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

• Engineering
• Creaturemagic
In summary, the student wants to create a circuit that will play a sound continuously and an LED will flash. However, he is unsure of how to connect the circuit and has questions about what specific components are needed. Additionally, he needs help with understanding how a flip-flop works.
Creaturemagic

## Homework Statement

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:
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".

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...

http://commons.wikimedia.org/wiki/File:Sr-latch.png

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...

#### Attachments

• Alarm.jpg
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## 1. How does a burglar alarm circuit work?

A burglar alarm circuit is a type of electronic security system that is designed to detect unauthorized entry into a building or area. It consists of several components, including sensors, a control panel, and an alarm sounder. When the circuit is armed, any movement or disturbance in the area will trigger the sensors, which will then send a signal to the control panel. The control panel will activate the alarm sounder, alerting the occupants of the building and potentially scaring off the intruder.

## 2. What is a 555 Flip Flop timer and how is it used in a burglar alarm circuit?

A 555 Flip Flop timer is an integrated circuit that can be used as a monostable (one-shot) or astable (oscillator) timer. In a burglar alarm circuit, the 555 timer is typically used as an astable timer to generate a continuous square wave signal. This signal can then be used to trigger the alarm sounder or to control other components of the circuit.

## 3. What are the advantages of using a 555 Flip Flop timer in a burglar alarm circuit?

One of the main advantages of using a 555 Flip Flop timer in a burglar alarm circuit is its simplicity and reliability. It is a widely available and inexpensive component that can be easily integrated into the circuit. Additionally, the 555 timer has a wide operating voltage range and can be used with a variety of different power sources.

## 4. Can I customize the timer settings of a 555 Flip Flop timer in a burglar alarm circuit?

Yes, the settings of a 555 Flip Flop timer can be customized to suit the specific needs of a burglar alarm circuit. The timing of the signal can be adjusted by changing the values of the resistors and capacitors connected to the timer. This allows for flexibility in designing the circuit to meet different security requirements.

## 5. Are there any safety concerns when using a 555 Flip Flop timer in a burglar alarm circuit?

While the 555 Flip Flop timer is a relatively safe component to use, it is important to ensure that it is properly connected and powered to avoid any potential hazards. It is also recommended to use a battery backup in case of power outages to ensure the alarm system remains functional. As with any electronic circuit, it is always best to follow proper safety procedures and consult with a professional if needed.

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