# Detecting Thumb Touching a Finger on Same Hand?

#### one_raven

##### God is a Chinese Whisper
Valued Senior Member
Would it be feasible to wear something on a wrist that could detect when the wearer's thumb is touching another finger on that same hand? Through changes in capacitance and/or resistance maybe – or something?
If so, would it be able to tell which finger the thumb is touching?
Where on the finger?

If not something worn on the wrist, perhaps a ring worn on the thumb?
Something else (other than visual)?

Sounds pretty straightforward. A conductive probe on fingertips and thumbtip would complete a circuit, activating whatever machinations you want, different ones for each finger.

Actually, different ones for each finger combo, giving you ... 16 possible configurations. Possibly more, depending on how the contacts are arranged.

Essentially binary counting with four placeholders.

Open = 0
Thumb-to-index-finger = 0001T
Thumb-to-middle-finger = 0010T
Thumb-to-index-AND-middle-finger = 0011T
Then
0100T 0101T 0110T 0111T
and finally
1000T 0101T 1010T 1011T 1100T 1101T 1110T 1111T

If the contacts were big enough that you could have combos such as 1001- (index-to-ring-finger, no thumb), then you have 32 options - enough for a basic alphabet.

Well, I dunno about you but I cant' touch my index to my pinkie, so somewhere between 16 & 32.

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No other contact point or devices.
Just something worn on the wrist (like a watch) or a single thumb ring.
Perhaps the wrist piece AND a thumb ring...

No other contact point or devices.
Just something worn on the wrist (like a watch) or a single thumb ring.
Perhaps the wrist piece AND a thumb ring...
You mean detect contact remotely - as in from the wrist, several inches away?

Trickier.

What would the device (wrist or thumb) do with this info? Presumably, something electronic.

Perhaps there are other ways to do what you want. (For example iamge pattern recognition might be able to tell what fingers are in contact) - it depends on what the device is and what it wants to do with the knowledge.

Exactly. Or detect from the thumb.

How about a ring AND a wrist device?

The wrist device might be able to sense either electrical activity or tension in the tendons/nerves in your wrist. Don't know how accurate it is though.

Both could be an option – as long as they're not hard-wired together.

I was just thinking touching your thumb to a finger should change some electrical property of the thumb that could be measured.

Perhaps if the ring were transmitting and receiving some electromagnetic signal... Would touching different parts of different fingers be potentially detectable? Kind of like the way a metal detector works.

Possibly.

My suggestion is that you go ask on physicsforums.com in the appropriate forum. The place is chockerblock full of electronics engineering experts.

That's often my next stop, if I don't find an answer here.

What if the ring transmitted a signal, using the thumb as an antenna, and the bracelet were a receiver?
Would touching a finger change the structure of the "antenna" and allow the bracelet pick up on the difference?

What if the ring transmitted a signal, using the thumb as an antenna, and the bracelet were a receiver?
Would touching a finger change the structure of the "antenna" and allow the bracelet pick up on the difference?
Well, there's the rub. If you can fit a transmitter or receiver into something the size of a ring, you might have something there.
I was toying with the idea of the thumb ring having a pinhole camera that could "read" which finger was pressed against it (having first been "trained").

But ultimately, the problem is there's only so much electronics you can pack into something as small as a ring.

Hmmm... Maybe if the ring were a simple NFC tag...

Hmmm... Maybe if the ring were a simple NFC tag...
Sure but that's only half the equation. You still have to detect the fingers.

But what if touching the thumb to another finger could cause a slight change in the effective length of the loop antenna in the NFC tag embedded into the ring?

But what if touching the thumb to another finger could cause a slight change in the effective length of the loop antenna in the NFC tag embedded into the ring?
Yeah. You'd have to do experiments.