Laser Beam Alarm System with Rechargeable Battery for Laser

Step 6 Second Half of the Final Schematic, The Alarm.

The main purpose of this half of the schematic is to replace a design floor that I noticed in kipkay's version, no offence dude; I really love your work by the way, awesome!! Anyway, the problem was that when the alarm was triggered in kipkay's it would only stay on for a brief moment after the laser was restored to the LDR. This was because all he had to power it was a capacitor.

I wanted my alarm to stay on even once the laser had been restored to the LDR, and this is what I have done. How it works is the transistor (I dont know what type, I think NPN, pros help me out please) keeps the circuit open. Once contacts one and two (refer to diagram to understand what Im talking about) make contact they trigger the transistor to allow current flow to pass, this flow of current in turn keeps the transistor open, meaning it will not close the circuit (keeping the alarm on) until someone physically flicks a switch to reset/turn it off.

Contacts 1 and 2 are closed using the relay that I was talking about earlier. With the LED from the first circuit replaced with the coils of the relay, when the LDR detects that the laser beam has been broken, current will flow into the coils of the relay. These coils generate a magnetice field which closes the reed switch inside the relay. This reed switch is contacted to contacts 1 and 2, closing them which will turn the alarm on. Now the alarm will stay on because, it has a power supply all of its own.

Very confusing, I dont even know if I fully understand it, but it works, and it works really well!!

Comments

[Dhinesh kumar]

        My hearty congratulations for you. It is a simple but effective piece of work. I really appreciate that. But, Does it really works...? I mean, Is it possible to connect the BASE of the transistor directly to 9V battery supply through just a 120 ohm resister....?
        And more over, how can a transistor be in ON position, if its collector terminals are OPEN...? You said that the LED keeps the transistor ON. But I really couldn't get the logic in it... Please can you explain me that.....? I am eagerly waiting for your reply....! Please.........

[revhead]

well my project is very dusty know and is sitting right in front of me and it work perfectly, apart from the fact that my sister dropped the laser housing and broke the laser diode!! i am not very experienced with electronics, i dont study it, it is a fun hobby of mine and was my year 12 systems engineering project, all i had to do was present a basic understanding of the circuitry and modify existing circuits to combine them into one project. from your comment you really seem to understand what you’re talking about, so if you have time i would appreciate it if you could explain how it "actually works" and what you would change?? as to your question if it works...yes, it does exactly what i intended it to do, laser shines on LDR, when laser is broken from LDR it activates relay, relay closes secondary circuit which activates alarm, when laser is returned to LDR relay opens secondary circuit but alarm stays on thanks to transistor which keeps a segment of the secondary circuit closed to power alarm! that’s pretty much my knowledge, if you could expand on that it would be great.
Cheers!

[handyman29]

I have a question. Can I hook up a siren circuit (two 555 timers, a few resistors and capacitors) that I made where the "BUZZER" goes? It makes a sound similar to a police siren. If so then maybe I can give you the schematic.