Soil sensor + ADC board

The soil sensor + ADC board pairs a capacitive soil moisture sensor with an MCP3008 ADC, the probe's analog output on channel 0. It costs $10 and is for plant projects: watering alarms, a pump controller, or logging how fast a pot dries out. Wire the SPI pins plus power and ground, then read channel 0 — the voltage falls as the soil gets wetter.

What's inside

Part	Qty	Price each
Soil moisture sensor	1	$4
MCP3008 ADC (SPI)	1	$4
Total	2	$8

The soil probe outputs an analog voltage that slides from its dry value (around 2.4–2.8 V) to its wet value (around 1.1–1.4 V).
The MCP3008 digitises that voltage to 10 bits over SPI; the Pi has no analog input to read it directly.

Exposed pin	Inside the kit
vdd	`adc:VDD`
gnd	`adc:DGND`
clk	`adc:CLK`
dout	`adc:DOUT`
din	`adc:DIN`
cs	`adc:CS`

vdd powers the probe and the ADC and must be 3.3 V so the output stays in the Pi's range; gnd is the shared ground. The kit has no chain ends; the SPI pins are clk, dout, din and cs.

Wiring it in circuit.py

circuit.py

from wiring import *

pi = Pi()
soil = kits.SoilADC()

pi.V3_3 >> soil.vdd
soil.gnd >> pi.GND
pi.SCLK >> soil.clk
pi.MISO >> soil.dout
pi.MOSI >> soil.din
pi.CE0 >> soil.cs

Driving it in main.py

main.py

from gpiozero import MCP3008
from time import sleep

soil = MCP3008(channel=0)

while True:
    print(f"probe {soil.voltage:.2f} V")
    sleep(1)

Calibrate before deciding anything: note soil.voltage in dry air and in a glass of water, then scale between the two. Higher voltage means drier soil.

Why buy the kit

Loose parts cost $4 + $4 = $8.00, against $10 — $2.00 for a short three-wire run and an ADC connection made properly. The probe is the part most likely to be wired to 5 V by mistake; the kit fixes it at 3.3 V, where its output and the Pi are both happy.

Simulator vs real hardware

The probe's output follows its zone's soil moisture, moving over a couple of seconds rather than instantly; run the pump in the same zone and the reading responds.
Each probe picks its own dry and wet voltages within the datasheet bands, so absolute readings differ between parts — the calibration advice is not just for real hardware.
The MCP3008 is simulated at the transaction level with 10-bit quantisation and noise, so gpiozero.MCP3008 works unmodified; keep the probe's electronics out of the water on a bench.