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How can I extend the wires of my soil moisture sensor without dropping analog output voltage?

My control box is 15 meters from the plants. I also have my 5v power supply for my sensor. I have 2 sensors.

Specification for control box:

Power supply: 3.3v or 5v Output voltage signal: 0~4.2v Current: 35mA

Pin definition: Analog output (Blue wire) GND (Black wire) Power (Red wire)

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Although you did not identify the sensor or what kind of wire you are considering, it seems quite unlikely that you will get any voltage drop unless you use very tiny wire.

Assuming the 35mA current rating is for the (undentified) sensor active electronics, regardless of the voltage (3.3V or 5V), any kind of decent cable will provide no significant voltage drop. And there will be no significant current flow in the signal wire because of the (assumed) very high impedance of the destination (a microcontroller or ADC?)

And even if there were some voltage drop, it is unlikely that would significantly affect the operation of the overall system. You could conceivably adjust for whatever detected offset it might cause.

If you want a more solid answer, we need to know:

  1. What is the sensor? Link to the specifications page.
  2. Wnat is the destination of the signal? Microconroller? ADC?
  3. What kind of wire (specifically the wire gauge) are you proposing?
  4. What kind of calibration procedure does your system use for the sensor?
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  • \$\begingroup\$ 1. SPECIFICATION for sensor This analog soil moisture sensor uses the two probes to pass current through the soil, and then it reads that resistance to get the moisture level. More water makes the soil conduct electricity more easily (less resistance), while dry soil conducts electricity poorly (more resistance). Power supply: 3.3v or 5v Output voltage signal: 0~4.2v Current: 35mA Pin definition: Analog output(Blue wire) GND(Black wire) Power(Red wire) Size: 60x20x5mm(2.36x0.79x0.2") Surface finish:Immersion Gold \$\endgroup\$ – john velasquez Nov 26 '16 at 7:25
  • \$\begingroup\$ 1. SPECIFICATION for sensor This analog soil moisture sensor uses the two probes to pass current through the soil, and then it reads the resistance of the soil. Power supply: 3.3v or 5v Output voltage signal: 0~4.2v Current: 35mA Pin definition: Analog output(Blue wire) GND(Black wire) Power(Red wire) Size: 60x20x5mm(2.36x0.79x0.2") Surface finish:Immersion Gold. 2. The destination of the signal is a microcontroller, a PLC. 3. 14awg is the wire \$\endgroup\$ – john velasquez Nov 26 '16 at 7:44
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The GND wire is the thing to worry about.

Check the current flowing on the blue wire. When it's below 100µA, the voltage drop on that one should be negligible. So make the GND wire as thick as possible.

schematic

simulate this circuit – Schematic created using CircuitLab

Or power the sensor locally instead through the wire.

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