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Sunday, 14 July 2019

A simple circuit for measuring electrical current with Arduino


The current measurement requires the use of an ammeter placed in series with the load. An ideal ammeter has no voltage drop, i.e. it is a short circuit. But most of the current sensors are based precisely on the measurement of the voltage drop on a resistor which, according to Ohm's law, is proportional to the current that passes through it.
For what has been said, a good real ammeter must have a very small voltage drop, this in order not to alter the circuit under measurement with its insertion. The sensor I present has a voltage drop of only 50 mV and uses easily available components.

The shunt
The most accurate system is to use a shunt, i.e. a resistor with very low resistance compared to that of the load and the generator. Of course today we also find on the market sensors based on the Hall effect, that is a device that measures the magnetic field generated by the current passing through a wire for the well-known Ampere’s law, but these semiconductor sensors do not have comparable stability and precision. Figure 1 shows the typical appearance of a 10A and 75mV shunt.

For low currents it is possible to realize the shunt with 0.1 Ω wire resistors, possibly put in parallel, or with a piece of constantan wire of adequate section and length.

Measurement techniques
There are two ways to insert the shunt:
1)      low-side: the shunt is connected between the load and the mass.
2)      high-side: the shunt is connected between the power supply and the load.
Figure 2 shows the two connection modes.

The first mode has as advantages a low common mode voltage, inputs and outputs related to ground and circuit simplicity but is affected by disturbances of ground returns
The second way is not affected by mass disturbances but the amplifier must work with high common mode voltage values and the circuit is more complex.

A low-side solution
There are several breakout boards available on the market ready to measure the current with Arduino, but for those who love self-construction and also to understand the functioning of the circuits, I suggest to carry out my project. It is a fairly accurate current sensor while using common components.
I use the shunt in low-side mode connected to a differential amplifier with Arduino-compatible output with internal reference, therefore with ADC overall voltage measurement range equal to 1.1 volts. With this solution a more stable and less noisy reference is obtained with a resolution of 1100/1024 = 1.074 mV.
Using operational power amplifiers with a single power supply there are problems if the signals approach zero. Even using special operational amplifiers called "rail to rail", whose output swing is closer to the rails, rarely they have outputs below 20 mV at zero input. A similar output can also be obtained using a most common LM358 suitable for working even with single power supply.
An expedient to overcome this drawback is to operate on a value higher than zero and therefore to subtract this constant at software level. To have a zero in output and also to eliminate the off-set I added a Vsh constant of about 100 mV. For this circuit I used a differential amplifier U1a, whose output is equal to:
Vo = (Vin+ - Vin-)R2/R1+Vsh
The second operational U1b, connected as voltage follower, generates the voltage Vsh by means of a divider on the Arduino Vref.
For correct operation the following relations must be respected: R1=R3 and R2=R4 with very low tolerances. I have selected them among metal film resistors with a 1% tolerance, now easily available, using a good ohmmeter. Since the double LM358 does not have a very low offset I preferred not to overdo the gain, so R2/R1 = 20.
The ADC converter has a full scale of 1100 mV and I take 100 out of it to make the zero, so I can have 1000 mV for the measurement, this means that the maximum input voltage must be 1000/20 = 50 mV.
The following table shows some shunt values and related flow rates.
I max [A]
Shunt [Ω]
0.5
0.1
1
0.05
5
0.01
10
0.005
It is also possible to use commercial shunts. Many of them have a voltage drop of 75 mV so we can vary the gain that becomes 1000/75 = 200/15 = 13.33 or calculate the new full scale which is equal to 2/3. For example, a 10 A and 75 mV shunt has a resistance Rs = 7.5 mΩ and a full scale I = 50 / 7.5 = 6.66 A.
To have a sensitivity of 75 mV, suitable for many commercial shunts, it is sufficient to set R1 = R3 = 15kΩ and R2 = R4 = 200kΩ, standard and easily available values.
Figure 3 shows the circuit and its connections with Arduino.
To test this circuit I used a 0.1 Ω shunt resistor and a 0.1 A current source.

Significant improvements are achieved by replacing the LM358 with a low off-set input like the OP290, OPA2196, OPA2277 and similar chips.
In figure 4 we can see the layout and the connections of the components of my prototype, made on a small pre-drilled pcb board.
The op-amp offset
Not considering here the problem of saturation already seen for the single power supply, a real operational amplifier has no zero-volt output with zero input, the input offset is defined as the voltage I have to give to the input to have zero in output.
Figure 5 shows the model for evaluating the offset of a differential amplifier.
Considering the Vos offset voltage, the output voltage with zero input is:
Vo = (1+R2/R1)Vos
For example: if an LM358 has an offset of 2 mV, with R2/R1= 20 there would be a difference of 44 mV (2x21 + 2, including that of U1b) between the voltage Vo and Vsh. The offset is temperature dependent and causes a thermal drift, this operational has an input offset voltage drift of about 7 µV/°C.
For this application the input offset is very important as this constant is amplified by the amplifier.
Ignoring the thermal drift, we can remove the offset using a trimmer on the divider that produces Vsh. The rest will do the software by subtracting a constant. For Vsh= 100 mV this number is equal to:
Nsh =100*1023/1100 = 93
This is the constant to be removed and we should set the Rp1 trimmer until zero is read for zero current. In the test phase I increased this value to 98 to work better with the trimmer.
Components list
component
description
component
description
R1, R3
11 kW ± 1% metal film
Rp1
500 Ω multi-turn trimmer
R2, R4
220 kW ± 1% metal film
Rs
0.1 Ω  shunt
R5
10 kW ± 2% metal film
C1
10 µF,25V Aluminum electrolytic
R6
1 kW ± 2% metal film
Arduino
Arduino UNO or Nano board
R7
10 kW ±5%
U1
Dual op amp LM358

The program
The example program is very simple, the values used are relative to my system. To calibrate the Rp1 trimmer, just run the program and turn the trimmer to read zero on the serial monitor with zero input current. If it is not possible to zero the trimmer, it is necessary to slightly modify the Nsh constant to be subtracted from the measurement.
The constant mVtomA is derived from experimental measurements using a good precision ammeter, a power supply and some resistive loads such as car bulbs or wire resistors of adequate power.

/* program ArduAmmeter.ino Arduino current meter
 Giovanni Carrera, rev. 11/07/2019 */

float NtomV;
const float VREF = 1095;// in mV, this value can be read on VREF pin
const int Nsh = 98;// shift value corresponding to about 100 mV
const float mVtomA = 2.17;// value obtained by a calibration

void setup() {
  Serial.begin(9600);
  analogReference(INTERNAL); // internal ADC reference input = 1100 mV
  NtomV = VREF/1023;// constant of conversion into millivolts
}

void loop() {
  int val = analogRead(A0)-Nsh;// read the current sensor and remove the shift
  float mvolt = NtomV*val;// convert to millivolt
  float mamp = mvolt/mVtomA;// convert to milliampere
  Serial.print("Vo = ");
  Serial.print(mvolt,1);
  Serial.print(" mV - Current = ");
  Serial.print(mamp,0);
  Serial.println(" mA");
  delay(1000);
}



References
1.       “AN1332, Current Sensing Circuit Concepts and Fundamentals”, Microchip DS01332B, 2011.
2.       “AN39, Current measurement applications handbook”, Peter Abiodun Bode, Zetex Semiconductors, 2008.
3.       “AN105, Current Sense Circuit Collection”, Tim Regan, Linear Technology, dec 2005.
4.       “Fully Integrated, Hall Effect-Based Linear Current Sensor IC with 2.1 kVRMS Isolation and a Low-Resistance Current Conductor”, ACS712-DS, Rev. 15,  Allegro MicroSystems – 2013.
5.       “VARDULOG - Data logger dei consumi di un apparato elettrico”, Giovanni Carrera, rivista FARE ELETTRONICA n. 361-362, Novembre-Dicembre 2015;
6.       “Progetto ArduWattmeter”, Giovanni Carrera, rivista FARE ELETTRONICA n. 367/368 ­ Giugno/Luglio 2016;
7.       “Op Amp Input Offset Voltage”, Analog Devices, MT-037 Rev.0, 10/08, WK.

Monday, 20 May 2019

How to make a supersize thermometer with a strip RGB led and Arduino.


The idea came to me by experimenting with intelligent RGB LED strips, that is equipped with a control microchip, like the NEOPIXEL based on a WS2812B chip. The interesting thing is that just one bit is enough to control numerous LEDs. The number of pixels is not limited by the signal level, always retransmitted by each chip, but by the transmission speed.
On the web there are several libraries, I have tested both the FastLED-master and the Adafruit_NeoPixel, both can drive various smart led and controller chips. To measure the temperature I tried three types of sensors but in the final version I used an LM35.
Of course this linear display is suitable for displaying other physical quantities and the number of LEDs can easily be increased.

The WS2812B LEDs

They can be mounted on a flexible and adhesive strip of printed circuit board, the appearance of which is shown in Figure 1. The density is 60 LEDs per meter.
Figure 1 - Typical appearance of a WS2812 strip.
The chips, powered at + 5V with C = 100nF by-pass capacitor, are connected in cascade according to the schematic shown in figure 2.
Figure 2 – Strip led wiring diagram.
The serial communication protocol is NRZ (Non Return to Zero) and the data is 24 bits, eight bits per color in the order GRB (Green, Red, Blue) with the most significant bit first. In this way up to 2^24 = 16777216 colors are realized. The bit rate can vary from 400 to 800 [kbits/s]. Figure 3 shows the diagrams of the packets transmitted and how they are interpreted by the individual chips.
Figure 3 - Data transmission protocol.
The first packet, sent by the MPU, is acquired by the first D1 chip that retransmits the second and subsequent ones. The second chip D2 acquires the second packet and retransmits the next ones, and so on. At the end of the cycle, a pause of at least 50 µs resets the system.
Each LED consumes up to 20 mA, so 60 mA maximum per chip. If I wanted to represent a light bar with 30 leds of white light, that is with the three colors on, I would have a consumption of 1.8 A with considerable dissipation and variation of current. In order to drastically reduce the current I prefer to turn on only one led at a time using one of primary colors RGB and also change its brightness.

My LM35 sensor prototype
It is an analog sensor, from National Semiconductors / Texas Instruments, with output proportional to degrees Celsius and a slope of 10 mV/°C, so the output signal is relatively low.

Its measurement range is from 2 to 150°C with a power supply from 4 to 20 volts and, with particular circuits, it can measure up to -55°C, figure 4 shows the sensor and pin layout.
If the Arduino ADC converter has Vref=5V (default), the resolution is about 5mV, or 0.5°C. But the supply voltage is not stable and varies significantly if Arduino is powered via USB or by Vin. If I program analogReference (INTERNAL) the resolution becomes almost 1 mV because the internal reference of the ADC is about 1.1V (from 1 to 1.2V) and is also much more stable than the default mode. For this reason I used this reference even if the display resolution is one degree.
To measure the temperature of a house it seems to me an economic sensor that also requires a simple software. The scale of my display has 30 RGB LEDs starting from 5 °C up to 34 °C. The schematic is that of figure 5.
Figure 5 - Electrical diagram of the thermometer.
As a precaution, I powered the LED strip with a separate regulator as, in the event of a fault more LEDs could be lit simultaneously, the small controller on Arduino Nano could overheat and even burn.
The average consumption of my prototype is around 50 mA, this also because I only turn on
 one led at a time and also reduced its brightness. The power supply is similar to that used for Arduino, preferably use voltages between 7.5 and 9 volts, even not stabilized.

I used an Arduino Nano because it is compact and with a 0.1" pitch, necessary for prototyping matrix board. The sensor is connected to pin A0 and I have connected a potentiometer on pin A1 to calibrate the thermometer without modifying the program. For the prototype I used a matrix board, as shown in figure 6. Notice at the top the sensor that comes out of the box.
Figure 6 - Aspect of the components of the prototype.
List of electronic components
component
description
component
description
R1
47 kW ±1% metal film
LD
RGB strip LED type WS2812B
R2
220 W ±5%
Arduino
Arduino Nano board
Rp1
5 kW trimmer
U1
LM35 sensor
C1
100 µF,25V electrolytic capacitor
U2
LM7805, 5V regulator
C2,C3,C4
100 nF ceramic capacitor

dissipator

Prototype construction
The simplest constructive solution is to buy the led strip aluminum profile complete with its diffuser and to transfer on it the thermometric scale numbers.
I used a "U" aluminum profile of about 15x10 mm, 600 mm long for a total of 30 LEDs with a scale from 5 °C to 34 °C. The strip has a density of 60 LEDs per meter, so the pitch is about 16.7 mm.
At the end of the bar I fixed a small box containing the electronics, while beside the led bar I glued an "L" shaped profile of about 10x20 mm on which I drew the thermometric scale with a normographe, then I painted it with a transparent varnish protection. This solution, quite "do it yourself" is visible in the introductory photo. The writing can also be done with a label printer or with self-adhesive numbers. The version with the special profile and relative diffuser is also aesthetically the most valid solution.

The program
For this program I used the Adafruit_NeoPixel library (https://github.com/adafruit/Adafruit_NeoPixel) for the LED strip, because it is simpler and more compact.
Nothing prevents you from using multiple LEDs to enlarge the scale, updating the program on the #define NUM_LEDS 30 line and also modifying the control of the LEDs. With some modifications we can use a scale in Fahrenheit and with hardware and software interventions we can extend the scale even to negative values.
The display resolution is one degree, so I convert the temperature that is of float type to integer and address with this the led to turn on.
I have used mostly only one of these leds and with moderate intensity, for example:
pixels.Color (0, 0, 70);
it turns on one of the RGB colors (the blue LED, in this case), with an intensity of only 70/255. This greatly reduces consumption and does not disturb the eyes too much. Only at the ends of the scale, to better highlight the exceeding of the limits, I use violet (red + blue) and yellow (red + green) colors that require two LEDs, but I turn them on only for one second every five. Reducing consumption also avoids heating the sensor and distorting the measurement.
The led chip[0] is the one at the bottom: it is lit blue for a temperature of 5 °C or flashes violet for lower temperatures. The led chip[29] is the highest one and lights up red for a temperature of 34 °C or flashes yellow for higher temperatures.
My program turns on the LED chips based on the following criteria:
·         Chip led[0] a violet flash of 1 second for T < 5°;
·         Chip led[0÷17] blue led on for 5° ≤ T <18°;
·         Chip led[18÷14] green led on for 18°T≤ 24°;
·         Chip led[25÷29] red led on for 25°T≤ 34°;
·         Chip led[29] e yellow flash of 1 second for T > 34°C.
Of course it is very easy to change the limits to better adapt them to personal wellbeing thresholds.
The function pixels.Color() sets the colors with three bytes corresponding to the primary colors, respectively: R (Red) G (Green) B (Blue). The intensity of the colors varies from 0,0,0 corresponding to the LEDs that are all off (black) up to 255,255,255 for the three LEDs that are on (white). Varying the intensity of the individual LEDs the displayed color varies. There are on-line computers that simulate the color generated.
As already seen, the program reads the potentiometer to make a correction of about -3° up to + 6°.
At the beginning I put VREF = 1050 mV, this value is obtained by measuring the voltage with a digital voltmeter, otherwise put VREF = 1100.

The code
/* program ArduTempLedLM.ino Arduino strip led thermometer
 use a LM35 or TMP35 as temperature sensor
 Giovanni Carrera, rev. 11/05/2019 */
#include <Adafruit_NeoPixel.h>

int Temp;
float NtomV;
const float VREF = 1050;// in mV, this value can be read on VREF pin

#define PIN  2 // used for strip led data
#define NUMPIXELS 30  // number of leds in strip (5° to 34°C)
Adafruit_NeoPixel pixels(NUMPIXELS, PIN, NEO_GRB + NEO_KHZ800);
#define DELAYVAL 500 // Time (in milliseconds) to pause between pixels

void setup() {
  Serial.begin(9600);
  pixels.begin();// INITIALIZE NeoPixel strip object delay( 500 );
  analogReference(INTERNAL); // internal ADC reference input = 1100V
  NtomV = VREF/1023;// constant of conversion into millivolts
}

void loop() {
  int val = analogRead(A0);// read the LM35 sensor
  float temperature = NtomV*val/10.0;// convert to Celsius
  val = analogRead(A1);// read the potentiometer for T correction
  float corr = NtomV*val/10.0 - 3;// adjusting factor, about -3 to +6
  temperature -= corr;// Temperature correction
  Serial.print("  Temperature = ");
  Serial.print(temperature,1);
  Serial.println(" °C");
  Temp = (int)temperature;
  if (Temp < 5){
    pixels.setPixelColor(0, pixels.Color(100, 0, 150));// bright violet color
    pixels.show();
    delay(1000);
    pixels.setPixelColor(0, pixels.Color(0, 0, 0));
    pixels.show();
    }
  else if (Temp < 18){// blue led for T<18°
    pixels.clear(); // Set all pixel colors to 'off'
    // pixels.Color() takes RGB values, from 0,0,0 up to 255,255,255
    pixels.setPixelColor(Temp-5, pixels.Color(0, 0, 70));// moderately bright blue color
    pixels.show();
    }
  else if (Temp >= 18 && Temp <= 24){
    pixels.clear(); // Set all pixel colors to 'off'
    pixels.setPixelColor(Temp-5, pixels.Color(0, 70, 0));// moderately bright green color
    pixels.show();
    }
  else if (Temp > 24 && Temp <= 34){
    pixels.clear(); // Set all pixel colors to 'off'
    pixels.setPixelColor(Temp-5, pixels.Color(70, 0, 0));// moderately bright red color
    pixels.show();
  }
  else {
    pixels.setPixelColor(29, pixels.Color(150, 50, 0));// bright yellow color
    pixels.show();
    delay(1000);
    pixels.setPixelColor(29, pixels.Color(0, 0, 0));
    pixels.show();
  }
  delay(5000);

}

References
1)      “WS2812B-V4, Intelligent control LED integrated light source”, http://www.world-semi.com/
2)      “FastLED Basic usage”, Daniel Garcia, https://github.com/FastLED/FastLED/wiki/Basic-usage,16 Aug 2017.
3)      “LM35 Precision Centigrade Temperature Sensors”, Texas Instruments, SNIS159H, August 1999–revised December 2017.