one-time 240VAC trigger switch

Discussion in 'The Projects Forum' started by terratoss, Mar 26, 2012.

  1. terratoss

    Thread Starter New Member

    Mar 26, 2012
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    Hi guys, I need help with this one. I would like to build a circuit that can allow me to do a one-time trigger to allow 240VAC to pass through.
    Example: A table lamp set on timer from 12am - 7am. On the next day when it is 12am again, I want to prevent the table lamp from turning on unless I physically press the trigger switch.

    Thank you very much in advance for any advise or solutions given to me! :)
     
  2. strantor

    AAC Fanatic!

    Oct 3, 2010
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    I would do it with an ordinary DPDT (240V coil) relay and a pushbutton. You run 240 through one set of normally open contacts and then to the relay coil. Place the pushbutton in parallel with the relay contacts, so that when you press the button, current flows through the relay coil, closing the relay, and then the current now passes through the relay's own contact and the relay remains latched. Now, use the other set of contacts to break the power to the light/timer.
     
  3. terratoss

    Thread Starter New Member

    Mar 26, 2012
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    Thank for the reply.

    In fact I am quite new to relays so I am a little confused here. Anyway, I did some research on the web and I saw SPST relays as well. Can I use those? If I did not read wrongly, the concern is about the triggering contacts might not be able to take 240vac. If that is the case I can use a resistor to step down the voltage right? I am sorry to trouble you but if you do have any schematics of the circuit that will greatly help me in understanding the circuitry better.
     
  4. terratoss

    Thread Starter New Member

    Mar 26, 2012
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    I did more read up about the relays. There is a part that I do not understand. For example is I use a SPST relay, I energize the coil on a SPST-NO relay, it closes the poles with electricity passing through it. But when I cut off the electricity to the coil will the poles stay closed or open?
     
  5. strantor

    AAC Fanatic!

    Oct 3, 2010
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    ah crap, I totally forgot to reply to you after post # 3. sorry. I'm getting offline now, so I will probably be able to reply sometime tomorrow.
     
  6. terratoss

    Thread Starter New Member

    Mar 26, 2012
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    no problem.. i will wait for your reply.. :)
     
  7. strantor

    AAC Fanatic!

    Oct 3, 2010
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    Here's a drawing I made for you. I have drawn the relay as it is actually physically configured so you can get an idea what's going on inside. it is a DPDT relay, drawn in the unactuated state. In the unactuated state, current is free to flow from the common on the left side to the Normally Closed on the right side top (nothing connected in this drawing). When current flows through the coil, it draws the contact (diagonal line) down and current is allowed to flow from the common on the left side through the normally open contact on the right side bottom.

    [​IMG]

    In this circuit, when the timer actuates, nothing happens except that Voltage is now present at the pushbutton. When you press the pushbutton, current flows through the relay coil and actuates the relay. Once the relay is actuated, it continues to feed itself current through it's own coil through it's own contact (bottom contact). The top contact permits current to flow through the lamp. When the timer elapses and shuts off again, power is removed from the circuit and the relay resets.
     
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  8. terratoss

    Thread Starter New Member

    Mar 26, 2012
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    I got it thanks. The dpdt 240vac coil is quite expensive over here. I thought of an alternative of getting a 5VDC dpdt and getting a 5V adapter to activate it instead.
     
  9. terratoss

    Thread Starter New Member

    Mar 26, 2012
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    This is my alternative circuit. I think it should work right?

    [​IMG]
     
  10. strantor

    AAC Fanatic!

    Oct 3, 2010
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    Yes that should work. I don't see the 240V neutral wire for your 5VDC converter, but I trust you understand that you need to connect it.
     
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