"Sensor" for communication within millimeter

Hi everyone

I'm new to the arduino world and might try to do my first project. To do so I'm looking for a way to communicate wireless over a distance of a few mm. Initially I thought, this could be done with the help of a special sensor (capacitive / inductive), that's why I'm posting in the "sensors" forum.

What I want to do is something similar to what I have attached.

Any ideas on how this could be done?

Thanks for your help.

Regards

You could have 1 LED shine onto a light sensor on the other side.
You can even go much distances that way:

Unfortunately this only works through air, but not with anything in between. And the second problem is that I need “one” sensor, which means that sender and receiver must be integrated in the same device.

From a physical point of view it should be possible with capacitors (maybe <10mm).

So you are after some 2-way RF means of transmission then.
Many options exist there
http://www.yourduino.com/sunshop/index.php?l=product_list&c=12

t_guttata:
over a distance of a few mm

How many millimeters ?
What kind of wood ? Cheap light wood, or heavy red wood ?
Can the sensors be glued to the wood ?
Is the wood moving ?
How big can the sensors be ? A plate of 10 cm * 10 cm ?
Is the wood dry or moist ?
Can you drill a hole in the wood ?

A gap of a few millimeters might result in a capacitor of only a few picoFarad. That is not reliable for a capacitive or touch sensor.
Trying to get a light or laser through the wood can be hard.
Inductive coupling is perfect for this. You can even use RFID (Adafruit RFID key). Glue the key in one part and the antenna in the other part. The exact location will not be accurate.
With a piezo element and high frequency sound might be possible, however the basic Arduino boards are not good with sound, so connecting a piezo element to an Arduino board as microphone is not ideal.
A mechanical knock has the same problem that a microphone is needed.
A RF signal goes in all directions. I also don't like a RF signal with capacitive pads. It will work, but a lot of extra electronics is required.

Tell us about your project. What is it ?

@ CrossRoads

I only need to transfer 1 bit in both directions.
IR sender / receiver would work, but not through wood.

Koepel:
How many millimeters ?
What kind of wood ? Cheap light wood, or heavy red wood ?
Can the sensors be glued to the wood ?
Is the wood moving ?
How big can the sensors be ? A plate of 10 cm * 10 cm ?
Is the wood dry or moist ?
Can you drill a hole in the wood ?

A gap of a few millimeters might result in a capacitor of only a few picoFarad. That is not reliable for a capacitive or touch sensor.
Trying to get a light or laser through the wood can be hard.
Inductive coupling is perfect for this. You can even use RFID (Adafruit RFID key). Glue the key in one part and the antenna in the other part. The exact location will not be accurate.
With a piezo element and high frequency sound might be possible, however the basic Arduino boards are not good with sound, so connecting a piezo element to an Arduino board as microphone is not ideal.
A mechanical knock has the same problem that a microphone is needed.
A RF signal goes in all directions. I also don't like a RF signal with capacitive pads. It will work, but a lot of extra electronics is required.

Tell us about your project. What is it ?

Thanks for your explanations.
It should work with at least 2mm in between, and it should be as small and cheap as possible (approx 1x1cm). Nothing is moving. Unfortunately for many of the other questions I don't have an answer yet.
I would need several identical "sensors", spaced maybe 5-10cm and obviously the sensors should not have any cross coupling. RFID will not work because at least the ID will be different, right?

One square centimeter with at least 2 mm spacing, I think you can forget about normal capacitive coupling. Trying to get a RF signal across might work. You would need a circuit for the radio signals. I'm thinking in the range of 200kHz to 1MHz. However, if you are going to use a circuit for radio signals, then I would go for inductive.

If everything must be the same, then every module would need both a RFID antenna and a RFID tag. Every tag being different is no problem.

Inductive coupling in that small space with that gap is no problem. Do you have enough depth on both sides ? Then you can use a coil with a ferrite core. I'm sure it will work, but I can not provide a schematic.
It would even be possible to transfer data with the VirtualWire/RadioHead library.

Koepel:
One square centimeter with at least 2 mm spacing, I think you can forget about normal capacitive coupling. Trying to get a RF signal across might work. You would need a circuit for the radio signals. I'm thinking in the range of 200kHz to 1MHz. However, if you are going to use a circuit for radio signals, then I would go for inductive.

If everything must be the same, then every module would need both a RFID antenna and a RFID tag. Every tag being different is no problem.

Inductive coupling in that small space with that gap is no problem. Do you have enough depth on both sides ? Then you can use a coil with a ferrite core. I'm sure it will work, but I can not provide a schematic.
It would even be possible to transfer data with the VirtualWire/RadioHead library.

A single bit of information is just an on-off signal. Perfect for inductive coupling. The sender will produce a 1000 hz tone to a coil on one side. The tone on-off will convey the 1 bit of information. An identical coil on the other side will pick up the tone due to inductive/magnetic coupling and send the tone on to your receiver where it can be determined to be either on or off. Does that work for you?
Paul

Thanks for your inputs, this sounds great. Yes, I think this could work out perfectly with a small coil and ferrite core. The coil could be for example D=5mm, L=10mm or even larger if needed.

Why 1000 Hz? Do you think there is something out there or would I need to design my own „sensor“ to work with the the distance I need? Basically 1 Bit should be enough, but you know, if more is possible it would be great anyway.

t_guttata:
Thanks for your inputs, this sounds great. Yes, I think this could work out perfectly with a small coil and ferrite core. The coil could be for example D=5mm, L=10mm or even larger if needed.

Why 1000 Hz? Do you think there is something out there or would I need to design my own „sensor“ to work with the the distance I need? Basically 1 Bit should be enough, but you know, if more is possible it would be great anyway.

Don't even need a ferrite core. Just a few hundred turns of # 26 enameled wire on both sides. 1,000Hz is just a convenient number. Any number will do. More than one bit of information means you need to design a protocol to identify the start of the bits and the end of the bits. Look at the worldwide standard for asynchronous communications.
Paul

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