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I'm hugely new to electronics. I'm about to place some outdoor sensors around the house and those should be accessed by a Raspberry-Pi which is in middle of the house. I'm aware about setting those analog sensors with RPI with ADC modules.

However, my concern is that all of the sensors are around 30m radius from the RPI. What are some good methods of connecting those sensors to RPI?

WIFI/Bluetooth: I would need to place a wifi/bluetooth capable module with every sensor.

Radio: This sounds complicated.

Cables: 30m cable could generate lot of noise level. But I'm open to use this method with proper knowledge, if someone like to offer.

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    \$\begingroup\$ There's a reason why industry uses 4-20mA current loops for sensors. \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Aug 25, 2017 at 3:00
  • \$\begingroup\$ What sensors? What bandwidth do you need? \$\endgroup\$
    – Curd
    Aug 25, 2017 at 10:39
  • \$\begingroup\$ Wi/Fi is radio and it needn't be complicated. Maybe you can provide some insight into the amount of information that is sent and how often it needs to be sent from all sensors. \$\endgroup\$
    – Andy aka
    Aug 25, 2017 at 12:11

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Try a current loop. Instead of using a voltage to represent a measured quantity, the quantity is represented by a current, typically between 4 and 20 milliamps.

  • Current is equal everywhere in a series circuit, so the resistance and consequent voltage drop over a long wire is not a concern.
  • The low impedance of the circuit tends to be less prone to noise.
  • The same two wires can also power the sensor.
  • Using 4 mA as a minimum reading means a measurement of 0 mA indicates a faulty or disconnected sensor.

This is a standard interface in industrial sensors, so you should have little difficulty finding ICs or discrete designs.

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If the sensor is analog, you can get an I2C ADC to read the sensor, and communicate via I2C to the Pi. Of course, you cannot run I2C over a 30m line. However, there are I2C to diff pair driver chips that can be used to communicate over differential pairs that solve your problem. An example is the P82B715.

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  • \$\begingroup\$ Somewhat related: Memoirs of an overgrown I2C bus. If the O.P. knows that he needs to go long distances, then from the beginning he can design-in a bus that's intended for long distances. \$\endgroup\$
    – Nick Alexeev
    Aug 27, 2017 at 5:24
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If you have SPI sensors, simply run the sensors at LOW DATA RATES, and place slow low-pass-filters right at the MCU.

Use twisted pair, professionally-made twisted pair, machine-made, so any external magnetic fields are more likely to be nearly nulled out.

Be aware of significant upsets to "GND" over that 30 meter distance. Perhaps fiber optic isolated SPI, so GND_MCU need not be the GND_SENSOR.

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