will this circuit work

Discussion in 'The Projects Forum' started by swty_todd, Dec 27, 2009.

  1. swty_todd

    Thread Starter Active Member

    Aug 3, 2008

    We have to make a miniproject as part of my engineering course. I chose the above intruder alarm (got the circuit from the net).
    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 R8 is given at non inverting pin of IC2.

    At standby mode R8 is made little more than LDR so that the output of pin 6 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.

    The output of IC2 will be high once LDR becomes more than R8

    This makes Q1 on.

    This makes a negative going pulse to trigger the IC1 which is wired as a monostable multivibrator.

    The output of IC1 will be amplified by Q2 (SL 100) to produce alarm.

    I have one more question should I use SL 100 as Q1?

  2. mik3

    Senior Member

    Feb 4, 2008
    Why do you need the 555?

    Do you want the buzzer to turn off after some time?
  3. swty_todd

    Thread Starter Active Member

    Aug 3, 2008
    I have used 555 for the following reasons:

    1.It was there originally(I took the circuit off the internet)
    2.I have worked with 555 before.
    3.I can't think of an alternative to 555. Can you suggest an alternative?

    Between thinking of your question , I am confused a little. I think the position of my buzzer is wrong. I think one end should be connected to the transistor and the other end should be connected to ground. And what is the difference between speaker and buzzers , they seem to be shown by the same symbols in circuits.
    Last edited: Dec 27, 2009
  4. DickCappels


    Aug 21, 2008
    I don't know if there is a common schematic symbol for a buzzer. You can connect it as shown, but check the "buzzer's" data sheet and determine if it needs a capacitor across its power supply connections.

    And of course "buzzer" makes me think of one of those electromagnetic buzzers, and if you use one, then put a diode across the coil with the anode on the transistor's collector and the cathode on the positive power supply.

    Have you thought about what you are going to use as a buzzer? The way you have it set up, it will get 9 volts across it when it goes off.
  5. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    Looks like the 555 is just a 10 second monostable. So if the beam is briefly broken, the buzzer will stay on for 10 seconds, not just chirp briefly. The wiring for the 555 output, the transistor and the buzzer appear correct.


    Senior Member

    Jul 1, 2008
    I think you would be better off with one of those radio shack piezo oscillators. They will pull much less current and they can be quite loud. No protection diode needed either. ;)
  7. Wendy


    Mar 24, 2008
    The 741 will not work, or if it does it is iffy. The power supply is way low, 741 requires +/- 15VDC typically. Use a LM3909 quad comparitor (don't forget the pull up resistor). If you must use a conventional op amp think LM324 quad op amp.
  8. SgtWookie


    Jul 17, 2007
    I think you meant an LM339 quad comparator.

    Senior Member

    Jul 1, 2008
    Here's an equivalent circuit (functionally) using a Picaxe microcontroller. Yeah, I know... You must be asking...where did all the components go? :confused::eek:
    • LDR.jpg
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  10. zhgart

    New Member

    Dec 28, 2009
    you can cancel the q1,and q2, but you hve to change 741 situation reverse. 741 can be comparator as 393 or 339. no problem. if buzzer has driver inside, you can save 555 but have to us q2.
  11. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    The LDR/R8 junction needs to be tied to the inverting input. The reference becomes the non-inverting input.
  12. Wendy


    Mar 24, 2008
    Yep, tired computing again.
  13. Audioguru

    New Member

    Dec 20, 2007
    An LM339 has 4 low power comparators and has a 14 pins case.
    An LM393 has 2 low power comparators the same as in the LM339 and has an 8 pins case.

    You can buy an SL100 transistor in India.