Sensor to detect small force on mirror

[ep.hobbyiest]

I m looking for sensor to detect that someone is writting on glass. Means i have area of 100x100 mm area. If we write on that area of glass then we should detect that we touch the glass with small amount of force which require the pen to write on glass.
So which kinda sensor i need?
is it pressure or force sensor?

 

[barry]

I would look at using a strain gauge. These can me mounted directly to the glass such that any deflection of the glass will cause the sensor output to change. You can purchase the basic gauges fairly inexpensively, but you'll need some circuitry to go with it. Or, you can purchase a complete package with conditioning electronics built in, but they're not that inexpensive.

Another thought (much cheaper) would be to use a piezo element. They are extremely inexpensive. The only problem with piezos is they only respond to CHANGE, not static forces.

 

[BradtheRad]

A microswitch has a pressure sensor built in. Its spring loaded.

The glass must be mounted so it can move slightly, forward and back.

 

[ep.hobbyiest]

thank you sir
but if i mount glass on sensor then sensor should not measure the that weight means this will be reference zero for that sensor and onward that weight should be measure.

 

[th3gr8]

You can also use a 4-wire resistive touch screen .. You can design your code and they can be easily interfaced with a microcontroller ..

 

[Harrie Nijhof]

You could use a kitchen scale, any movement on top will generate some signal on the display(pins).
or perhaps the sensors from it. Like barry's answer, but with preinstalled matched straingauges.

 

[KlausST]

Hi,

i like the touchscreen idea.

Anoter possibility is to detect the presence of a pen or finger with a capacitive sensor. This could be easy, because the mirror surface usually is highly conductive (silver?)
Just generate a some kHz signal and measure current or phase shift.
Or build an oscillator using the mirror surface and measure the frequency.

could be a very simple solution.

Klaus

 

[BrunoARG]

Hello.

KlausSt, I've heard that mirrors have silver to reflect the light but then I read that silver was used a long time ago (that's why mirrors were so expensive in the 1600s). I think that nowadays mirrors are made with aluminum since it's used in CDs (and it reflects the light pretty well).

ep.hobbyest, I would recommend you to use a "digital" sensor, like pressed or not, instead of a analog one which would be linear and so expensive. Yes, you could make a capacitor between the mirror back part and measure its capacitance variation but I don't know how to build it. Maybe two pieces of aluminum and a thin paper sheet as dyelectric. I don't know if it would paractically work but it should, theorically.

 

[ep.hobbyiest]

Thanks sir,
But sir touchsceen should detect because i am going write it with marker pen which is bit soft.

 

[BrunoARG]

Do you want to detect just the pressure on the whole surface or detect where the marker is "writing"?

You could detect it optically, with an emitter on the pen and two receivers behind the glass. One of them might detect X axis movement and the other the Z axis one. Then, with some software calculation you could calculate the emitter position.

 

[ep.hobbyiest]

i want to detect the pressure that is something getting written. Becasuse on glass i am going to keep black glass.