Proximity Switching

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Hi,

I am looking into proximity switching to be used on door handles. I am thinking of using an Analog Devices AD7746, which is a capacitance to Digital converter. This seems like overkill to me does any one else have any ideas, it has to be reliable, i.e. work with any user. I have worked with capacitance switches before and they were user dependant.

thanks

James

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Are you trying to sense that the door handle has been touched? If so, there is an easier solution. The touchbars that you find in banks that release a maglock use a very simple circuit. I have seen tha board and it has an lm324 quad op amp on it and a few external components and a relay to drop power to the lock. I remember that the bar was made by ingersoll-rand so you should be able to get a schematic somewhere

Jim

I would rather attempt something great and fail, than attempt nothing and succeed - Fortune Cookie

 

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Atmel Studio6.2/AS7, DipTrace, Quartus, MPLAB, RSLogix user

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Thanks Jim

I'll do a search see what i can find

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Jim,

While I've never tried a design for this, I've attached a schematic for a water level sensor that I use in one of my designs.

Granted, you're not detecting water level, but the principle of detecting a high impedance to ground is similar.

My circuit monitors four water levels (or four door knobs). You may need to play around with R86 to determine the trip point or the pull-up resistor on the input.

Does this help? Let me know if you have questions about it.

Regards,
Paul

Attachment(s): 

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Cool design!!!

I dunno if the circuit would work, but it is worth a try. the touchbar does not operate on the premis of a closed circuit as our shoes isolate us from ground. My guess is that the touchbar adds/decreases capacitance to the circuit knocking it out of whack. I am going to look for the schematic as well. Now i want to know

I am going to play with your schematic too as it looks like a good design

Jim

I would rather attempt something great and fail, than attempt nothing and succeed - Fortune Cookie

 

"The critical shortage here is not stuff, but time." - Johan Ekdahl

 

"Step N is required before you can do step N+1!" - ka7ehk

 

"If you want a career with a known path - become an undertaker. Dead people don't sue!" - Kartman

"Why is there a "Highway to Hell" and only a "Stairway to Heaven"? A prediction of the expected traffic load?"  - Lee "theusch"

 

Speak sweetly. It makes your words easier to digest when at a later date you have to eat them ;-)  - Source Unknown

Please Read: Code-of-Conduct

Atmel Studio6.2/AS7, DipTrace, Quartus, MPLAB, RSLogix user

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Hey, this might be a really wild idea, but I've done some experimenting on an AVR how to do capacitance measuring like Qprox devices do.

All you need is two AVR pins, one 100nF or so capacitor, and a 10k resistor.

----+     Rs=10k
   A|--+--[]-------------O Touchplate
    |  |                 |
    |  = Cs=100n         = Cx unknown capacitance 
    |  |                 | (free air, human finger)
   B|--+                GND
----+
  |
 GND

So basically this all boils down to the fact that you measure how many times a charge stored to unknown capacitance Cx (not an actual component) needs to be transferred to known measurement capacitance Cs before there is enought charge in Cs so that there is enough voltage over it so the AVR reads the other side of the cap as logic 0 and the other side as logic 1.

Steps:
1: Discharge Cs, A=output low, B=output low, wait 100us
2: Let Cs float, A=input highz, B=input highz
3: Charge Cx to supply voltage, B=input highz, A=output high, wait 10us, A=input highz
4: Transfer charge from Cx to Cs, B=output low, wait 10us
5: increase counter variable
6: repeat steps 3..5 until A reads 1 or counter has grown too large (eg. 10000)

Heavy filtering is required for the acquired counter variable, due to noise etc.. But that is another story.

Use in hobby projects only, as this "charge transfer" seems to be patented technology.

- Jani

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Paul,

Thanks for the circuit I will have a look see if I can use it, I suspect I will have the same problems I have had in the past, with inaccurate switching. The problem I always seem to have is the effects of the environment and changes with the person operating the switch, I can get a good switch if I use it but when my manager tries it out it never works.

Jani,
Seems like a good idea but this is for a production lock so would have to pay for the design. I may have a play with it thought just to see the idea work wish I was clever enough to come up with something like this.

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Although you can check Cypress Cap Sense uC. Pretty nice uC with this feature inside, used in the i-Pod keypad.

Guillem.

Guillem.
"Common sense is the least common of the senses" Anonymous.

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Cypress chips are used in iPod touchwheel too. Some time ago they changed from Qprox chips to Cypress.

- Jani

Edit: I once had to use an AVR to use QT510 to emulate a quadrature rotary encoder.

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I just recieved a Eval kit from Quantum research group, and there capacitance switching is very good. Using the QT100 chip which is very small and few external components i get very reliable switching, they also appear to be cheap. If you are looking for capacitive switches you should try them
WEB:www.qprox.com

They can also be powered off a micros output for very low power consumtion.