Archive | 23:38

Soil Moisture Meter with a 555 Timer

5 Feb

For the past two days, I have been thinking about and building a low-cost electronic soil moisture meter for house plants that does not involve using a microcontroller.  To accomplish this, I envisioned being able to blink a light (LED) to indicate when a plant requires water using a cheap 555 timer and two nails to measure soil resistance.

Soil Moisture Sensor based on 555 Timer

Soil Moisture Sensor based on 555 Timer

555 timers determine their output signal based on two resistors and a capacitor.  I used the Astable 555 Square Wave Calculator to determine blink speeds for lighting an LED and for the 555 schematic.  To figure out what my capacitors were rated in Farads, i used the Electrostatic Capacitance Converter to convert uF to F.  My 2.2uF capacitor was 0.0000022F; that’s 2.2 times 10^-6.  I used a 100,000 ohm resistor for R1 and 10 ohm resistor for R2 to achieve a frequency of about 6.56 hz.  It looked like this:

Astable 555 Timer Circuit with Two Resistors

Astable 555 Timer Circuit with Two Resistors

To create a soil moisture meter, I used 2 parallel holes (1 inch apart) in a small piece of wood to put 2 bright common nails with wires attached (with solder) through the holes.  Speaker wire worked well here because it’s flexible, there are two wires within it, and it can easily be split apart.  I then replaced the 10 ohm R2 resistor with my soil moisture meter.

Astable 555 Timer Circuit with One Resistor and Soil Moisture Sensor

Astable 555 Timer Circuit with One Resistor and Soil Moisture Sensor

Here’s a short video of what it looks like in action:

The timing isn’t quite right, but it shows promise as a prototype that can be refined.