light sensitve alarm

Discussion in 'The Projects Forum' started by arun the curious, Aug 7, 2012.

  1. arun the curious

    Thread Starter Member

    Aug 6, 2012
    60
    3
    need some help in creating a light sensitive wakeup alarm...
    already created a circuit(from the book electonics for dummies) but speaker isnt responding apart from a pop sound initially while giving connections
    also iam not sure about speaker's rating...The circuits says a 8 ohm 1W speaker but nothing is marked in my speakers...:confused:
     
  2. jgunn

    New Member

    Aug 7, 2012
    4
    1
    Hmm, that's not a lot to go on for someone to help troubleshoot.

    - Can you post a schematic?
    - Are you sure you wired everything properly?

    Where did you get the speaker? Even if it's not an exact match, it should be doing something if your circuit is wired correctly. If it's way too big of a load, the output might be quiet, and if the speaker were undersized you might blow it, but if you're getting a pop when you connect it, the voice coil is still moving.
     
  3. arun the curious

    Thread Starter Member

    Aug 6, 2012
    60
    3
    schematic is posted as an attachment please view guyz..

    It doesnt say anything about values of photoresistors....what abt deir values..?
     
  4. wayneh

    Expert

    Sep 9, 2010
    12,156
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    The circuit

    [​IMG]


    IC1: LM555 Timer IC
    _ Q1: 2N3906 PNP transistor
    _ R1: 100K potentiometer
    _ R2: 3.9 Kohm resistor
    _ R3: 10 Kohm resistor
    _ R4: 47 Kohm resistor
    _ C1, C3: 0.01-⎧F disc (non-polarized) capacitor
    _ C2: 1.0-⎧F electrolytic or tantalum (polarized) capacitor
    _ Speaker: 8-ohm, 0.5-watt speaker
     
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  5. arun the curious

    Thread Starter Member

    Aug 6, 2012
    60
    3
    i have done everything as per the circuit...but its not giving any result...
     
  6. absf

    Senior Member

    Dec 29, 2010
    1,493
    373
    If you connect a wire from pin 4 of 555 to pin 8, do you hear a tone?
    What's the voltage on V+ ?

    Allen
     
  7. arun the curious

    Thread Starter Member

    Aug 6, 2012
    60
    3
    do u mean connecting 4 and 8(shorting them with a jumper) or is it as shown int he schematic....
    i have applied 9v as the Vcc(using standard battery)
     
  8. #12

    Expert

    Nov 30, 2010
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    Yes. When pin 4 is at low voltage, the chip will not run. Connect pin 4 to +9 volts to put it in "run" mode.
     
  9. arun the curious

    Thread Starter Member

    Aug 6, 2012
    60
    3
    ah...finally it worked...but iam hearing only a single freq sound, how can i change its freq...can anybody suggest any modifications in design..?
     
  10. #12

    Expert

    Nov 30, 2010
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    Replace R4 with a potentiometer wired as a rheostat (the center pin shorted to one of the end pins).
     
  11. arun the curious

    Thread Starter Member

    Aug 6, 2012
    60
    3
    a pot cant we connect in the normal way...taking wires from one end pin and the middle pin...why short mid pin with one end pin?
    and what ohm rating pot is to be used....can i get a police siren from this circuit...?
     
  12. wayneh

    Expert

    Sep 9, 2010
    12,156
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    When the pot slider temporarily loses contact with the resistive material track, the circuit sees a suddenly very high "open" resistance. Connecting the slider to the opposite pole ensures that the maximum resistance in this situation is the nominal resistance of the pot. So connecting the opposite pole is a way to stabilize the output. Somewhat optional, but good form. Something you always do unless you have a reason not to. ;)
     
  13. #12

    Expert

    Nov 30, 2010
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    There are only 2 places that R4 connects to. Where would you connect the third terminal of a potentiometer?

    The pot would be labeled as 50 K ohms to replace the 47 K ohm R4.

    No, this is not a police siren design. That would require another 555 chip to make a controller for this single frequency oscillator design.
     
  14. arun the curious

    Thread Starter Member

    Aug 6, 2012
    60
    3
    thank u guys for the reply...
    is it necessary that all terminals of pot shud be employed cant we use 1 end pin and i mid pin....and there isnt any sliding arrangement in pots...now did u mean a rheostat?
     
  15. wayneh

    Expert

    Sep 9, 2010
    12,156
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    You can. I said the technique of using all 3 is optional.
    And just how do you think they work?
     
  16. #12

    Expert

    Nov 30, 2010
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    Two Hall effect sensors detect the position of the adjustable magnetic shaft and a microprocessor outputs a resistance value. :D
     
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