Laser Trip Wire Alarm(LDR) Question

[kefalonitis]

Hi all.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.
Does any one know a SIMPLE(im not an electrician) circuit that I can use?

This is what im using now.

 

[lurchman]

A simple answer is to replace the relay with a multiple contact relay with normaly open and normaly closed contacts.

If you arange for the relay coil to be powered via the relay contacts and a momentary push button to reset it then it will stay in the active state.

That is let us assume the relay is normaly held on and a normaly closed pair of contacts are used to drive the bell (ie fail to alarm). If the power to the relay coil also goes through a normaly open pair then when the relay drops out the circuit to the coil is broken. A simple push to make switch across this contact pair will act as a reset and allow the coil to pull in the relay thus closing the normaly open contacts to hold the circuit after the switch is released.

 

[dr pepper]

If you understood that circuit enough to get it to work, then allthough maybe not a leccy, you are definately an electronics enthusiast.
The system described above is in common use in motor starters, and most relays tend to be double pole so should be easy to do, in fact you can do it with a single pole if the transistor is a high enough rating.

A couple of drawbacks with your system would be that if direct sunshine fell onto the photocell it would saturate and the system wouldnt trigger if the beam was broken, you might want to put the photocell at the end of a tube so that the sun cant hit it direct, another issue would be someone shining a torch on the photocell would easily be able to overide it and walk past the beam without triggering it, a solution to this would be a toneburst system, where the laser is modulated at a set frequency, and the photocell circuit made to only respond to this.
Kinda complicates things though, and maybe not appropriate, non the less you might be interested to know.
If you made the tube over the photocell long, like 2 feet it woudl be very difficult for anyone to shine anything down it without first breaking the beam and triggering it,