This is a very simple and easy to construct LASER Trip wire circuit. It uses light detecting resistor (LDR) to detect a LASER beam and trigger alarm when it is lost. Just like the one you see on movies like Entrapment 1999 featuring Sean Connery and Catherine Zeta Jones.

This circuit is designed to work against severe ambient conditions (ambient lighting). And it is has also very fast response. It can even detect beam cutting by a falling coin, hair comb or a fly !!!

Step 1: Components You Need

Receiver Portion (This is the one which reciceves the LASER an Triggers alarm when the beam is cut)

1 LM358 Operational Amplifier
1 LDR (Gives almost 100K ohms at dark , 300 ohms at bright light)
1 20K ohms resistor
2 Similar resistors (20 K ohm can go very well)
1 DC Relay (12 volts for the coil , Up to 220 volts / 6 Ampers for the contact)
1 Diode ( LED can fit very well and be cheap as well)
1 Door Buzzer (You must understand how to connect it very well)
1 6 volts (or more, less than 15 volts) DC Source to supply the circuit

Transmitter Portion (The one which sends the LASER beam to the transmitter)
One Class B LED laser emitter (The red LASER pointer will do the job)

Notes : There are different classes of LDR depending on the values of bright and dark resistances, the one used here is almost 5 millimetres in diameter

You will also need a soldering iron to solder the parts to a PCB , or you can use a bread board to test if the circuit works first before soldering.

This is a video that shows the circuit in action (Update 2)

Step 2: The Layout

Here is the Layout of the reciever circuit. For connecting the LM358 IC note that
1- Each leg of the IC is numbered , number goes up clocwise starting with the one with a carved circle next to it.
2- Use a low power soldering iron so as not to destroy the IC
3- Each leg of the IC is named for right connection in the layout
4- VCC indicates positive pole of the DC source, the ground symbol (three horizontal line of decreasing length) indicates the negative pole of the DC source.

The LM358 IC is an operational amplifier, here it is used as a compare circuit. When the voltage at the positive input is higher than the voltage on the negative input, we have an output. Having two resistor connected in series is called a potential (voltage) divider (Voltage accross two series resistors is divided according to their values, bigger resistor has bigger voltage), when the LDR is lit by LASER beam its resistance is very low and thus its voltage, when the beam is cut, its value goes up and thus its voltage so the op-amp will produce its output to trigger the Relay. The diode connected across the relay is a free wheeling diode which protects the IC from the high voltage produced when the relay coil is off due to its high inductance.

Relay contact used is the normally open contact (NO) , it acts as the push button used for the door buzzer.

When the LASER Beam lights the LDR , the contact is open and the buzzer does not work and vice versa.

Better to connect the door buzzer and the wall socket source after you're done with the IC circuit connection and testing it. To test the circuit , connect as before, once you connect the power source, you should hear the relay giving a tick sound which indicates it closes the contact. when you light the LDR with LASER it should give another tick sound indicating the opening of relay contacts.

Step 3: An Update

This update is to make the circuit latching. Normally closed (NC) contact of the relay series with the LDR will open when the alarm is triggered and relay is energized keeping the relay ON. Normally closed (NC) push button series with the relay coil can be used to reset the circuit bu cutting the output going to the relay de-energizing it so the NC contact series with the LDR will close again and the circuit is off until the beam is cut again. Read the precaution carefully.

Enjoy

Step 4: Update 2

This update is to make the circuit more reliable. If someone manages to cut the power off the circuit it will raise the alarm, since the relay is energized in normal condition and de-energized in trip condition then it is more resistant to manipulation.

Latching is done by NO contact of relay, which will be closed in normal operation as the relay is energized.

Starting the circuit and resetting it is done by NO push button parallel with it which overrides the NO relay contact and make the circuit produce output as the LDR is lit by LASER.

Read the precaution carefully.

Enjoy

Step 5: An Alternate to Update 2

This alternate update is the same as update 2 in function.

You can try either depending on different values of resistors you have.

Remember you can adjust the sensitivity of the device to LASER absence by changing the value of the resistor connected in series with the LDR.

For update 2 and its alternative, you can use the NC contact of the relay to control the door buzzer directly, or better you can use the NO contact (which will be closed during normal operation) and use it to energize another relay and take it NC contact (which will be open during normal operation) for running the buzzer.

The idea of using the NO contact which is energized during normal operation is that if somehow the wires between the relay and the AC relay fails, the alarm will still be triggered which gives more safety to the circuit.

If you want to connect the system to a PLC or a micro controller or Arduino, you can use any relay contact you want. But I recommend the NO contacts that will be closed during normal running and open during intrusion thus the system will always be alert, even an internal damage could be detected as an intrusion.

Tip for running long wires from relay contacts: A floating wire (not connected to anything) for a long distance tends to develop a voltage from magnetic induction of wires surrounding it. The solution is to make it either connected to a voltage source (a live connection, intentionally energized) or when de-energized is to be connected to the earth (earth terminal in an electric panel or any large metallic object buries in earth around).