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Homework Help: Burglar Alarm Circuit - Help needed with 555 Flip Flop timer.

  1. Aug 22, 2013 #1
    1. The problem statement, all variables and given/known data
    I have to construct a circuit for my Physics class. The circuit must be tripped by a CDS Cell (Detects changes in light) and, once being 'tripped' it must play a continuous sound and make an LED flash on and off.
    This picture is of the circuit board we are allowed to use, and of the 'Burglar alarm' found in the instruction book. The alarm is connected to terminals here, and is tripped by one of the wires being pulled out. I already re-configured it to be connected to the CDS cell, and now when I pass my hand in front of the CDS cell it activates the speaker.

    However once I move my hand from the CDS cell range, the sound stops. I need it to continuosly loop until the circuit is turned off. I also need a flashing L.E.D attached.

    2. The attempt at a solution

    I understand that I will need to use the Integrated Dual Flip-Flop circuit, because it's a timer and allows me to loop the sound and turn the L.E.D on and off. However I am unsure of how to connect it. I've done some research but none of the things I've read deal with a dual flip-flop.
    I'm just unsure of how to connect the Flip-flop circuit so it loops sound.

    Thank you so much for any help!
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Aug 23, 2013 #2


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    Staff: Mentor

    You will probably need only one of the JK flip-flops, not both. The CdS detector will control an input to one JK flip-flop, and a output of that flip-flop will control your buzzer. Do a google search if you are unsure how a JK flip-flop works. (BTW, it is unlikely to use a 555 IC.)

    A description of a flip-flop's operation won't include the verb "loop".
  4. Aug 27, 2013 #3


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    Gold Member

    What NascentOxygen said.

    Think through the problem. I believe you have two not one.

    The first is not really a problem with the sound generator, it's a problem with the signal from the CDS. The problem is that the signal from the CDS is transitory (when you remove your hand it goes away). What you need to do is use the CDS signal to SET a latch. The latch can then be used to control both the sound and the LED.

    So you could arrange for the latch to be SET by CDS and RESET by a power cycle. You could use one of the JK latches or you could build a simple SR Latch from two nand gates...


    Google "Power on Reset" for a simple way to do that. Typically an RC circuit is used.

    The second problem is flashing the LED. Presumably you want this to flash at a relatively slow rate not at the sound frequency? So you may need to build another oscillator that runs at the LED flashing rate also controlled by the output of the latch.

    I shouldn't design the circuit for you but here is a block diagram of how I would approach it. Some blocks you already have...

    Attached Files:

    Last edited: Aug 27, 2013
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