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Weight Sensor

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hello everyone,

I am looing for a weight sensor (or any initial design), which does not cost too much or i can make it by myself (might be piezo-electric...).

All I need to do is to detect if there is a weight of few Kgr... (whatever)
Preferably need Digital output logic 0 and 1, if the indication is Weight (+ve) or Not (-ve). Alternatively 0V and 5V analog!! Any... :idea:

Please help if you can.
Thanks :D
Maybe a plate with adjustable springs and a microswitch?
Or - i've see in small scales - differential condenser.
Some thoughts - there are devices that change resistance in response to strain or a force on them - they are usually called strain gages. Some of the surplus houses sell them at very low cost. You adhere or otherwise fasten them to something that will elongate in response to weight.

Another device is similar to a potentiometer - either linear or rotating. The amount of extension or rotation determines the resistance. I'd be inclined to homebrew something out of a potentiometer - might not be pretty but it would work. Linear or slider type pots can be had - as well as traditional rotating pots. With springs to allow amounts of deflection needed, for the load intended, you'd have a simple scale.

Two metal plates separated by an air space will behave as a capacitor. If the space is decreased as weight is added then this could become a sensor. An electrical circuit that can respond to the changes would be used as a sensor.

I am not entirely sure if a piezo sensor would work - at least as I'd expect it to. Current will flow when the piezo element is being deformed but once in a static state the current flow stops. The piezo element behavior might change at various states of static deformation - and as long as you had a means of measuring/indicating that it could work. I know they make piezo pressure sensors - and my guess is that they work that way.
I think you are right about the piezo-sensor.

I will have a look at your thoughts, thanks anyway!!

Basicaly, i wanna put this sensor or detector... or whatever it is,
at the top of a seat (from a car) and detect weather or not someone is sitting on the seat!! therefore, i need a digital outpt of 0 and 1.
Simple Lever Switch

Go to and do a search for: D2FW-G071M-ND

The unit is a simple waterproof leaf type switch built for automotive applications which would be relatively easy to install in a seat and would give you your logic 1/0 if configured correctly.

Also do a search for "leaf switch" or "lever switch" or "spring switch" to get some other configurations. There are literally thousands of configurations available that may work for your purpose.

Well, you got some interesting suggestions but now that you told us exactly what you want to do, you might want to hear about something more practical.
The suggested weight sensing switches would have to be fitted somehow to the car seat mounts, tricky and potentially dangerous if it involves altering the seats frame.

What you are looking for is a "pressure switch". You have seen versions of that under floor mats at shops where a beep goes off as soon as someone steps on it, thus alerting the shop staff (who might be busy behind the counter).

A pressure mat is thin and flexible and could easily slide under your seat covers, you just need to find one of the right size.
They could either work by sensing air pressure and the switch is a membrane that triggers a micro switch or they simply have some conductive foam sandwiched between the covers and the foams resistance changes to give you an output. The mat I have here is rated 1 Amp @ 30 Volts - you should be able to interface that easily enough to a digital input device.

You did not really want to "weigh" anything, just sensing the presence of a weight. It sent a few respondents up the wrong path so to speak :wink:
first of all, thanks everyone for your replies!!

But i think Klaus u r right,
because this is wht i really need...
a detector sensing the presence of a weight, which is able to fit on a car seat!!
I will really start loking for it 2nite but what do u think about a micro switch? or do u suggest anything specific, that i can have a look at?? :idea:

Thanks anyway, i'll let u know what i'm gonna do!!
hello...i neeed help for my project.

so if i want to measure the weight, do i use both the strain gauge and piezo sensor? If yes, where do you convert the output of the piezo sensor to? can u kindly tell me how it works? thanksss
There is a Company called "Interlink Electronics" and they make "Force Sensing Resistors" (FSR's). Basically these devices are a Thin, flexable Wafer and they change resistance with force or weight.

They ar Not accurate enough to be used for a Weight Scale, but are good for many other applications.

**broken link removed**

or Phone: 805-484-1331

Hope this helps.......Gary
Id use a simple proximity switch with MOSFET's.... Or Id go to the junk yard and buy an "occupancy sensor" from an old seat. Many late model cars (eg BMW's) with SRS airbags use these paper thin sensors under the passenger seat cushions. Theyre very cheap and just what youre looking for.
Am also looking for a sensor which can measure milligrams for my project. can anybody suggest me some cheap sensor..

regards & thanks
Hi, you may need a device like this: Strain Gauge to USB Convertor . Its useful for strain gauge data acquisition such as weight and pressure.If youre looking for a really cheap solution then I'ld concur with hyedenny's comment.
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