Many saw in movies of the alarm system which work when overlapping laser beam. It is not fantasy, such devices really exist. Moreover, they are quite available to independent production by house masters.
Instruction
1. Buy the cheapest low-power red laser pointer in newsstand. Take the unnecessary charging device from the cell phone and the resistor with a resistance of 200 Ohms and 0.5 W. Connect pointer to the charging device via this resistor, observing polarity (plus - on the body). By means of piece of plastic and wide adhesive tape fix the button of inclusion of pointer in the pressed situation. Turn on the charging device in network - the laser beam has to appear. Do not send it to eyes at all. Current through pointer has to be about 35 mA. If it differs from this value, pick up the resistor more precisely. You make connection and shutdown of measuring tool at powered off.
2. For power supply of reception of part use the second charging device from phone. Take the photoresistor and the special sensitive relay called polarized. Connect them consistently and connect to energy source. Such relay has two contact blocks, one of which becomes isolated at one voltage direction on winding, and another - at another. Define by ohmmeter what of these groups becomes isolated when lighting the photoresistor. If reaction of system to emergence and disappearance of light is indistinct, pick up other combination of the photoresistor and the polarized relay. Surely make sure that both elements of chain at long lighting do not overheat.
3. The polarized relay cannot directly operate call by two reasons. First, both of its contact units - normally opened, and are required at least one normally closed. Secondly, they are not calculated on the big current consumed by call. The intermediate relay will help to solve both problems. It has to be calculated on tension in 5 V and current in 20 mA. Connect it to energy source through that contact unit of the polarized relay which works about lighting of the photoresistor. That contacts of the polarized relay did not burn, winding of the normal relay to zashuntiruyta in the return polarity (the cathode - to plus, the anode - to minus) the KD243Zh diode or import type 1N4007.
4. Use call from set of the children's electrodesigner. Some such calls demand assembly - in this case, carry out it, being guided by the instruction to the designer. Connect ready call to energy source through normally closed contacts of the normal relay, and its winding also to zashuntiruyta the diode in the return polarity.
5. The transmitter and the receiver arrange so that the beam of the laser fell on the photoresistor. Make sure that every time when the beam is crossed, the call rings. If desired, the beam can give irregular shape by means of the mirrors located at an angle. Do not allow reflection of beam not back in the laser - it can put it out of action. As the self-made laser alarm system is not certified, it can be used only as demonstration installation, but not for protection of real objects.