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A Sensor under seat of chair.

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Alexander Prad

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sensor switch using qt100

Hello All!

I need to build a device like this: there is a chair whith wooden seat (no upholstery). When somebody sit down on this chair electronic devise must play a message. Please help me to find out a type or design of sensor to solve a problem.
Thanks.
 

pdf of sensor chair in library

there are several ways to do it example:
-switch
- strain guage.
-ldr, photosensor.

it depends on the application.
Let me know if you need more help. If so elaborate your thoughts
 

Dhis devise is for my friend painter. He wants to paint a picture on the seat of chair. When somebody seats up on the chair it must to be a short audio-message (about 5 seconds). That is his conception.
The chair is wooden and no possible to make drills in seat.
 

Alexander, I'm almost certain that you can achieve what you want with a capacitive sensor.
http://www.analog.com/library/analogDialogue/archives/40-10/cap_sensors.html
**broken link removed** QTouch and QMatrix are commercial names of capacitive sensor signal conditioners
http://www.capsense.com/capsense-wp.pdf more general and theoretical

Also, I’d like to invite your question into a group dedicated to sensors. I’m curious what other experts there will suggest **broken link removed**

Best,
Nick
 
If its only on and off... use a switch!! Engineer some how, to put a switch under one of the chair's leg when someone sit on it, the switch is pushed but not crushed. Build a padding from styrofoam or something.
 

rikie_rizza said:
If its only on and off... use a switch!! Engineer some how, to put a switch under one of the chair's leg when someone sit on it, the switch is pushed but not crushed. Build a padding from styrofoam or something.
Click to expand...

This work is'nt so easy for me. I prefer to build some electronic instead of carpenter's work.

Added after 24 minutes:

kender said:
Alexander, I'm almost certain that you can achieve what you want with a capacitive sensor.
http://www.analog.com/library/analogDialogue/archives/40-10/cap_sensors.html
**broken link removed** QTouch and QMatrix are commercial names of capacitive sensor signal conditioners
http://www.capsense.com/capsense-wp.pdf more general and theoretical

Also, I’d like to invite your question into a group dedicated to sensors. I’m curious what other experts there will suggest **broken link removed**

Best,
Nick
Click to expand...

Thank you Nick, your's help is useful for me.
 

Alexander
Capacitance of an object will change to several factors.
Touching the sensor with your bare hand will have an effect diff. than a cloth.

On the other hand you have to check if the capacitance will be effected by the type of cloth one is wearing.....

Do not forget also that capacitance is also effected by humidity.


So I think strain gage is the best.
they are very fine piece of wire. when a strain is put on them the resistance of the wire will change.


check these:
https://www.sensorland.com/HowPage002.html

**broken link removed**

**broken link removed**
 
pouchito said:
Alexander
Capacitance of an object will change to several factors.
Touching the sensor with your bare hand will have an effect diff. than a cloth.

On the other hand you have to check if the capacitance will be effected by the type of cloth one is wearing.....

Do not forget also that capacitance is also effected by humidity.


So I think strain gage is the best.
they are very fine piece of wire. when a strain is put on them the resistance of the wire will change.


check these:
https://www.sensorland.com/HowPage002.html

**broken link removed**

**broken link removed**
Click to expand...

Thank You for links.

There is 2 capacitence solutions - QT100 from Quantum ResearchGroup and AD7150 (Analog Devices). These chips are quite smart. Long-term changing of capacity (due to humidity or somth) has no influence to normal operating. Only quick changing of capacity has effect. (See datasheets). So it is good solution (auto-calibrating, low power consumption ets..) But unfortunately I can't find it in my area.

As regards to strain - do You know some special IC to build an application with strain?
 

you decided to go for capacitance or strain?
by the way di you know what is ldr or how do they work?
I suggest first to know how they work and what are most suitable for your application.
For sure I am here to help just ask
 

pouchito said:
you decided to go for capacitance or strain?
by the way di you know what is ldr or how do they work?
I suggest first to know how they work and what are most suitable for your application.
For sure I am here to help just ask
Click to expand...

I decide to use QT113 capacitence IC. For strain it is need to build some good bridge and good amplifier. And I suppose it is not possible to find strain in my city :(.

LDR - You mean Light Dependent Resistors ?
Like this **broken link removed** ?
 

yes you are right.
when someone is on the chair there will be no light on the chair therefore the resistance of the ldr will changed
 

pouchito said:
yes you are right.
when someone is on the chair there will be no light on the chair therefore the resistance of the ldr will changed
Click to expand...

It can work but...
1. Not possible to make hole in seat for LDR (there must be a picture)
2. If somebody switch off a light in room there will be a sound ?
 

where are you going to put the capacitance sensor?
the same is for the ldr. Then sitting on it will have more effect than the lights off.

I advise you first to purchase that sensor (if cheap) try to put it in contact with several clothes and measure the capacitance.

You have to do such tests since you do not want your design to fail down right!!!
 

you should do the testing quickly with different conditions. cloth type, humidity, bare hand.....
so that if the results are not as you have expected to switch to another sensor.
GOOD LUCK. Let me know the results
 

for message you can use isd14xx - sound recorder chip , about qt100 .... i think this is the best choise for proximity sensing they are very intelegent .

regards from bulgaria
 

Hello I doubt cap switch will be really working. Normally you need to touch with bare skin to activate cap switch. I have some cap switches in different portable devices and they react very poor or even never. They are really designed for completing a connection over the human body to ground. Any clothes changes that path impedance and affects the performance.

I would say optical sensors are better idea. Just you have to design a good filter cascaded.

Best regards
Hakeen
 

maybe Hall effect sensors can be used. magnetic strips on the chair and sensor under the chair.
 

hakeen said:
Hello I doubt cap switch will be really working...
Click to expand...

Hakeen,

I have no doubt that the capacitive sensor will work in this application even without touching with bare skin. Take a look at the brief experimental report, which is attached (I've posted it earlier to **broken link removed** in response to this question, which was mirrored there).

- Nick
 
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