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.
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