor with positive feedback and a momentary reset switch to short out the feedback path to turn it off? EDIT: might want a diode to stop the positive feedback from going into the output of the beam detector though... i didn't draw that.
 

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crutschow

Joined Mar 14, 2008
24,298
Why not just run the alarm trigger signal through a transistor with positive feedback and a momentary reset switch to short out the feedback path to turn it off?
Because it won't work.
You can't use positive feedback from the emitter to base of a transistor to keep it on.
The emitter voltage is always less than the base voltage.
 

Bernard

Joined Aug 7, 2008
5,394
Minor changes depending on LDR but operated with 3 different LDRs, with one had to reduce R1 to 620
ohms. With this SCR load needs to draw 22mA to latch. Some buzzers will not keep SCR latched but can be paralleled with a resistor.
Scan.jpg
 

Thread Starter

baykojt

Joined Jun 19, 2019
7
Hey guys! Thanks for the help so far. I finally had time to draw the current set up to the best of my abilities. I’m not 100% sure I got everything correct on my drawing; again, I’ve never done this before. Any help would be greatly appreciated! If you’re able to respond the best way would be to pretend that you’re responding to someone with zero knowledge about electronics because that’s exactly the case haha. I’m just a simple man who like gadgets and doesn’t want to pay through the nose for something I can build myself with a little help. Thanks!
 

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