Weight sensor to act as a button?

I had the idea for a prank I would love to try at some point, but I'm having some issues with the basic idea...
What I need is a way to hook up either a sensor or button to a scale. What I want to have done is when someone steps on the scale, and exceeds a certain weight (say 50 pounds, just to make sure a pet won't set it off) it will act as a button, and toggle on a small speaker which will just scream at them. Cruel, I know, but it'll be a fun laugh if I can get it working.
Any help is greatly appreciated!

The weight sensor could be a switch mat. This is a kind of switch that is meant to be concealed under a floor mat for example, and it commonly used in security/burgler alarm type appications. You can make your own custom switch mats easily. They are just two metal surfaces separated by spacers. When a force large enough to flex the metal contacts enough to contact, the switch is closed and can be used to indicate that someone stepped onto it. The metal contacts can be aluminum foil over cardboard, and the spacers can be cardboard. I made something like it for use with a floor mat video game controller. Here is a link to that step in one of my instructables:

I've seen other good designs on the web, and there are probably some on this site too.

If you made a custom mat that would fit entirely underneath the bathroom scale, it shouldn't be too noticeable. No modification of the scale itself would be needed. You'd have to experiment with the spacers and switch construction to make sure that the weight of just the scale itself doesn't close the switch.

As a word of caution, don't use such a homemade switch to directly control high voltages or high current.  !!

Have fun.
DoctorWoo (author)  LargeMouthBass4 years ago
Clever idea!
By chance, do you happen to know of any sort of material-to-weight ratio? Like what material will cause a 25 pound load to trigger.

Also, just to make sure...in the event that I would choose to stick and ardunio into this: said arduino would be able to use this as a button, right?
It will be difficult to design to a very precise activation weight, but is should be fairly easy to make one that requires the weight of an adult person to activate and would not activate from pets, etc. That's where you'll have to experiment a little.

The device I described is really nothing more than a single pole, single throw (SPST) momentary pushbutton switch. So, this should be very easy to interface to the input of an Ardunio or any other microprocessor or digital logic.