I am making a laser door alarm in which whenever somebody breaks the laser path, a continuous alarm should be generated. Although the design which i used is working properly, the problem is that buzzer only beeps when somebody breaks the laser beam and stops as soon as the laser beam falls on the phototransistor again.
A thyristor can be a perfect choice for solving the problem. But I don't understand how to choose the correct model of a thyristor for a particular circuit (especially where it is used for latching)
The circuit design is similar to what is given here:
electroschematics.com/4929/laser-based-door-alarm​

 

Attach a schematic here so that we can see what your circuit looks like.

hgmjr

 

Never mind, I see you have supplied a link in your first post.

hgmjr

 

You are missing a latching function in the design. The way it is working is the way the circuit you breadboarded was designed to work.

There are probably some solutions that better suit your application at www.discovercircuits.com.

hgmjr

 

A recent post uses 2N5060's to latch LED's & a buzzer. Buzzer was in anode ckt, reset by momontary interuption of + supply to anode. Look up Otaku, Latching LEDs question, Pg 6 & 52, Projects Forum