3 Smart Laser Alarm Protection Circuits

The post discusses the making of 3 simple yet effective smart laser alarm protection circuit using the IC 555, for securing a specified restricted zone from human interventions. The idea was requested by Mr. Collins.

Circuit Objectives and Requirements

1. I just want to tell you what an amazing passion and dedication to electronics you have and thank you for helping others like me. My name is Collin from south Africa.
2. We have a huge problem of safety here.
3. There are countless number of robberies and house break inns its so unreal. I was hoping you can help me with regards to a circuit for a very cost effective and reliable product.
4. I have built a laser tripwire alarm using a 555 IC timer but the circuit design lacks a lot of features.
5. I need something that as soon as an intruder enters my property I will be alerted before they even get a chance to try and get into my home.
6. The circuit needs the following: Once alarm is activated it needs to sound for a few minutes and then go off
7. and arm again automatically. Don't want it ringing for hours in end if I am not at home to reset it.
8. It should not be triggered by pets or flying debris in the yard.
9. Easy alignment of the sensors. The laser alarm works OK but its very difficult to set the pointer on the LDR that keeps moving off. Any tips if we are going than route?
10. It doesn't have to be a laser system I am open to whatever you think will solve my problem.
11. Thanks a lot for everything and feel free to add whatever you think I missed out with regards to all the features.

1) The Design

Parts List

R1, R4 = 100K
R2 = 1M
R3/C2 = TO BE CALCULATED (SEE TEXT)
C1 = 4.7uF/25V
IC = ANY 555 VARIANT
C3 = 10nF
C4 = 0.33uF
ALARM = 12V, 200mA PIEZO ALARM.
LDR = any standard

IC 555Pinouts

Circuit Operation

The proposed smart yet simple laser alarm circuit using IC 555 can be witnessed in the following image, the functioning can be understood with the following points:

1) The IC 555 is configured in its basic monostable mode.

2) Pin#2, which is the trigger input of the IC can be seen connected with the emitter of a PNP BJT via a blocking capacitor C2.

3) We can also notice a couple of parallel LDRs secured within opaque pipes and their leads hooked up with the base of the PNP BJT, such that as long as the LDRs remain illuminated together through the laser light focus, the BJT remains deactivated. This happens because in the presence of laser light the LDR's combined resistance drops to around 30K, which keeps the base of the PNP more positive than the ground bias from R2.

4) The inclusion of two LDR ensures a fool proof alarm set-up such that only a human presence is able to interrupt the LDRs, while these remain unaffected by other smaller irrelevant intruders such as animals, birds etc.The two LDRs could be placed at a distance of around 2 feet so that only taller objects such as a human being gets detected.

5) Therefore whenever an interruption in the laser beam is detected, the LDRs go through a sudden rise in their resistances causing T2 to switch ON, which in turn triggers pin#2 of the IC via C1.

6) This prompts the IC 555 to activate its pin#3, which finally activates the connected alarm unit.

7) Since the IC 555 is configured in its monostable mode, the pin#3 remains activated only for a period determined by the RC network at pin#6 and pin#7 of the IC, or by R3,C2.

These timing components can be appropriately calculated using the IC 555 calculator software for accomplishing the desired length of time for which the alarm may remain switched ON.

How to Set Up the Laser Transmitter Units

The laser transmitter devices could be installed near the LDRs itself and focused back to the LDRs using mirror reflectors as shown below:

The installing of the laser devices near the LDRs allows the entire unit to be installed inside a single enclosure and also allows the lasers to be powered from the circuit itself.

This also facilitates securing the lasers and the LDR units firmly and accurately so that both the counterparts are unable to move or deviate from their positions even in an event of a mechanical shock or other vibrational interferences.

The mirrors could be positioned at some specified distance, exactly opposite to the laser installation , making sure that the laser beams intersect the restricted zone and the presence of a possible intrusion is detected without fail.

This concludes the making of the proposed IC 555 based smart laser alarm protection circuit, if you have any doubts please feel free to put them forth in the below given comments box.

Video Test Results

Using BJTs for Reducing Current Consumption

The above design can be further enhanced to work with lower standby current, and also with a battery backup during of power failures, through the following upgraded schematic:

Laser Security using Single LDR

If you intend to simplify the implementation for a single LDR operation, in that case the following concept can be tried:

2) Laser Alarm Circuit for Protecting Field Crops against Animals

The second idea explains another simple laser alarm circuit which can be installed in farms and fields for detecting all possible intrusions, either by a human or an animal and alarming this to the owner, and ensuring an effective protection to the crops against such intrusions. The idea was required by Mr. Mohammed and Mr. Daniel.

Circuit Request#1

Congratulations for all amazing work please give me any circuit for agricultural fields, need to protect the crops from uninvited guests like animals i want to achieve an Active infrared barrier [laser] circuit with distance range of 100 meter or above.

Circuit Request#2

sir please my grandfather is an orange farmer and he is having a problem with thief's so he ask me to do something that to protect the farm or anything that can alert him when any thief's or any person try to take away any thing from the farm so i decided to install a CCTV camera,so i need a circuit without a camera or if you can help me out sir i have said this but you can create something that can help me out thank you hope to hear from you soon

Circuit Operation

 The Design

The proposed laser alarm circuit for protecting crops from animals and intruders can be seen in the figure above.

The idea appears extremely straightforward, and employs an IC 555 based monostable multivibrator stage and an LDR detector.

As shown in the design, the laser beam is generated through a laser pointer circuit held at some parallel point across the field fence which requires to be protected.

Since a laser beam has the property of focusing over a given point undistorted and on a straight line, regardless of the distance, the focus in this application is adjusted over an LDR across a specified distance, as indicated in the diagram.

The LDR is enclosed inside a light proof box with a tiny hole which allows only the laser beam to enter while obstructing most of the ambient light around.

As long as the laser beam is held focused over the the LDR, the resistance of the LDR is enabled at a minimum level which could be around 10K to 50K approximately depending on the particular LDR specs.

The low resistance from the LDR ensures a high potential at the base of the associated BC557 transistor rendering it inactive. This in turn keeps the pin#2 of the IC555 monostable at a high potential and the output of the IC at logic zero, so that the relay is held switched OFF.

Now in an even when an intruder (a human or animal) makes an attempt to cross the protected line of fence, cuts or obstructs the laser beam, which instantly causes the LDR resistance to increase and trigger the BC557 device via the 1M resistor.

The BC557 responds to this and conducts grounding pin#2 of the IC and activating the monostable action.

The above procedure forces the pin#3 of the IC to go high and switch ON the relay, the relay contacts possibly being connected with an alarm, sounds the alarm, alerting the field owner regarding the intrusion.

The alarm continues to sound for some moment of time depending on the values of R and C, whose values are directly proportional to the length of the alarm switch ON period.

The above discussed laser alarm circuit can be installed across all the corners of field for ensuring maximum and all round protection for the valuable crops and for ensuring a peaceful sleep to the field owner.

3) Laser Beam Security Alarm Circuit

We have often seen laser alarm system as an integral part of security solution especially for a location which deserves hi-level security.

The Circuit Concept

Right from a museum preserving priceless historical possession; to a safety bank vault; and even in thriller flicks where the protagonist is seen on and often tied in the ray of red light beams, while trying to reach a safe vault; one is well acquainted with the usage of laser light beams.

In fact it is also considered as a security device even in household today to ward off burglary or theft.A laser beam is not a simple beam, but has the efficacy to respond when faced a distraction.

This means that if the light ray gets interrupted with any objects, the photodiode receives resistance which in turn activates an alarm.

The laser alarm system is an economical option when it comes to consumption of power, as the receiver needs power supply less than 10 mA on average.

Setting up a laser alarm system is relatively easier as the laser and the receiver can be setup in one box, on a single power input.

Below is a circuit design diagram which illustrates the flow of a laser alarm system.

How it Works

Referring to circuit diagram it is seen that a TL072 op-amp (IC1.A) is configured as a voltage comparator, placed in between the adjustable voltage driver P1/R4 and the light-dependent voltage which consists of photodiode D1 and R3 – a fixed resistor.

As the laser beam receives a distraction from a foreign agent, the voltage on comparator pin drops to 2 below that at pin 3, thereby enabling the agent swing to positive voltage supply and enable an alarm situation.

As the laser alarm can detect any foreign agents, so is the alarm can be set in a more sophisticated fashion, where it can bypass accidental interrupts from agents, like an insect.

This is accomplished as the Resistor R2 provides a level of hysteresis, thereby prevent oscillation when two comparator input voltage rests in almost equal state. In order to set a faster response method, it can be achieved by reducing the value to 1 µF.

How to Set up the Alarm

A laser beam system is easier to setup, either as a single or a separate entity.

If a single box is maintained to setup the alarm, then it is to be made sure that the photodiode can’t have direct contact with the laser beam.

Assembling the components and the circuit in a breadboard, it should be mounted in a black box having a hole.

A black drinking straw should pass through the hole to enable light flow from the direction of the laser beam to enter. Setting up the system properly makes the laser beam act even with direct sunlight, as it cannot affect the operation of the photodiode.

Article Written and Submitted by Mr. Dhrubajyot​i Biswas

About the Author

I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials. If you have any circuit related query, you may interact through comments, I'll be most happy to help!