Laser alarm circuit

I am searching for a circuit which can drive a siren circuit like this and can be activated by a laser pointer.I have tried a lot of these circuits but all had the same problem:The siren sounds only for a few seconds(As long as the interruption of the laser beam...)

Thank you in advance!

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freethetech6 years ago
I would just create a latch using a relay -

If you wanted to take it a step further, I'm sure you'd have no problem creating a logic gate so that if the latch output is high and it is activated by the laser it turns low, and vise-versa.
pizza1993 (author) 6 years ago
Thank you for your advice!What i really need is a time circuit that can be turned on with a instant high voltage(5volts+) and the output voltage remains for a calculated time period,but cant be turned on by a low voltage(0.5volts)
Goodhart6 years ago
What you then need is, according to your description above, a "latching circuit" OR just a latching relay. It goes on, and stays on, until you turn it off, or until some other signal turns it off, however you set it.
caitlinsdad6 years ago
Then you need the photorelay to trigger some kind of timing circuit .
pizza1993 (author)  caitlinsdad6 years ago
A trigger is a quick changing in voltage,isn't it?Can i power up a timing circuit by a transition from 0.5volt to 5volt or the timing circuit's input voltage must be a transition from 0volt to +(...)volt?
look on your circuit site for that burglar alarm circuit. It should help you out. Good luck.