Multiple Analog Inputs Using One Analoge Pin

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Introduction: Multiple Analog Inputs Using One Analoge Pin

Hi!

In this instructible I will explain how to read values of more than one sensor by using only one analog input pin.

You might wonder why you want to this.

When you are using a Nodemcu board, as illustrated in the picture, only one ADC pin is availible. For a small weather station I would like to read values of multiple analog sensors: a rain sensor and a light intendsity sensor. This is unfortunately not possible with this board.

A solution is to multiplex the sensors. This works as follows: You will turn a sensor A on, read sensor A, then turn sensor A off. After this you will do the same for sensor B.

The idea is based on the instructable of Breagan. As his code was written in LUA I have writen a sketch in C++. This is handy when you are programming your Nodemcu in IDE (the arduino environment). For ease I tested the sketch and wiring on the arduino UNO/duamilanove.

please see also the intructable where I used this technique!

Step 1: Wiring

For this project you need the following:

  • Two 10K ohm potentiometers;
  • Two diodes;
  • One 10K ohm resistor;
  • Arduino;
  • Broadboard;
  • Some wires.

Connections:

  • Connect one side pin of each potentiometer to ground;
  • Connect the other side pin of potentiometer A to pin 10 of the arduino;
  • Connect the other side pin of potentiometer B to pin 11 of the arduino;
  • Connect the pin in the middle of both potentiometers via a diode to the analog pin 0;
  • Add a pull down resistor (10K ohm) between the ground and the analog pin 0.

The potentiometers do have the same characteristics as analog sensors and are easy to use for testing. The diodes are the key for isolating the sensor circuits. Diodes restrict current to only one direction. Without diodes the sensors still work, however they interact with each other.

Step 2: Coding

In order to multiplex, only one sensor can have a complete circuit at a time. By setting the the GPIO pin 10 to HIGH, we are sending 5v to sensor A and completing the circuit. The other pin (GPIO11) is being set to LOW and is therefor sensor B is turned off.

The sketch will read both values and print a value between 0 and 1023 in the serial monitor. You can open the monitor in the IDE software by pressing CNTRL+M.

As one can see in the illustration above the first potentiometer (A) is fully open while the second one is closed. After this potentiometer A is slowly closed until both values approach zero. After this potentiometer B is fully opened.

Have fun trying this out!

Vincent

Untitled Sketch 2016-04-23

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2 Questions

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how i can know how many resistor for sensor ?

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Hello I have a question how many potentiometers can I plug into one analog input ? And if I have one set of 16 potentiometers axial and another set of 16 pots horizontal is it neccessery to use 2 diodes 1resistor for every analog input ? or is there a "easier" solution need clarification on this thank you and great post thanks alot

27 Comments

Hello I have a question how many potentiometers can I plug into one analog input ? And if I have one set of 16 potentiometers axial and another set of 16 pots horizontal is it neccessery to use 2 diodes 1resistor for every analog input ? or is there a "easier" solution need clarification on this thank you and great post thanks alot

you really saved my life !!

but node MCU supply 3v , what if a sensor usess 5v ?

Edit, the last technic with only the power supply defeats the purpose of this instructables

You can either have a transistor in a switch configuration to switch higher voltages with only 3.3V (or less) needed to activate, or you take the easier way and buy a 3.3V to 5V step-up converter. What you can also try is just to have an external 5v power supply connected to the sensor and just have the Output wires connected to the nodemcu, but remember to connect ground of the power supply to that of the nodemcu.

if i use other diodes, not the 1A 50V diodes, would it still work?

Please give me the suggestions.
1. Would it work if I connect the pin in the middle of both analog sensors directly to the analog pin 0 without a diode?
2. Would it be possible to use other resistor rather than 10K ohm for the pull down resistor? What will happen?
Thank you.

Or you could just buy a quad ADC and use it for four inputs, no multiplexing needed. After researching this today I picked up a quad ADC from Aliexpress for $2.25. Just search for ADS1115 for 16 bit and ADS1105 for 12 bit. You can read the channels with I2C.

Without the diodes you'd have both sensors in the circuit at the same time, and the one powered down could be shorting the output of the other to ground.

Thank you for your reply.
Is that means sensor values would be unreliable?