I Need Your Help In These light alarm

Belal 111 · Sep 16, 2010

Hi
In the Attachments There is a light alarm circuit but there is a problem in these circuit:
When a person cut the light (pass through the laser beam) the alarm will work only if there is no light, in my case these will take less than a second thus I will not hear the alarm, so how can I let it work until I disconnect it in myself.
Thanks...

LTX71CM · Sep 17, 2010

The contents of the above attached .zip file

MrDEB · Sep 17, 2010

what ICs are those?
perhaps a simple op amp circuit would be better connected as a voltage comparator.

Reloadron · Sep 17, 2010

MrDEB said:
what ICs are those?
perhaps a simple op amp circuit would be better connected as a voltage comparator.

The ICs seem to be a pair of 555 timers. I agree as I would not use the 555 but rather a comparator like the LM339 (or similar family) and have it latch a D flip flop like the 4013. The 555 will work and can be configured as either a Light or Dark detector but I don't see it as the best choice. If I did use the 555 I would still have it drive a flip flop that can latch. Then manually reset the F/F after it was triggered.

Ron

MikeMl · Sep 17, 2010

Rewire the first 555 like this:

Belal 111 · Sep 17, 2010

MrDEB said:
what ICs are those?
perhaps a simple op amp circuit would be better connected as a voltage comparator.

These ICs are NE555

Belal 111 · Sep 17, 2010

MikeMl said:
Rewire the first 555 like this:

This is What I Want, But I Replaced "R1" by "LDR"

Thank You Very Much "MikeMl"
And Thanks To All For Help.

MikeMl · Sep 17, 2010

Belal 111 said:
...I Replaced "R1" by "LDR"

So you want a "beam-break" detector? Then swapping the fixed resistor and the LDR would do that. Resistance of LDR drops when illuminated, so breaking the beam would cause the resistance to rise, causing the voltage at the TRIG input to fall, causing the first 555's output to go high until reset with the switch.

Belal 111 · Sep 18, 2010

MikeMl said:
causing the first 555's output to go high until reset with the switch.

Yes This is What Happened, But No Problem.

I Need Your Help In These light alarm

Belal 111

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