If it works, is it correct? (newbie here)

Discussion in 'The Projects Forum' started by doggonepony, Jul 30, 2016.

  1. doggonepony

    Thread Starter New Member

    Jul 28, 2016
    4
    1
    I am a self taught beginner. I have put together a doorbell type circuit that I modified and messed around with til I got it to do what I wanted. My prototype works, but that doesn’t necessarily mean it’s correct, right?

    Can anyone look at this and tell me if I have made any errors that might cause problems? before I solder it up.

    Purpose of circuit: pressing a momentary pushbutton will activate either a flashing LED or, a flashing LED AND a beep from a buzzer depending on the position of the power switch.

    Example here:

    When the button is pressed momentarily, the LED flasher (a special flasher with built in resistor and flasher) will flash approximately 5 times, but will flash continuously when button is pressed continuously.

    If the power switch selection is on LED/buzzer, the same as above will occur, but the buzzer will only beep one brief time whether the button is pressed once or continuously. The buzzer will reset after about 5 seconds so it can be triggered again.

    Attached is the schematic.
     
  2. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    The only thing that looks odd is the diode across the switch - it doesn't do anything, but it won't stop it working.
    To do what you want I would expect the second '555 to be wired as a monostable too, triggered by the first one but, hey, if it does what you want go for it.

    ETA: I can't see whether that diode is actually wired as it is shown on the schematic.
     
  3. doggonepony

    Thread Starter New Member

    Jul 28, 2016
    4
    1
    The diode across the switch is so when the switch is set for LED flashing only, it won't complete the buzzer part of the circuit.
    I wasn't sure of any other way to accomplish that, but would love to learn!


     
  4. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    Just try removing the diode and see if it still works.
    The switch connects and disconnects the buzzer.

    EDIT: No hang on a minute. Does the switch have a centre off position?
     
  5. RichardO

    Well-Known Member

    May 4, 2013
    1,231
    382
    You need a resistor in series with the LED. Since this is a battery powered circuit I would try 2.7k for the resistor, giving the LED a low current of about 2 ma.

    There are probably a million ways to do it a little bit better but the diode is the easiest and cheapest way I know of. For instance, I think that a MOS-FET connected as a diode would work but hardly worth the trouble.
     
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  6. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    The LED is one which has a built-in flasher circuit - it needs no external resistor.
     
  7. doggonepony

    Thread Starter New Member

    Jul 28, 2016
    4
    1
    Yes it is an on-off-on. But it's not on the prototype. I just switched where I was hooking the battery ground.

    EDIT: No hang on a minute. Does the switch have a centre off position?[/QUOTE
     
  8. RichardO

    Well-Known Member

    May 4, 2013
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    Oops. I just assumed that the 555 was doing the flashing. I have never used a flashing LED. Do you have any feel for how much current they draw?
     
  9. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    OK, the diode is fine. Get the soldering iron out :)
     
    doggonepony likes this.
  10. AlbertHall

    Well-Known Member

    Jun 4, 2014
    1,893
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    I guess that is going to depend on size but is likely to be similar to normal LEDs.
     
  11. doggonepony

    Thread Starter New Member

    Jul 28, 2016
    4
    1
    If I remember correctly, 20 mA. I don't have the packaging handy so many might be wrong.
     
    DickCappels likes this.
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