Damp Wall Sensor - Homemade ( MegaOhmMeter - AntiMold Sensor )

How to build a sensor for recognizing if a wall has a certain level of moisture reached
(dangerous mold could grow, there could be a broken pipe…)

< 0,20 Euro

What you need:
2 flathead thump tacks
2x 10MOhm (10,000,000 Ohm) resistor (=20MOhm)
Some wires
Solder iron

How it works:
The resistance of the wall gets lower when its getting damp.
This can be used by a Voltage Divider with 2 resistor (one the wall, one a electronic device resistor) showing a change in Voltage when the wall gets dryer / more damp.
With the ADC (analog read) we can read this change.

Thumb tack corrosion:
You should use a kind of AC Voltage, if not, the thumb tacks will corrode very quick.
If you use two digital outputs for supplying and toggle them after the same amount of time
you have a squarewave AC.
It is easier to use only the measured values of one polarization, with the other one the values will be “inverted”
The wire its insulation itself can have a lower resistance than 50MOhm
For the wires connecting to the Thumb Tacks better don’t use twin wires which are near to each other, use single wires.

When I measured my walls I had Values between 100MOhm and 5MOhm
I decided to take >30Mohm as dry >22.5MOhm as damp and >5MOhm as wet.
(-> around >613 , <460, <280 when “resistor1” is connected to ground)
I don’t know if these values are to high/low or if they depend of the kind of concrete your walls are made of.

How to build:

  • Solder a wire to each thumb tack (when applied to a wall with 3cm distance, they have the resitance of the wall beetwen them )
  • Solder one wire of each 10Mohm resistor together (so you have 20MOhm)
  • Connect one free wire of “resistor1” with one free wire of “resistor2”
    -where the two “resistors” meet, solder a wire for the ADC (analog input)
    -on the other free wires of the “resistors” you can connect the voltage supply/toggling output pins

Maybe if of interest I will try somewhen in future to make a library for this kind of sensor, using one analoginput and two digital outputs.

Kind Regards

Any advice for improving / correcting mistakes is welcome!

Finished Project with this sensor:

Can't belive this is me just finding this post! thank you! been looking for something like this!! :slight_smile: