Weight-activated switch circuit design

[Tahmeed]

I am designing a circuit, in this simple circuit, when a weight isn't applied, the switch will go off and the circuit won't work until a certain weight is applied, how can i design it?? i am totally new and doing this for a science club, i have never designed any circuits with censor. so please be elaborate in your description, if possible, please give a simple drawing of a possible design and i will modify it according to my need.

Thanks.

 

[anorlunda]

A search for "weight sensor" on Amazon.com returned 88,673 hits.

 

[berkeman]

I am designing a circuit, in this simple circuit, when a weight isn't applied, the switch will go off and the circuit won't work until a certain weight is applied, how can i design it?? i am totally new and doing this for a science club, i have never designed any circuits with censor. so please be elaborate in your description, if possible, please give a simple drawing of a possible design and i will modify it according to my need.

Thanks.

Try doing a Google Images search on Limit Switch:

http://3.imimg.com/data3/YP/XH/MY-3059229/limit-switch-250x250.jpg

You could improve the Google Images search by adding the term Circuit to the end of it to get some typical schematics...

 

[jim hardy]

Weight is force.
One can buy force sensors.

Force will compress or stretch a spring, changing force into movement.
You could detect movement with a switch like the picture above
or with the optical wheel sensor from a junkshop computer mouse

to change force into movement with some accuracy,,
check out Hooke's Law
http://www.britannica.com/science/Hookes-law

 

[davenn]

Weight is force.
One can buy force sensors.

yup, commonly called Load CellsDave

 

[meBigGuy]

There are many approaches, some alluded to above. It depends a lot on your mechanical constraints, which you have not been clear about. Are you allowed to have something move, or does it have to be an essentially solid/steady platform? If it can move, how much?

Putting rocks in a basket connected to a spring is a different problem from putting rocks on a tabletop that is essentially solid. Or are you pouring water is a bucket? Adding people to a platform? We have no idea.

But is does boil down to either:

a load cell (or strain gauge) that drives a detection circuit. This requires a load cell amplifier and comparator to drive a relay or something. Probably not what you want.
OR
a switch (mechanical or optical) that detects spring displacement (or the like).