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I have made dark detector using this following circuit diagram:

enter image description here

Whenever the LDR is closed with finger,the LED would glow. Now all I want is to keep that LED in ON state for a while or alternatively stay ON until the reset button is pressed (if possible), so that it can be used as laser burglar alarm (but with LED instead of buzzer). How can it be done using a 555 IC?

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    \$\begingroup\$ When the LDR is closed with a finger, it's dark. If it's dark, why would the LED glow if it's suppose to be "automatic light detector"? Maybe it's the opposite..? \$\endgroup\$
    – Dor
    Aug 30, 2013 at 10:01

3 Answers 3

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A 555 isn't really required for a simple task like this. You can make the LED latch with another transistor and a few extra components:

enter image description here

When the voltage at the junction of the photoresistor and R4 rises high enough, Q1 turns on, turning on the LED. Q1 pulling low also turns on Q2, which charges C1 and latches Q1 in its ON state. Discharging C1 with the switch turns the LED off, and assuming it is sufficiently light (the resistance of the photoresistor has decreased), Q1 and the LED will remain off.

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From: http://www.instructables.com/id/quotDark-Detectorquot-using-a-555-timer-IC/

You can use this circuit but replacing the buzzer with a LED. Be careful that the 555IC drives out the same voltage output as you have as voltage supply (9v in this example, the IC allows 4.5 to 15 V).

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You can use this circuit. Its set when triggered by the LDR, the output stay in high level for 11seconds, or you can reset by the switch SW1.

You can adjust the sensibilty of the LDR by the R1.

The time you can control by R3 and C2.

R4 will vary depending from the voltage that you will apply in the source. You need guarantee the correct current to the LED depending the source voltage that you will apply.

schematic

simulate this circuit – Schematic created using CircuitLab

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