Help! Question about a pressure sensitive switch

[Chris Stewart]

People that know more than I do,

I have a simple project in mind that I could use some guidance on. Imagine taking an item out of a box that activates a pressure sensitive switch underneath the item removed that activates a battery powered string of LED christmas lights. See enclosed illustration.

Example here of lights here:

https://www.1000bulbs.com/product/56398/HLS-415BATCLRW.html

What kind of switch would I look for? Any freelance hobbyists out there that would be interested in taking on such a project? I would need around 30 of these switches created and integrated into the LED lights to replace its built in switch.

Thoughts? Any help is appreciated!

 

[Anon_LG]

Welcome to electronicspoint!

You could make a simple pressure plate out of tin foil and cardboard, this would turn the switch on (close the switch) when something was on top however, so (unless you were to make the pressure plate turn off (open the switch) when something was on it) you could feed the output from the plate into a NOT gate (or inverter) and feed the output from this into the base of a NPN bipolar to switch the load (in this case the lights) on and off, assuming they are not mains powered. I could draw a diagram if this sounded like an idea.

 

[hevans1944]

...

You could make a simple pressure plate out of tin foil and cardboard ...

I would buy some brass shim stock (it comes in a roll) so you can solder wires to it. You will need to cut with scissors two small brass strips and insulate them from each other with cardboard. Form a "J" hook in the bottom piece so that it will contact the top piece and complete the circuit when it springs up. You may not even need a spring between the two pieces as shim stock is pretty "springy" anyway.

 

[Arouse1973]

If you just want to sense when something is removed, could you could use a micro switch with a springy lever you could use this to switch on the light also. Sandwich this between two plates with small springs in either corner.
Just an Idea.
Adam

Picture of a lever micro switch.

 

[KrisBlueNZ]

If the item is always going to be lifted in a lit room, you could detect light falling on a light-dependent resistor (see http://www.digikey.com/product-detail/en/PDV-P8103/PDV-P8103-ND/480610).

But I think Adam's suggestion is the best. You just need a microswitch recessed into a sheet of polystyrene or something similar, with the actuator sticking out so that the item pushes it down when it's present. Assuming it weighs more than a few grams.

Edit: You could also have a recess for the battery pack for the lights, if the polystyrene sheet is thick enough. You could glue or double-sided-tape the microswitch to the side of the battery pack to keep it in position.

 

[Arouse1973]

I also like the light sensor idea Kris!
Adam

 

[hevans1944]

A CdS photoresistor is a good idea. Their resistance decrease from total darkness to even moderate illumination is huge, but probably not enough to serve as a switch for a string of LED lamps. You will probably need to add a transistor switch.

If you can find a small, inexpensive, microswitch that would be ideal.

I suppose you have about 20 presents to be contained in the boxes and you want the lights to come on to surprise the gifted? So, I guess cheap and simple (less than a buck?) would be ideal?

 

[Gryd3]

A couple years back I had a problem with my Aunt opening gifts early...
I took a smoke alarm, and took off the cover so that a could bend a metal tab to make contact with the 'test' contacts on the board. Confirmed it worked by the screeching in my face, and slipped a little piece of plastic under the tab (breaking the connection). This tab was then tapped to the gift, so that when it was removed, the plastic slipped out from under the metal tab and the 'test' button completed it's circuit. This was difficult to reset, as I needed to slide the little plastic tab back under the metal tab, but it still worked great.

The only problem... I made the gift too tempting my half tying the bow which made her suspicious so she actually waited for XMas day to open it... Judging by her reaction, and the explanation I had to give, I'm certain she will always think twice before opening a gift from this side of the family early.

Long story short, aside from an electrical solution, you could always use a mechanical switching method. (Think how you buy toys from the store with the little plastic tab sticking out of the battery compartment)

 

[Chris Stewart]

Wow, I didn't expect so much feedback. What a great community. Thanks for all the help!