Laser alarm system not working correctly..

[neba39391]

Hi guys, i am a first year college student and this is my first project to build by myself.. and i need help.
I build this laser alarm system and it works good. the Alarm sounds when the laser is broken and it stops when the laser is not interrupted. But the problem is when the laser is disturbed the alarm sounds but as soon as the laser touches the photocell again the alarm stops..
i was thinking, is there any way to make the alarm sound constantly non stop ones the laser is broken, Is it possible to make the alarm sound continuously without non stop unless it is stopped manually?
Please look at the schematics and help me..
thank you

View attachment 41667

 

[Hero999]

Yes, you could use an SCR (thyristor) which will turn on and remain on until the anode current is interrupted or you could use two transistors configured as a bistable multivibrator.

 

[neba39391]

I went to radio shack to buy the scr and the told me they don't have it in store and I can order it online and when i went online there are alot of options, can someone tell me which to buy and some people told me I could use two transistors to make the current flow continuesly if so where do I add the second transistor? Thank you very much

 

[Hero999]

I went to radio shack to buy the scr and the told me they don't have it in store and I can order it online and when i went online there are alot of options

There are other better places to buy components from than Radio Shack.

can someone tell me which to buy and some people told me I could use two transistors to make the current flow continuesly if so where do I add the second transistor? Thank you very much

Type bistable multivibrator into a search engine.

 

[neba39391]

I was working on this project and i connected everything according to the schematics, but i am facing one problem. when i supply power to the circuit(with no laser on the photocell) nothing is happening when it was suppose to sound, and with out changing anything to the connections when i apply power to the circuit again(everything the same) the alarm sound.... one time it sounds and one time it doesnt.. I am guessing there is a problem with the resistors and their amount, but i am not sure.. can you please help me find the problem and tell me how to correct it. Thank you
I attached the schematics... 

View attachment 41669

 

[diyvn.net]

I also have a project on laser alarm system.
I will soon update it here.