safety switch

Discussion in 'General Electronics Chat' started by badow24, Aug 9, 2011.

  1. badow24

    Thread Starter New Member

    Aug 9, 2011
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    Hi I am wondering is it possible to make a circuit where one switch must be pressed first aka safety switch before another switch can be pressed to pass the power through to the next circuit? I want this to be so that you cannot press the red button before the safety. Only works one way, pressing the red button before then the safety will not complete the circuit, only safety first then the red will only make it work.

    Thanks
     
  2. Crispin

    Member

    Jul 4, 2011
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    You could just have two NO switches in series. They would both have to be held down.
    If you want it to latch, i.e. press the 1st button then let go, then the second then let go, you could use two relays for that or SCRs.
     
  3. Jaguarjoe

    Active Member

    Apr 7, 2010
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    The golden rule of safety circuits is KISS-Keep It Simple S(I forgot what the other S was).

    You may be able to get away with 2 simple N.O.'s in a row. Keep in mind that if someone sets a rock on top of one switch to continually keep it energized, the circuit will no longer be safe.

    Adding components that can possibly fault leaving the circuit in an unsafe condition is a big problem.

    Stamping presses have (or had) two button interlocks, one for each hand. Maybe you can find write ups on how they do (or did) it.
     
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    It is possible sure, but how is this going to turn off?

    One way would be to make the first switch turn on a self-latching relay, which is basically a switch turning on the relay, with a set of contacts parallel to the switch so once it is on it stays on. You then take this output to drive the second switch.

    But you still need some way to turn it off.
     
  5. t06afre

    AAC Fanatic!

    May 11, 2009
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    The last S stands for stupid. At least what i remember from my army time. The KISS rule was something to remind us that complex and fancy plan. Was most likely to fail. But back to topic. If you want some buttons pressed in a sequence I think this can be done by two relays. And you can ad a third button a kill switch.
     
  6. Crispin

    Member

    Jul 4, 2011
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    To turn it off you can have either a NC switch inline or a relay which breaks the supply for a short period.

    To stop the "placing a rock on the safety switch" a timer could be employed? is still depressed after x seconds, trip.
    Adding to the complexity though...
     
  7. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    This sounds like the perfect use for an 8-pin DIP microcontroller like a PICAXE. Relatively cheap, minimum components, multiple I/O, complex logic, timing, BASIC programming...:)

    Ken
     
  8. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Why waste all of those pins! 2 inputs and 1 output could be handled by a PIC10F in a 6 pin SOT package.

    It even has the spare input for when the OP comes back to tell us he also needs to turn the damn thing off!

    :D
     
  9. badow24

    Thread Starter New Member

    Aug 9, 2011
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    okay thanks for the reply's. This is for my spud gun. I got the electrical box for the spark and all i had before was a red button for the fire, so i figured make it safer and cooler if i made a safety switch so ppl just don't go and hit the red button. So i just cut one wire going to the red switch and added another switch. Only thing is it works i guess since to have to hit the second button otherwise the red button does'nt complete the circuit but you can technically hit the buttons in any order and it will still fire. Kinda be more realistic if i had to actually hit the safety first then hit the red. But it is safer in the sense if the bump the red button it cant go off.
     
  10. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    one of most common 'safety' arrangements is a 2 position maintaned NC switch that when depressed, maintains on open circuit.
     
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