It uses a LDR as a sensor and 555 ic as the brain.
The circuit is very simple.
it uses two 555 timer ic one as Bistable and other Astable .
Step 1: Components Needed:
the components needed are very few.
1. 2*555 IC.
2. 2*8pin DIP socket.
3. 1*8ohms 0.5 watt speaker.
4. 1*Push Button.
6. 1*PCB board.
7. 1*6v relay.
10. 1*100 k preset.
11. 1*100 ohms resistor.
12. 1*22 k resistor.
13. 1*1 k resistor.
14. 1*2.8 k resistor.
15. 1*0.1 uF cap.
16. 1*0.01 uF cap.
17. 1*10 uF cap.
18. 1* Laser.
Hot Glue Gun
Step 2: THE CIRCUIT
A LDR and a 50k preset are connected across the voltage and Ground.
The resistance of the LDR decrease when light falls on it.
Hence when light is falling on the LDR the resistance of the LDR decreases and hence the voltage across it.
When the light is not falling on it the resistance of the LDR will be high and hence voltage across it.
The voltage across the LDR is connected as input for the not gate.(using 2n2222a NPN transistor).
Hence the output of the transistor is low when light is not falling on it. And the output is high when the light is falling on it.
This output is connected to a 555 timer circuit used as Bistable multivibrator.
The output of the 555 goes to high when pin 2 is triggered low and the output remains stable until the pin 4 is connected to low.
when light is not falling on the LDR the output of the 2n2222a goes low which is connected to the pin 2 of 555 ic. which makes the output to go high this powers a RELAY and a 555 Astable multivibrator circuit which makes a loud noise .
The 555 ic's output remains high till the push button is pressed.
The circuit can be powered from 6v to 15v.
One can find many circuits of the same type.
But the difference is this circuit keep on making the sound if triggered once until the push button is pressed.
Step 3: Build It
After all my soldering work was done I used some jumper connectors to connect Relay, DC jack, Push button, LDR and speaker to the PCB. Then I closed the box and secured it with some masking tape.
Step 4: Setuping the Laser
I attached the laser to a Usb chord.
I used a screw and nut to attach everything and focused the Laser on the LDR which is a bit difficult task if it is placed in longer distance.
You can see the laser Holder design
In that design the laser can be adjusted to focus at various heights which makes the work easier.
you can get it 3d printed.