creating a latch switch

Discussion in 'General Electronics Chat' started by itel933, Sep 7, 2010.

  1. itel933

    Thread Starter Member

    Aug 26, 2010
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    I am using the 555 timer to make a minute timer. Once the time is up I want to press a momentary button to turn the LED off (this LED indicates that the time is up) I was told that i need to make a latch switch.

    My projects is very small and if I use a relay it will be too big for the housing I made. From what I found I need a relay to make this. Is there a way to make it without using a relay?

    I am attaching the schmatic I used.
     
  2. BillB3857

    Senior Member

    Feb 28, 2009
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    Just don't connect the discharge pin of the 555. Instead, put your switch across the cap.
     
  3. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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  4. itel933

    Thread Starter Member

    Aug 26, 2010
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    bill what do you mean put my switch across the cap? How do I do that
     
  5. BillB3857

    Senior Member

    Feb 28, 2009
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    Just hook the two terminals of the Normally Open switch to the two wires of the timing capacitor. As the cap starts to charge, the comparators in the 555 monitor the voltage built up. When the voltage on the cap equals 2/3 of the supply, the output toggles. This also causes the discharge line of the 555 to go to a low state. Normally, this would cause the capacitor to start its discharge cycle. When the voltage on the cap reduced to 1/3 of the supply voltage, the output would toggle the other way, and the discharge line would go "open", allowing the cycle to start again. By not connecting the discharge line of the 555, once the cap charges, your output will do its thing and since the discharge line is not connected, the output will stay in that state. Whenever you push your reset button, it will discharge the cap and timing will start over. You will need to play with the resistor value to get you timing set the way you want it. Do a little research on the 555. A very versatile device.
     
  6. itel933

    Thread Starter Member

    Aug 26, 2010
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    so how do i connect the timming cap to the 555 (one lead threshold to other lead ground) with a push button connected to the two leads
     
  7. BillB3857

    Senior Member

    Feb 28, 2009
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    Do a little research on the 555. A very versatile device. There are many articles available both on the web and on this site. Use the search function.
     
  8. itel933

    Thread Starter Member

    Aug 26, 2010
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    attached is what I did. Is this what you were talking about? Once the timer elapsed and the red led and the buzzer came on, i pressed the button and nothing happened. Does it matter that I have other parts connected to the IC? there is nothing connected to the Discharge of the chip, as per your suggestion.

    I experimented and I noticed that if I put a swith inbetween the + and - the circuit starts over when I press the switch, this is not what I want. I just want to shut the buzzer and the red LED off when I press a button.
     
  9. BillB3857

    Senior Member

    Feb 28, 2009
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    Why don't you just put a power switch on your circuit? Turn it on, it starts timing. Light comes on, buzzer sounds. Turn switch OFF. Light goes off, buzzer stops.
     
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