Does this burglar alarm circuit design function correctly?

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Discussion Overview

The discussion revolves around the design and functionality of a burglar alarm circuit, focusing on its theoretical and practical aspects. Participants explore the circuit's components, behavior under different conditions, and potential modifications to improve its reliability and sensitivity.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Exploratory

Main Points Raised

  • One participant describes the circuit's operation, detailing how the LDR and comparator IC2 interact to trigger an alarm when an intruder is detected.
  • Another participant questions the mechanism by which the LDR's resistance changes and seeks clarification on the role of R9, suggesting it may be a photoresistor.
  • Concerns are raised about the potential for undefined states in the comparator if the input voltages are set exactly equal, emphasizing the need for a definite "off" bias.
  • There is confusion regarding the output state of IC2, with one participant pointing out a contradiction in the description of the output transitioning from low to low.
  • A participant clarifies that R9 is indeed an LDR and mentions an error in the previous figure, indicating that the output transitions from low to high.
  • Suggestions are made to adjust the circuit to reduce sensitivity to ambient light changes, including modifying the resistance of the 47k pot to be greater than that of the LDR.
  • Questions arise about the appropriateness of using the SL 100 transistor for Q1 in the circuit design.

Areas of Agreement / Disagreement

Participants express differing views on the circuit's design and functionality, with no consensus reached on the effectiveness of the proposed modifications or the clarity of the original circuit description.

Contextual Notes

Participants note potential limitations related to ambient light sensitivity and the undefined state of the comparator, which may affect the circuit's reliability. Specific assumptions about component behavior and circuit operation remain unresolved.

Who May Find This Useful

Readers interested in circuit design, particularly in alarm systems, as well as those studying engineering concepts related to comparators and light-dependent resistors.

swty todd
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We have to make a miniproject as part of my engineering course. I chose the above burglar alarm. I thought that there were some mistakes and I have made some changes (therefore some things are written in black). I did be grateful if you guys could confirm whether the circuit will work theoretically and practically. Let me know if I have made some mistakes. Here IC2 uA 741 is wired as a sensitive comparator ,whose set point is set by R6 & R7.The voltage divide by LDR and R9 is given at non inverting pin of IC2.At standby mode these two voltages are set equal by adjusting R9.Now the out put (pin6) of comparator will be low.Transistor Q1 will be off.The voltage at trigger pin of IC1 will be positive and there will be no alarm.When there is an intruder near the LDR the shadow causes its resistance to increase.Now the voltages at the inputs of comparator will be different and the out put of IC2 will be high.This makes Q1 on.This makes a negative going pulse to trigger the IC1 which is wired as a monostable multivibrator.The out put of IC1 will be amplified by Q2 (SL 100) to produce alarm.
 

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When there is an intruder near the LDR the shadow causes its resistance to increase.
How does the occur? What is R9; a photoresistor?
 
Here are a few comments which I hope may help:

  • I cannot see what you mean by
    I have made some changes (therefore some things are written in black).
    What exactly have you changed?
  • To define the standby mode you should adjust for a definite "off" bias. Making the voltages at the comparator input exactly equal would give an undefined state, subject to the input voltage offset of IC2.
  • An alarm working directly on brightness would be very sensitive to changes in ambient lighting e.g. due to varying daylight or changes in mains voltage affecting electric light. (That's not your problem if your instructor has told you to use this circuit.)
 
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You have me confused here, you seem to be saying that the output of IC2 is going from low to...low?

At standby mode these two voltages are set equal by adjusting R9.Now the out put (pin6) of comparator will be low.Transistor Q1 will be off.The voltage at trigger pin of IC1 will be positive and there will be no alarm.When there is an intruder near the LDR the shadow causes its resistance to increase.Now the voltages at the inputs of comparator will be different and the out put of IC2 will be low.
 
R8 is a 47k pot and R9 is an LDR. I made those changes in the above figure(wrote them in black) but seems like they weren't saved.Extremely sorry about that, I have posted the correct figure. Also output goes from low to high

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Adjuster said:
Here are a few comments which I hope may help:

  • I cannot see what you mean by What exactly have you changed?
  • To define the standby mode you should adjust for a definite "off" bias. Making the voltages at the comparator input exactly equal would give an undefined state, subject to the input voltage offset of IC2.
  • An alarm working directly on brightness would be very sensitive to changes in ambient lighting e.g. due to varying daylight or changes in mains voltage affecting electric light. (That's not your problem if your instructor has told you to use this circuit.)

To avoid situations you have stated in your second and third comments, any idea how I should modify the circuit.One way I can think of would be to adjust the 47K pot so that its resistance is little more than the LDR. By increasing the value of the pot , I can also make the alarm less sensitive (to avoid situation no.3 ), though am not sure how much a particular change will affect its sensitivity.

Also is it okay if I use SL 100 for Q1?
And thanks a lot!
 
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