Is there a switch that turns on when it detects a "pull" of a few grams on a string?

Question

I'm working on my first Raspberry Pi project. I'd like to turn my project "on" when a force, of a few grams, is applied to a string (i.e. "pull the string" to turn the unit on, the unit turns off when the string is released). It looks like one way to solve this is to use a a mini load cell. As long as the strain gauge detects a force above n gram, the unit remains on.

Is a load cell/strain gauge the best way to go for this functionality? I haven't really worked with electronics before so I'm not even sure I'm searching for the right terms.

Answer 1

What you are looking for is a mechanical limit switch or micro switch of some type. These are switches that require a set force to turn on and then release once that force drops below a different value.

Here is a random switch that I found online with a generic search.

If you look at the details, you can see that it has an operating force of 25 grams, and a release force of 6 grams.

I'll have to leave it up to you to identify a switch that meets your specific requirement.

Answer 2

If you want to control the force / deflection of the switch over a range that's not obtainable in commercial switches, then you can sense the position of a spring with a magnet+Hall switch.

You can make a spring out of almost anything. I used a strip of PCB material for such a switch.