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I'm designing a circuit that will need to be off until the user decides to 'arm' the circuit. It should then always be closed (and hard to hack into the on position).

Does a certain component exist for something like this purpose?

If not can you suggest a strategy?

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    \$\begingroup\$ Can you define what you mean by "hack"? Difficult to deliberately turn off or accidentally turn off? \$\endgroup\$ May 27 '14 at 23:11
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Since you used words like 'arming' the circuit and 'hacking' into, the feeling I got is that you want to be able to arm the device (turn it on) and have it impossible to be turned on/off by outsiders. Again I'm not sure if this is actually your intention, but if it is you could use a simple key switch. - ie something like this. Its just a normal on/off switch, but you need a key to operate it.

enter image description here

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Here's a method to create a permanent "latching" action from a switch and a relay:

schematic

simulate this circuit – Schematic created using CircuitLab

Once the button (SW1) is pressed, it charges the relay coil and closes the relay contacts. As usual, this bring power to the load. But, in this case, it also brings power back to the relay coil. So, even when the button is released, the coil is still energized and the load stays powered.

The load will stay powered until you remove the voltage source. If you need a method to turn off the circuit, you could add a normally-closed pushbutton in the path from the load back to the relay coil.

This could easily be modified to use BJTs or MOSFETS, if you would like a solid-state solution.

Is this what you're looking for?

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  • \$\begingroup\$ +1 The self-hold relay could likely be opened by a sharp tap in the right direction. Your suggested BJT/MOSFET variation would prevent that. \$\endgroup\$ May 28 '14 at 15:04
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I'm a little confused by your use of "off", "on", "armed", and "closed".

Perhaps an "emergency off" button as used in machine control systems might meet your needs. These buttons are normally on, but when pressed, open the circuit, and must be twisted to return to the normal position. Some require a key to reset the switch to normal. Some may include an SPDT switch, so could be used as either normally open or as normally closed.

edit: such buttons are also called Emergency Stop (or E-stop) buttons. They provide a mechanical solution for turning something off quickly and intuitively, then they keep the off state and making accidental turn on unlikely.

enter image description here (datasheet)

There's nothing special about this particular model. There are plenty of buttons like this.

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