Lesson 18 discovery

Projects Discussion and Showcase Device Hacking
1

Greetings to all.

First post here so please be gentle in response :relaxed:.

Newby alert! (probably been done already)

While finishing the 18th lesson about photocells acting as a pot on LED's, I was beginning to disassemble the various parts when something caught my eye.

It was a strange flicker of the LED's. As I approached the board things were happening. I had removed the 1k resistor and photocell, but the rest of lesson 18 remained intact.

As my hand drew near the wire from A0 to the breadboard, the LED's would react lighting up 0 to 3 or 4 depending on the proximity of my hand. Then used forefinger and thumb and lightly squeezed the wire and all 8 LEDs lit up quite nicely.

Now I must admit I do not understand what is causing this other than the fact that an electrical field is generated in the A0 wire and that my hand is producing a triggering resistance of some sort similar to a photocell. This only seems to work with the A0 wire and nothing else.

If anyone has an explanation I would like to hear it. Onward to #19!

Thanks.

Lesson 18fieldeffect.jpg
Lesson 18fieldeffect.jpg3082×1851 730 KB

LED wiring schematic.PNG
LED wiring schematic.PNG748×543 335 KB

2

Search the forum for "floating". You will see that when you removed the 1k resistor and photo cell, you left the arduino pin "floating". That is, neither connected to ground or connected to 5 volts.

When the pin is floating, it is susceptible to any voltage floating in the vicinity, and your body supplied that voltage. You are acting like an antenna for the 50/60 cycle power radiation coming from your house wiring. Technically, you are capacitively coupling the Arduino pin to the house wiring. Very normal.

Paul

3

Paul_KD7HB,

Thanks for the explanation and identifying the principle too.

As I understand you, I effectively became a conduit through which the house electrical field was passed to the A0 wire and triggered the LEDs. Hence becoming a human antenna.

Very interesting, I will do the search you recommended for 'floating'.

Edit: Found it! Floating ground - Wikipedia

Regards,
Scott