Do you need a portable alarm system? Something cheap but very effective? Then this covert laser alarm system is the one for you. I have seen a few laser alarm instructables, but they all only trigger the alarm while the beam is broken. This alarm is set up to use any regular laser pointer and the receiver. When the laser beam is broken (even for a second) It will trigger the alarm indefinitely (until the power is cut off). The alarm can range from a simple buzzer to a silent LED to even a full blown high power alarm.
This alarm system is relatively easy to do if you have basic electronics knowledge in reading schematics.
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Step 1: Schematic and Parts
I made this circuit based on two basic circuits, an op-amp comparator and a NOR Gate latch. The basic principle (in electronic language) is that R2 sets the reference voltage, while the photoresistor and R1 create the input voltage. Originally, the op-amp output is high when the photoresistor is illuminated, but the PNP transistor reverses this and allows the signal to go low for the latch. I could have switched the inputs for the op-amp too but for some reason this didn't work. With the output reversed, when the photoresistor is dark, the output to the latch is high. This allows the latch to trigger the buzzer or any other alarm no matter if the photoresistor is dark or illuminated. S1 resets the latch circuit only if the photoresistor is illuminated. You can use a key switch for this purpose so only you can reset it.
*When power is given to the circuit the alarm is triggered automatically. To prevent this, S1 must be closed. (explained later)
R4-100 Ohms (optional)
T1-Any PNP Transistor
T2-Any NPN Transistor
IC2-Dual Input NOR Gate (4001)
Cheap Laser Pointer
An enclosure to put the receiver circuit in
*additional items will be mentioned later
Step 2: Building the Circuit
After obtaining all the required components, you will need to assemble the circuit board.
Most of these components can be found can be found at Radioshack (I got the Dual Input Nor Gate at Fry's electronics).
The circuit is quite simple and assembling it on a prototype board should be pretty easy. At this point you should begin thinking about where you want to mount the receiving circuit.
Step 3: Making the Receiver
After putting together your circuit board, you should mount it in a box or case. I chose this box from Radio Shack that barely fit everything in (including a 9 volt battery). Make the appropriate holes to mount the switches, L.E.D., buzzer, and light sensor.
Step 4: Using the Laser
The laser I used was the cheap red ones for like 2 bucks. To make it more discreet I wrapped it with black tape. Wherever you choose to mount it, make sure it can be adjusted easily and that it can be left on. A good idea is to modify those metal flexy lights so you can attach the laser. I used a hose clamp to leave the on button pressed.
Step 5: Setting Up the Alarm
To test your alarm first align the laser with the sensor.
When you turn on the receiver, first press the prime switch while turning on the power switch to prevent the alarm from sounding.
As long as the prime switch is pressed, the alarm will not sound.
If the laser is aligned correctly the green LED will be on
Once the LED is on, you can let go of the prime switch and the alarm will be set.
If the laser beam is broken, the buzzer will be triggered and will ring until you turn the power off or you reset it.
Step 6: Going Further
This alarm system has many uses ranging from anti theft, to espionage, to paintball. This circuit can be adapted to go anywhere and be used in a wide variety of circumstances.
Have fun and enjoy :)
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