This project involves a printed circuit board design to measure the resistance between two metallic probes that can be placed inside soil as a measure of the soil humidity. It uses AC current to sense the resistance between the probes, preventing corrosion of the probes which otherwise would happen when a DC current is used instead. The design is based on the SMX sensor by EME Systems.
The sensor can be supplied with either 3.3 V or 5.0 V and utilizes an onboard 3.0 V regulator powering a TLC555 timer. The output is a pulse train with an amplitude equal to the supply voltage and of a variable frequency that correlates to the measured resistance. The pulses are at a 50% duty cycle.
Contains EAGLE, Fusion360 and LTSpice project files.
The PCB production Gerber files can be found here.
| IC1 | CMOS low-dropout voltage regulator, 3.0V 250mA | MCP1700-3002E/TO | |
| IC2 | CMOS 555 timer, input 2V-15V | TLC555CP | |
| R1 | 4.7 kΩ 1/4W 5% | Resistor, carbon film | |
| R2 | 100 Ω 1/4W 5% | Resistor, carbon film | |
| R3 | 150 kΩ 1/4W 5% | Resistor, carbon film | |
| R4 | 390 Ω 1/4W 5% | Resistor, carbon film | |
| C1 | 1 μF 50V 10% | Capacitor, ceramic | KEMET C320C105K5N5TA91707301 |
| C2 | 0.1 μF 50V 10% | Capacitor, ceramic | KEMET C320C104K5R5TA7301 |
| C3 | 1 μF 50V | Capacitor, aluminium electrolytic | |
| C4 | 0.1 μF 63V 5% | Capacitor, metallized polyester film | TDK B32529C0104J |
| C5 | 4.7 μF 25V 10% | Capacitor, ceramic | KEMET C320C475K3N5TA91707301 |
| C6 | 4.7 μF 25V 10% | Capacitor, ceramic | KEMET C320C475K3N5TA91707301 |
The frequency-resistance relationship of the simulated idealized circuit:
Dennis van Gils, 2022