Touch sensor: strip of metal, throw relay?

Quixote_1

Active Member
I was wondering if anyone knows if it is possible to use a strip of metal (the length of a standard door and about .5 to 1" width) to sense a human's touch. I read that a human body will add resistance to a circuit like that just by touching the metal. I would like to run a strip of metal down a door and switch a relay when someone places their hand on the metal.
Is this something that I could buy or use another piece of equipment to hack and use parts from?

Thanks for any leads.
 
Here are some links for that.

Thanks -- I found a few of those links myself, but I was hoping for something very simple, which I have yet to find. I guess my next question is if there is anyone here that is really adept with electronics and soldering that I could commission to build me one of these circuits in a nice little box?
 
You should be able to buy something close. . . I'm looking at one right now that is still in its package-

Westek replacement TouchTronic Dimmer Universal touch lamp control

I've had it for a while- I'd say try looking where lamp repair parts are, like the dimmers that go in cheap torch lights.

This is an AC thing, but if you used a small relay after it instead of a bulb, I'm sure it would do the trick. Mine says $5.49.

Markd
 
Ah ha! As Markd mentions, I have seen products in places like HD, Lowes, and Menards that let you touch-enable a lamp.
 
Those are both very interesting and great solutions. I wonder how sensitive they can be. What I want to do is have a strip of metal running down the side of a door so that if the door is grabbed from the side, a switch will turn on. This is a safety feature for my door entry project, so it has to be fail-proof. Also, it has to run on 12Vdc.
I guess ultimately there's only one way to find out!

A big thank you to all of you.
 
I believe that there are two types of touch switches, the most common being the touch lamp kind and I believe they use the fact that when you touch it your body is acting like a big 60hz antenna and they detect this. I would be curious to know if adding a large metal strip will make them false trigger. Actually, have you taken into account that you might have false triggers at the best of times?
The other type of touch switches are the newer kind that use capacitive sensing for consumer electronic devices that respond to your presence without electrical contact.

just as a side humor note, my kids have grown up in a very automated house with their room lights dimmable and controllable through IR remote control but the first time they saw a touch lamp they thought it was the coolest thing ever...bah!
 
Those are both very interesting and great solutions. I wonder how sensitive they can be. What I want to do is have a strip of metal running down the side of a door so that if the door is grabbed from the side, a switch will turn on. This is a safety feature for my door entry project, so it has to be fail-proof. Also, it has to run on 12Vdc.
I guess ultimately there's only one way to find out!

A big thank you to all of you.

I don't think I'd trust my life to the one I have. . . its also AC. It is hooked to a piano hinge that runs the width of the headboard on our king-sized bed and controls the overhead reading lights, so that is a pretty long strip. Sometimes you do have to touch it more than once to get it to react- but then I don't think you are going to find any solution that will always work- for instance, what if the person is wearing gloves?
 
Capacitive touch sensors (normally lamp touch switches, etc) are typically made for smaller sensors. A metal strip down the length of the door would be problematic for a couple of reasons. First it acts like a giant antenna and you would probably blow the circuitry. Second and related problem is that the capacitance of the sensor can not be over a certain level which you are sure to exceed with a giant metal strip. Typically the sensor capacitance should be in the picofarads (measure this with your multimeter) while the chip itself handles a cap up to around 20 nanofarads (not a very good explanation but hopefully you get the idea. . .the sensor capacitance must be much less than the chip handling capacitance)

Possibly a thin cat5 wire loop running the length of the door might work. . . but you would have to check capacitance first.

Can you explain a little more detailed what you are trying to do and I might be able to help a little better. . . .a diagram would help. Does the sensor need to be the length of the door? etc.

Thanks
Craig
 
Capacitive touch sensors (normally lamp touch switches, etc) are typically made for smaller sensors. A metal strip down the length of the door would be problematic for a couple of reasons. First it acts like a giant antenna and you would probably blow the circuitry. Second and related problem is that the capacitance of the sensor can not be over a certain level which you are sure to exceed with a giant metal strip. Typically the sensor capacitance should be in the picofarads (measure this with your multimeter) while the chip itself handles a cap up to around 20 nanofarads (not a very good explanation but hopefully you get the idea. . .the sensor capacitance must be much less than the chip handling capacitance)

Possibly a thin cat5 wire loop running the length of the door might work. . . but you would have to check capacitance first.

Can you explain a little more detailed what you are trying to do and I might be able to help a little better. . . .a diagram would help. Does the sensor need to be the length of the door? etc.

Thanks
Craig


Well, after a little thought I've decided that a touch sensor is not the way to go. The sensitivity issues, the possibility of failure and the realization that gloves would prevent it from working are all factors. What I'm planning on doing now is to build a piston on the end of the linear actuator with a strong spring that, when forced against, will extrude a rod that will activate a push button.
That's if no one else has any better ideas, but I think I'll start another thread on it if someone volunteers a better plan.

Thanks again.
 
Capacitive touch sensors (normally lamp touch switches, etc) are typically made for smaller sensors. A metal strip down the length of the door would be problematic for a couple of reasons. First it acts like a giant antenna and you would probably blow the circuitry. Second and related problem is that the capacitance of the sensor can not be over a certain level which you are sure to exceed with a giant metal strip. Typically the sensor capacitance should be in the picofarads (measure this with your multimeter) while the chip itself handles a cap up to around 20 nanofarads (not a very good explanation but hopefully you get the idea. . .the sensor capacitance must be much less than the chip handling capacitance)

Possibly a thin cat5 wire loop running the length of the door might work. . . but you would have to check capacitance first.

Can you explain a little more detailed what you are trying to do and I might be able to help a little better. . . .a diagram would help. Does the sensor need to be the length of the door? etc.

Thanks
Craig


Well, after a little thought I've decided that a touch sensor is not the way to go. The sensitivity issues, the possibility of failure and the realization that gloves would prevent it from working are all factors. What I'm planning on doing now is to build a piston on the end of the linear actuator with a strong spring that, when forced against, will extrude a rod that will activate a push button.
That's if no one else has any better ideas, but I think I'll start another thread on it if someone volunteers a better plan.

Thanks again.


Please tell us alittle more of what you are trying to acomplish and maybe it has already been done a different way
 
well...here's one idea. Use a red laser pointer mounted about a 1/2 inch off the wall that shines up the length of the wall (next to the door) to a receiver and then you just have to place your hand (or elbow) against the wall to break the beam and trigger the relay. The red laser would make sure it shows up in the dark! ;)
Seriously, the laser probably wouldn't have the life span that you would want but you could use an electric eye kit also known as a photocell alarm kit
 
The laser pointer idea is interesting, but a little too complicated for what I want to do.

Here is the background that I've cut and pasted from another one of my threads:

I'm building an automatic door entry system that will use keyless entry and incorporate a door-strike, two electric deadbolts and a linear actuator to open and close the door. The system will run on a marine deep-cycle battery so that I will still be able to get in if there is a power failure and will communicate wirelessly with my computer to keep track of who is arriving and departing so that it can arm my alarm automatically when there are no occupants in my flat.


Now I need to set up a safety device that will reverse the polarity of the actuator. The electronics should be simple enough to find, but the switch is what's getting me. I want something simple, robust and reliable. It has to be idiot proof so that none of my guests, roommates' guests or my roommate amputate their fingers in the door.
 
One way to do it is to monitor the current draw of the actuator. It will rise if it hits resistance. . . I think that is how car and garage doors work.
 
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