Proximity sensor

[jmbengtips]

I am looking for a way to make a switch which is activated by moving your hand in the vicinity (<5cm) of a steel plate (dimensions +/- 500x40x0,5mm).
My initial idea is to generate a (magnetic?) field around this sheet and detect if there are any changes in this field. Since I have very little experience in these matters my questions are:
- how do I generate this field?
- how do I measure changes in this field?
- will this field actually change when I move my hand closer to the sheet?

Maybe there is a better solution to obtain the desired result?

 

[Skogsgurra]

Magnetic doesn't work - unless you are an armoured knight or something.

Capacitive will work quite well. But it doesn't discriminate between a hand or any other body part. It will also react to animals. A prescence indicator like the ones used to light lights when people enter a room will also work.

But, if you want that shape, an HF osillator that feeds the plate and a means of detecting the capacitive current flowing to it can work. There may be weather issues.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[jmbengtips]

Thank you for your reply Skogsgurra!
It is no problem that it also reacts to other body parts or animals, and also the weather is not a problem since it will be for indoor use only. A presence indicator is not a solution, I want to implement the ability to detect the presence in the object itself (so no external cameras looking if something is approaching the plate).

Could you please elaborate on the HF oscillator + detection which you suggest? I have had my basic electricity/electronics training at university a long time ago, so assume I'm an almost complete newbie on the subject

Many thanks in advance for helping me out!

 

[RyreInc]

Don't capacitive sensors require contact?

What about a light curtain?

http://www.bannerengineering.com/en-US/products/6/Machine-Safety/45/Safety-Light-Screens-Type-4

 

[IRstuff]

Ultrasonic could work. Your microcontroller could filter out longer distance returns and only respond to short distance returns.

https://www.sparkfun.com/products/8504

is an option; note that anything under 6 inches away will not resolve to much below 6 inches.

TTFN
faq731-376

 

[itsmoked]

Generally you measure the capacitance of the 'sensing shape' rapidly and continually. If you measure a significant change in capacitance between readings you register that as a 'touch'.

Go to Atmel.com

They have many touch products. You can easily do this with what they have. You can also create sliders, buttons, and anything else you can conceive of. They have several application notes describing the technology.

Microchip.com is a late comer to the same subject but they're all over it now too, possibly with even more ability. They actually include the feature in most of their latest offerings by adding the CTMU (Charge Time Measuring Unit) into their processors.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Skogsgurra]

Some capacitive devices work with contact. Most don't.

An early example of non-contacting devices working over long distance (feet) is the Theremin (google). But that was a bitt too elaborate. It can be done much easier. Either by letting the hand capacitance influence frequency or disturb coupling between plates.

Many easy solutions if you google capacitive contactless sensor. Or just make one after your own head.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[jmbengtips]

I think the Theremin is the way to go. I've known this instrument for a long time but this is the first time I made the connection with my current project. Let's say my plate is the theremin, I want the switch (plate) to work like a dimmer on a lightbulb: brighten the light when the hands are moving away from the plate, and dim the light when the hands come closer to the plate (as if you were extinguishing it).
I will look further into it, if anyone has any information on hand about the inner workings of the theremin, feel free to share.
Thanks for the helpful advice!

 

[jraef]

The Theremin needs relatively long cumbersome antennae.

There are already plenty of capacitive sensors out there on the market, there are even a few capacitive analog distance sensors, Balluf makes one that I have used extensively. But the distance is very short, like 8mm max at most. Capacitance sensing is difficult to project with any reasonable distance without singnificant increase in the antenna size.

Decent explanation of capacitance sensing.

http://www.youtube.com/watch?v=QItuf6lNvmI

There are however plenty of options with IR and laser analog sensors out there, the distance can be hundreds of feet.


"Will work for salami"