Any bio-physicists here?


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A long way from where I started
Topic Author's Original Post - Jul 16, 2017 - 07:38pm PT
Once upon a time, the method for allowing electricity to flow -- or to stop that flow -- was a mechanical switch. The primitive version was a big, clunky, physical thing, the modern version much more sleek and tiny.

But, primitive or modern, there was a physical action required to open or break the circuit.

And now? Well, now it's different. There is no mechanical action needed. Rather, when you want to turn on whatever it is you want to turn on, you touch a certain area on the appliance, and... somehow it senses the presence of your flesh, and the light goes on. Or the burner heats up. Or the whatever-it-is starts up.

Which is just fine. Who cares whether turning on a light-bulb requires moving a lever or touching a sensor.

Unless you're me. I can move a lever. No big deal here. Flip the light switch up or down and I can make the light turn off or on. Ditto for the stove. Move that dial clockwise or counter-clockwise and I can increase or decrease the heat going into the burners.

But touch-sensitive switches? Forget it. Oh, sure, if the "touch" they're sensitive to is pressure, no problem. But if what they're sensing is the presence of a living being... Well somehow I'm not on the list.

Except early in the morning. If it touch the sensor on my kitchen scale shortly after waking up, it turns on. Ditto for my induction cooker. But try it later in the day, and I might as well be dead.

So. Any bio-physicists out there?

Ice climber
Jul 16, 2017 - 07:42pm PT
Not me. However in light of your moniker, do you cast a shadow?

Trad climber
Seattle, WA
Jul 16, 2017 - 08:08pm PT

Hey ghost;

I don't think it's really a biophysics question, but a materials science / physics question (ain't they all?).

My understanding: : touch screens are based on changing thermal or capacitance properties. Thermal is pretty intuitive, if it (your finger) is warm , something happens. I've always been a confused by capacitance. I wait for Ed H. to correct me, but electrical charge concentrates at surfaces (e.g. capacitors, but still why, because its's a surface, wft?), you touch the screen and change the change the charge state.

I wait to be corrected.

Trad climber
South Pasadena, CA
Jul 16, 2017 - 08:21pm PT
Haha Ghost, I know EXACTLY what you are talking about! So annoying when climber (or guitar) calluses get in the way of touch screen detection. The issue I think is that the sensors rely on the change in capacitance caused by our wet skin acting like a dielectric to permit the transfer of charge across the dielectric like in a capacitor.

But those product designers didn't properly account for rock climbers or guitar players with thick dry skin chunks!

Quick summary of a capacitor:
It's like a resistor but the resistance varies with the frequency of the electrical signal. Low frequency stuff (e.g. DC current) a capacitor looks like an open switch (no connection), but at higher frequencies (depending on the value of the capacitor) it passes more of the electrons with a lower effective resistance until very high frequencies it just looks like a bare wire.

If you are electrically challenged, here is an analogy with water in a hose. Think of electrons in a wire like water in a hose. A resistor is like a screen or sponge in the line that slows down the passage of water, but it can be overcome if you turn the water pressure high enough (this would be like increasing the voltage). Here is where the analogy breaks down a bit... electricity can be a steady "direct current" like from a battery, or it can be a constantly changing wave that alternates between a positive and negative charge (AC = "alternating current") like in a wall outlet. In the water analogy, it would be like the water pressure quickly cycling between pushing forward and sucking back, and repeating this many times per second. Now a capacitor would be like a special screen or sponge in the line that changed its behavior depending on how fast the water pressure changed back and forth. If it stayed constant, the screen/sponge would block all flow. But if the pressure changed at a really high frequency (sort of like rattling a doorknob) it would break down and let the water flow.

So the touch screen device is detecting a change in the electrical properties as if a capacitor value changes when your finger presses it. But this doesn't change much for thick dry skin.

from out where the anecdotes roam
Jul 16, 2017 - 08:35pm PT
tin foil flags on antenna (like rabbit ears) don't work nearly as well as standing next to the struggling receiver. turn away and noise overtakes signal

A long way from where I started
Topic Author's Reply - Jul 16, 2017 - 08:39pm PT
turn away and noise overtakes signal

So if I want to weigh the flour and water after 08:00 I should turn my back on the scale and reach behind me to touch the sensor?

Mostly the next place
Jul 16, 2017 - 10:39pm PT
So Marx was right: Dialectrical materialism is the electromotive force of history!

Big Wall climber
Seattle, WA
Jul 16, 2017 - 10:46pm PT
use your other hand and lick your finger. Then switch.

Trad climber
under the sea
Jul 17, 2017 - 12:57pm PT
Here it's a brief summary of how capacitive touchscreens work for those interested.


Ice climber
Jul 17, 2017 - 07:50pm PT
Almost as complicated as healthcare. Thanks matty - I did it know that.

I'm waiting for a presentation on Apple's algorithm for changing my spelling.

A long way from where I started
Topic Author's Reply - Jul 17, 2017 - 08:01pm PT
Yeah, I know all that sh#t about body capacitance, and changing the electron balance in the screen. And also about licking my finger and turning around thrice widdershins.

But the fact remains that within an hour or so of waking, I no longer have any effect on these touch-sensitive appliances.

Boulder climber
Andy Cairns
Jul 17, 2017 - 08:06pm PT
Try dipping your finger in a jar of negative and positive charges. Maybe dip it in honey or peanut butter first.

Ice climber
Jul 17, 2017 - 08:11pm PT
What more can anyone say? Get your TSH level checked!


Mountain climber
The Other Monrovia- CA
Jul 17, 2017 - 08:30pm PT
Stayed in a London hotel where EVERYTHING was that shizz. Since I'm not of
anti-matter it worked but what a PITA. How expensive is a friggin light
switch to make?

Ice climber
Jul 17, 2017 - 08:40pm PT
It's not the on/off it's the gauge and motion of the wire and as always the labor cost.
Ed Hartouni

Trad climber
Livermore, CA
Jul 17, 2017 - 11:30pm PT

I wait for Ed H. to correct me, but electrical charge concentrates at surfaces (e.g. capacitors, but still why, because its's a surface, wft?)

VC = Q

Ice climber
Jul 18, 2017 - 02:23pm PT
Wire parallel to wall

Credit: zBrown

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