💬 Battery Powered Sensors

[hiddenuser]

I have used resistor ( 470K+1M ) to measure the voltage on a 5 minute interval. The Soil sensor is out in the garden where the container housing the arduino pro min gets exposed to sun. The voltage reading is high during 1 PM to 4 PM, when its under the sun. I am not sure if this is because of the heat.

The code is given below. Please note I multiply my actual voltage with calibration variable. however during the high voltage time the calibration variable does not seem to work.

• Battery powers the soil sensor.
• The voltage regulator has been removed.
• MCU powered using vcc pin.

//#define MY_DEBUG
#define MY_RF24_PA_LEVEL RF24_PA_LOW
#define MY_BAUD_RATE 38400
#define MY_RADIO_NRF24
#define VIEW_READING

#include <MySensors.h>
#include <SPI.h>
#include <math.h>


#define round(x) ((x)>=0?(long)((x)+0.5):(long)((x)-0.5))
#define N_ELEMENTS(array) (sizeof(array)/sizeof((array)[0]))
#define NUM_READS 10
#define CHILD_ID_MOISTURE 0
#define CHILD_ID_BATTERY 1
#define SLEEP_TIME 300000//10000 // Sleep time between reads (in milliseconds), was 10000
#define STABILIZATION_TIME 500 // Let the sensor stabilize before reading default BOD settings
#define VOLTAGE_PIN A0


int index;
long buffer[NUM_READS];
const long Known_Resistor = 4700;

/// @brief Structure to be used in percentage and resistance values matrix to be filtered (have to be in pairs)
typedef struct {
  int moisture; //!< Moisture
  long resistance; //!< Resistance
} values;


MyMessage soil_msg(CHILD_ID_MOISTURE, V_LEVEL);
MyMessage voltage_msg(CHILD_ID_BATTERY, V_VOLTAGE);


void presentation() {
  sendSketchInfo("Soil Moisture", "2.0");
  present(CHILD_ID_MOISTURE, S_MOISTURE);
  present(CHILD_ID_BATTERY, S_MULTIMETER);
}


void setup() {
  pinMode(6, OUTPUT);
  pinMode(7, OUTPUT);
  digitalWrite(6, LOW);
  digitalWrite(7, LOW);
}




void loop() {
  //float dryLevel = readNoMoisture();
  long moistureLevel = readAggSoilMoisture();

  //float coeff = 100.00 / float(dryLevel);
  float voltage = readVoltage() * 1.57368; 

  float batteryPcnt = voltage / 3.3 * 100;

#ifdef VIEW_READING
  Serial.print("--Voltage:"); Serial.println(voltage);
  Serial.print("--Battery %:"); Serial.println(batteryPcnt);
  Serial.print("--Soil Sensor value:"); Serial.println(moistureLevel );
#endif

  send(soil_msg.set(moistureLevel, 1));
  sendBatteryLevel(batteryPcnt);
  send(voltage_msg.set(voltage, 3), 1);



  sleep(SLEEP_TIME);
}

float readVoltage() {
  analogReference(INTERNAL);
  fakeRead(VOLTAGE_PIN);
  int sensorValue = analogRead(VOLTAGE_PIN);
  float voltage = sensorValue * 0.003363075;
  analogReference(DEFAULT);
  fakeRead(VOLTAGE_PIN);
  return voltage;

}

void fakeRead(int pin) {
  for (int counter = 0; counter < 5; counter++) {
    analogRead(pin);
    delay(STABILIZATION_TIME);
  }
}


// Averaging algorithm
void addReading(long resistance)
{
  buffer[index] = resistance;
  index++;
  if (index >= NUM_READS) {
    index = 0;
  }
}

long average()
{
  long sum = 0;
  for (int i = 0; i < NUM_READS; i++) {
    sum += buffer[i];
  }
  return (long)(sum / NUM_READS);
}

int readAggSoilMoisture()
{

  measureRawSoilMoisture(6, 7, A1);
  long read1 = average();

  measureRawSoilMoisture(7, 6, A2);
  long read2 = average();
  long sensor1 = (read1 + read2) / 2;
  return int( ((sensor1 / (float)Known_Resistor) * 100.00));
}



void measureRawSoilMoisture (int phase_b, int phase_a, int analog_input)
{
  // read sensor, filter, and calculate resistance value
  // Noise filter: median filter

  for (int i = 0; i < NUM_READS; i++) {

    // Read 1 pair of voltage values
    digitalWrite(phase_a, HIGH);                 // set the voltage supply on
    delayMicroseconds(25);
    int supplyVoltage = analogRead(analog_input);   // read the supply voltage
    delayMicroseconds(25);
    digitalWrite(phase_a, LOW);                  // set the voltage supply off
    delay(1);

    digitalWrite(phase_b, HIGH);                 // set the voltage supply on
    delayMicroseconds(25);
    int sensorVoltage = analogRead(analog_input);   // read the sensor voltage
    delayMicroseconds(25);
    digitalWrite(phase_b, LOW);                  // set the voltage supply off

    // Calculate resistance
    // the 0.5 add-term is used to round to the nearest integer
    // Tip: no need to transform 0-1023 voltage value to 0-5 range, due to following fraction
    long resistance = abs(Known_Resistor * (supplyVoltage - sensorVoltage ) / sensorVoltage) ;

  
    delay(1);
    delay(STABILIZATION_TIME);
    addReading(resistance);
 
  }

}

[Moebius Lutching]

Using voltage divider still might be necessary if you don't have battery directly connected to the MCU VCC, for example using step-up/down regulator to power the MCU. In this case you still can use a voltage divider and have a P+N Mosfet to control the current going through the voltage divider, so no leak to ground.

In practical terms you basically use another GPIO to enable or disable it the Mosfet when needed. I saw that on the Whisper Node board I'm using and seems to be effective (reference: https://bitbucket.org/talk2/whisper-node-avr#markdown-header-voltage-monitor)... In any case using high value resistors (over 100K) will reduce any current draw. Finally a small capacitor can be used to stabilize the voltage.

[Jarno Burger]

I have a DHT11 + NRF24L01 + Pro Mini 3.3v 8Mhz
All is working fine when on usb cable.. but it fails when connected to 2x 1.5 AA batteries..
what could be wrong ?

[gohan]

Dht 11 operating voltage is 3.3 to 5v, so 2 AA batteries are not enough. Better look at other more reliable sensors that can operate at lower voltages

[Jarno Burger]

I did a test with a variable power supply , and I can confirm that the pro mini does weird stuff at smaller then 3v, but it works at 3.3v.. I have now also ordered those 3.3v up-boosters (because I already bought the battery mounts) mentioned here.

[maghac]

What would be the best approach if I wanted a battery powered node using a 5V sensor? I want to build a secret knock sensor but I want it to be battery powered.

I was thinking about using 3xAA batteries (3x1.6 would be 4.8V max) and a 5V step-up converter and then power the sensor and the arduino (on the raw pin, since it's a 3.3V arduino). The radio would be powered from the VCC on the arduino.

[gohan]

I think that isn't going to be a very efficient way. Are you sure there isn't any 3.3V sensor you could use?

[ileneken3]

@maghac

Consider this option:
Use 2 AA batteries.
Change the BOD on the arduino to something lower than the 2.8V default.
Power everything from the batteries except the sensor.
Use the 5V step-up converter only for the sensor.

[maghac]

@ileneken3 Good idea, i think I'll design it that way. I also had a closer look on eBay and found another sensor that runs on 3.3v.

[mpp]

"Disconnect or desolder the 3.3 VDC regulator because it is not needed." => Why it isn't needed? I assume it is needed when connecting a sensor that requires 3.3V (e.g. HTU21d or even the RFM69)? I assume the assumption made here is that you're using 2 AA 1.5V batteries? I'm using 3 LR44 (3x1.55V) so I suppose I still need the regulator.

[mfalkvidd]

@mpp the line below the one you are quoting says "Power the device with two AA batteries connected in series". So yes, your assumption that 2 AA batteries is used is correct.

[FatBeard]

Hi, i've got a barebones arduino circuit set up with a dht22 sensor. It's powered off 2 aa batteries. All works well with the two batteries even when they are running at about 3.0 volts combined (it would probably run at lower voltages but batteries haven't gone down that far yet). If i power directly from usb with my ftdi interface all works. However, when i add the 3.3v step up, the radio doesn't get a response from the nrf gateway anymore. I have a 4.7u capacitor on the nrf. The gateway is receiving some data but not all as i can see "mygateway1-out/0/255/0/0/18 2.1.1" in my mqtt broker every couple of seconds but the mysensors client never seems to get fully initialised. I've tried two or three of the step ups and checked the voltage with a multimeter and i'm getting circa 3.3v. One thing i did notice is that when i swapped in one of my 3 dht22's it worked initially but then stopped, the other two wouldn't (all work without the step up). I think this is a bit of a red herring but putting in here for information. Any thoughts?

[rozpruwacz]

share your schematic. from what you are describing it seems like the step up converter is not able to provide enough current.

[gohan]

could be also noise generated from the booster, in fact you could run the NRF24 directly from battery since it can still work down to 1.9V

[rozpruwacz]

but the dht22 would not work, it requires at least 3.3v (however i succesuffly used it with 3V). I think that NRF has some decoupling capacitors onboard, so unless the boost converter design is not totaly wrong it shouldn't be a problem. schematic would be helpful.

[FatBeard]

@rozpruwacz

Here is my breadboard design, i'm afraid the schematic in fritzing isn't really in a state to post here. It's unreadable. The resistors shown in the diagram wouldn't have the correct values i used. The values i used are from the arduino site for creating an arduino. The ones shown are used for the sake of creating a pcb. The DHT22 goes on the 4 pin header, the NRF goes on the 8 pin header.

[FatBeard]

@FatBeard I should point out that the step up was an afterthought and it was the ground and vout of the stepup were connected directly to the power rail along the bottom. It powers the whole circuit.

[rozpruwacz]

@FatBeard said in 💬 Battery Powered Sensors:

goes on

ok, but what about boost converter ? as i understand the problem is when You use the boost converter ? is it some kind of module ? or your design ?

[FatBeard]

@rozpruwacz Ya, it's a module and it's the 3.3v step up module recommended on this page. Thanks for your help by the way

[rozpruwacz]

@FatBeard said in 💬 Battery Powered Sensors:

recommended

which one ? could you paste a link ?