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I have some battery powered LED lights to go in a cupboard; and I have a 5-terminal magnetic alarm sensor to turn them on when the cupboard door opens.

The problem is that the lights turn on when the sensors are close together (i.e. the door is closed) and off when they're apart. I've tried hooking up the sensor the opposite way around, and I've tried attaching it to both cables out of the battery pack, but it's always the same.

What's the easiest way to reverse this?

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    \$\begingroup\$ Don't you think that we might need a switch part number, datasheet or at least a photo? \$\endgroup\$ – Transistor Aug 19 '17 at 20:38
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    \$\begingroup\$ There is no easy way to reverse this. You need sensors with the opposite "openness". \$\endgroup\$ – Ignacio Vazquez-Abrams Aug 19 '17 at 20:54
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You're using the wrong sensor. The type you have is designed to detect a scum bag breaking into your house by breaking an electrical loop. So it's normally open, only closing when the magnet is touching it.

Chuck it away. Get a sensor with a normally closed arrangement. Like this one. It's not a burglar sensor so operates in the reverse configuration. You might be able to find alternatives, just remember normally closed. You might also encounter some that have N.O and N.C contacts in one switch. Just use the correct one.

Otherwise you'll have to mess with adding a 12V coiled relay to the circuit to invert its operation.

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  • \$\begingroup\$ Ok, thanks. I was afraid it would be something like "whoops, bought the wrong components"; fortunately I'm not in a rush and they were pretty cheap. \$\endgroup\$ – Guy F-W Aug 20 '17 at 8:21

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