Laser Trip Wire Alarm(LDR)

[kefalonitis]

I am looking for a circuit that would allow a laser to start an 12v alarm when the beam is cut, but i want the alarm to keep on as the beam hits again the LDR .Mainly my problem is that when the beam hits again the LDR the alarm stops. I don’t want it to stop. Or maybe use some 555chip and make it stop after 20sec or so…
Does any one know a SIMPLE(im not an electrician) circuit that I can use?

This is what im using now.
**broken link removed**

 

[ericgibbs]

hi,
What you need is a 555 connected as a 20sec monostable triggered by the relay.

Does that help.?

 

[kefalonitis]

Kind of.. Any schematic? Or other way. Sorry , im a beginner.

 

[ericgibbs]

Kind of.. Any schematic? Or other way. Sorry , im a beginner.

hi,
Look at this link.
With Homework questions we will only give hints, when you have looked over the link, come back with questions.

https://www.electro-tech-online.com/blogs/ericgibbs/125-simple-7555-cmos-timer-calculator-v1-2.html