I have an aluminium box (dimension about 10 × 10 × 10 cm) with a lid closed with 4 screws (such as this: https://www.hammfg.com/electronics/small-case/diecast/1590).

I would like to put a simple electronic circuit inside the box with a small battery such that the circuit is activated as soon as the screws are opened and the circuit is deactivated when the lid is pushed tightly in place on the box.

One possibility could be to use some kind of switch installed between the box and the lid such that the switch is activated when the pressure is released when removing the screws.

Is there such a simple switch or any alternative that achieve the same goal? It is not sufficient to activate the circuit when the lid is (fully) removed.

  • \$\begingroup\$ What is the problem with the switch approach? You could also use a reed switch and a magnet... \$\endgroup\$ – Ron Beyer Jul 21 '20 at 21:04
  • \$\begingroup\$ silvaconsultants.com/… \$\endgroup\$ – Bruce Abbott Jul 21 '20 at 21:16
  • \$\begingroup\$ I would place microswitches on each side of the box. This way even if the lid is lifted on one side, the circuit can activate \$\endgroup\$ – w7sbc Jul 21 '20 at 21:20
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    \$\begingroup\$ So you want to detect when the lid is loosened (the screws are removed) before the lid actually moves away from the body of the box? If so, you need to change the title of the question. \$\endgroup\$ – Elliot Alderson Jul 21 '20 at 21:26
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    \$\begingroup\$ the end of the screw could depress the switch ... switch would get released when the screw was loosened \$\endgroup\$ – jsotola Jul 22 '20 at 0:43

The conventional approach to the simple switch to detect box lid removal is to use a microswitch. These come with through mounting holes so that you can use a couple of machine screws to mount the switch to the inside edge of the box. They also come with a convenient lever that the cover can press down when the cover is closed. Finally many microswitches have push on lugs that allow easy wiring with crimp quick connect terminals.

Here is a picture of the typical styles. As you can see one has just the sensitive button.

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Picture Source

  • \$\begingroup\$ I will have a look at this, but I wonder if these microswitches are sensitive enough as the circuit should activate as soon as possible when someone starts to unscrew the screws attached to the lid, not when the lid is removed. (I edited the question to clarify this point) \$\endgroup\$ – Bau Jul 21 '20 at 22:09
  • \$\begingroup\$ @Bau This is probably the most reliable method. Maybe you can add a spring to the lid, so that as the screws are loosened the lid will move up, activating the switch. \$\endgroup\$ – Aaron Jul 21 '20 at 22:34

You seem to be looking for a switch that activates "when someone starts to unscrew the screws"

This can be accomplished by positioning the tip of the lever of a microswitch at the end of the screw's shaft. As the screw is turned it travels up its threaded shaft, and the tip gradually releases pressure off the microswitch, which eventually tips the switch.

It requires some fine grinding and drilling, of course, to get the switch in the right position. But it does provide the exact action you want. You can trip all 4 screws with this: when any screw is opened, its switch is activated.

As an aside, this is a common anti-tampering trick in exterior alarm sirens. There is a double housing: an inner lid and an outer shell. When one (or two) of the outer shell's screws are unscrewed, their microswitch is activated, thus opening the normally-closed 24hr anti-tamper circuit and triggering the sabotage alarm.


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