I want to connect 2 soil sensors to one board as well as a am2301 sensor, the soil sensors are powered externally, however I want to know if the Arduino can power the 2 rs485 modules as well as the am2301 sensor?
What do the sensor specifications tell you?
Note that Arduinos are not designed as power supplies - so, if these sensors draw significant power, you'd be better to power them separately.
What Arduino?
If it's an Arduino with a linear regulator, the limiting factor is often the power dissipation in the regulator - rather than the current itself.
Yes you can connect 2 of the NPK type sensors to 1 Arduino. However, and I can't stress this enough - you need the exact documentation for the NPK sensor you have bought, or intend to buy.
I offer you this advice as I have seen several variations of what forum users call an NPK sensor. Some just provide N, P & K values, others add temperature, moisture content and conductivity etc. They can have differing register addresses for the parameters. Without the documentation, it's guesswork as to the registers used.
Why do you think you need 2 RS485 modules? RS485 is a bus that supports several devices, so you can connect both NPK sensors to one RS485 module.
However, if you want to use 2 or more NPK type sensors on the same RS485 bus, then you need either the manufacturers PC program or the documentation. You need this because you need to be able to change the address of one of the NPK sensors. They will likely all be configured for address 01.
Of course, if you have a MEGA2560 based Arduino, then you can run 2 completely separate RS485 busses and keep both NPK sensors at their default address.
The Arduino board should be able to power the 2 RS485 modules and the AM2301 (aka DHT21 temperature / humidity ) sensor. A website I found said that the AM2301 draws around 1-1.5mA but you need to verify this.
Also take note of the advice above:
Do your sums to determine the sum total of the requirements of your RS485 modules and AM2301 sensor.
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