# What kind of sensor that can detect prolonged contact?

i'm currently working on project that needs an implementation of certain touch/force-presence sensor on human body surface. I want to place the sensor on my backpack shoulder straps, so that the electrical system of my smart backpack will know, whether the backpack is currently being worn or not.

I have already tried to use piezo electric sensor, but it seems like it can only detect an impulse signal; output signal HIGH when a force is momentarily detected, then go LOW even if the force is still detected (only detect the 1st contact).

I'm curious, is there any type of sensor that can detect prolonged contact, so that the sensor will always give HIGH signal as long as the force is detected? Thanks!!

Edit: There's also space constraint, so i need the sensor to be as thin and small as possible

• It sounds to me like you can easily solve it in software by having an extra variable that remembers whether the piezo went one way or the other way. Or you could just do the software solution in hardware, meaning you keep the piezo and use some very simple logic to remember the direction it last went with. – Harry Svensson Apr 14 '19 at 8:03
• @HarrySvensson thank you for your reply. Can you explain me a bit more about the 'went one way or the other way'? I'm still confused hehe – Marchio Kevin Apr 14 '19 at 8:07
• Sounds as if a simple microswitch would do the job. – henros Apr 14 '19 at 8:44
• @MarchioKevin I agree with henros solution seeing as it is ridiculously simple, and usually the simplest solution is the best solution, though that would require to actually get another physical object, compared to writing a few lines of code. - But if you have an impulse train that looks like this: 0000100 where 1 means you put it on, and another pulse train that looks like 000000(-1)0000 when taking it off. Do you see here how a simple check with 0.5 and -0.5 to set another variable to 1 or 0 can be achieved? – Harry Svensson Apr 14 '19 at 8:50
• @henros Actually, there's is space constrain for the placement of the device, so unfortunately microswitch is not suitable for my project. But considering the 'software solution', i will try it harry – Marchio Kevin Apr 14 '19 at 9:03

You could use a force-sensitive resistor. They come in a variety of shapes and sizes. Their resistance will vary based on the actual force, so with appropriate thresholds you should be able to detect whether the backpack is being worn.

There are also sensors that detect bends, known as flex sensors, but I'm not sure they would be appropriate here.

I would suggest you fit a thin tube in the contact region that is closed one end and terminated at the other end in a pressure transducer. The transducer can be remote and protected and it will measure the compression on the tubing.

A different approach would be to use an accelerometer to detect when the backpack is moving around, and assume that means that someone is wearing it.

More of less clever algorithms could be implemented to detect if the person stands stills or is walking with the backpack.

Bonus: This will inevitably be much more robust and less error prone over time as there is no mechanical slidtage compared to the more mechanical/physical solutions suggested.

• Except that you really can't tell whether someone has stopped moving or taken the backpack off. What if they stuff the backpack under their seat on the bus, so the backpack is itself moving but not being worn? – Elliot Alderson Nov 9 '19 at 19:08
• I totally agree. This solution is of course not valid for all use cases - the original question did not make it clear how this was intended to be used. – Jakob Halskov Nov 9 '19 at 19:09

You could measure the presence of the human body using a capacitive touch sensor. A capacitive sensor can see the difference between the presence and absence of the human body even when it separated from it by several layers of fabric.

You could also use an accelerometer as an additional sensor. A backpack which is worn by a human is going to have characteristic orientation with respect to vertical. It may also "see" characteristic motion.

• Right! Thank you. i have ever use a capacitive keypad for my project, but it seems like the sensor is sturdy. Is there any capacitive sensor that is flexible? – Marchio Kevin Apr 15 '19 at 4:49
• You do not need to put the sensor on straps. Put it near the back which remains flat. – Whiskeyjack Apr 15 '19 at 8:20

Disclaimer: this response is based on a 30 year old memory.

3M used to make a pressure sensitive conductive film (actually a sandwich of two films). The harder you pressed, the lower the resistance. Other than samples, I don't think there were any standard products; they produced it only in volume for custom designs. I suggest looking into this technology.

• I see, so the sensing is based on resistance. Cool! I will certainly read more about it, thank you – Marchio Kevin Apr 15 '19 at 4:48
• "Quantum tunnelling compound" can be used in this way. QTC is a rubber sheet with conductive particles embedded in it. Compression of the rubber sheet causes the resistance to drop considerably and in a roughly linear way. It is also highly flexible which I think would be a bonus in this application. – Wossname Apr 15 '19 at 9:01