💬 Battery Powered Sensors

aclertant

Do you think that the use of a piezo electric switch is possible ?
I found one that deliver 24v and 0,2A... The goal would be to create wireless switch...

AWI

@aclertant Interesting idea... Energy harvesting with piezo-electric components is certainly possible. From MySensors point of view the energy (very very little) has to be stored and boosted to power the radio and mcu for message processing. There are a few examples of "harvesting" remotes (i.e. Philips HUE) but I am not sure if these use a piezo element.

hiddenuser

I removed the both the LED and the Voltage Regulator of an Arduino pro min 3.3v . The simple sketch on it's own works fine. However when I connect an NRF24L01 it does not communicate back to the gateway.. :(

gohan

@aclertant
If you look at enocean products there are already commercial energy harversting switches in case you don't find a way to make it.

aclertant

@gohan yes it's exactly what I'd like to do ;)

I should forget to use an arduino in fact... just send one command using... a pic ? or something else...

The enocean switch is really cool, but quiet expensive :(

gohan

Like all commercial products they cost because they have been developed, tested and produced. So it's up to you if you want to spend time tinkering or go with an already working solution :-)

aclertant

@gohan @AWI
we need something like this but for NRF24L01 module.
https://github.com/SmingHub/Sming
Do you think that possibly exist or there is a huge problem ? Like no eeprom or something like that ?

gohan

You see, enocean protocol has been developed from the start to be used with devices that use energy harvesting, so you can't think to use general purpose HW (like esp8266, FRF24, Arduino boards) that is not optimized very low power consumption (look at all the mods required to make a mini pro last months on battery with a reed switch and a nrf24 module); with piezo-electric components the amount of energy is really really tiny so you need super optimized HW to work with that.
The link you posted is about something that works on a ESP8266 but that works over wifi, that is for sure not the best energy efficient system.

napo7

There is a much more efficient way (and cost-less) to measure VBATT :
https://provideyourown.com/2012/secret-arduino-voltmeter-measure-battery-voltage/
It doesn't need any external resistor, so there will be no current flowing even when the Atmega is asleep.

AWI

@napo7 This has been mentioned in this topic. @Yveaux Vcc library is the easiest way to handle this.

napo7

@AWI Ahh Sorry ! Didn't seen the comment !
I thought it was never mentioned since the how-to still refers to resistor divider method (which is, IMO a bad method since it will draw current...)
I'll have a look at Yveaux's lib.

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.

0_1488370532353_Voltage Graph.JPG

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.

💬 Battery Powered Sensors

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