An isolated analog output for Arduino Uno

This project uses the Arduino PWM Uno or other systems to realize a fully isolated analog output with a range of 0-5 volts or more, changing only the reference voltage.

Introduction

This project completes the series of my articles about the Arduino analog I/O with the aim to use it as a controller of small automation systems.

In control systems of the industrial plants it is always advisable to isolate both the inputs and the outputs coming from the field. This prevents disturbances caused by power surges, lightning strikes or other EMI sources and also by ground potential differences.

Arduino Uno, or systems based on the ATmega328 chip has no a true analog output, but it may be realized using a PWM output averaged with a low-pass filter.

The use of an averaged PWM signal with 8-bit setting is not comparable with a real DAC, but in the insulation case presents undoubted advantages of simplicity since it is sufficient to use an optocoupler for isolating the PWM digital signal. Recently I designed another circuit to generate a 4-20 mA current with Arduino, that experience gave me the idea for this new project.

The Arduino PWM

Arduino Uno has several pins (3, 5, 6, 9, 10, and 11) that can be configured for PWM output. For this project I used pin 9 because the others were used by various devices (LCD, SD and RTC) in my Arduino system.

The PWM signal on pins D9 and D10 is generated by Timer# 1 of ATmega328. It has a prescaler which divides by 1, 8, 64, 256, 1024, controlled by the three least significant bits of the register TCCR1B. The default value of the prescaler set by the Arduino IDE is equal to Np= 64 (TCCR1B, bits 2-0= 110), which provides an output frequency:

PWM frequency = CPUClock/(2´Np´TOP) = 16000000/(2´64´255)= 490.196 Hz

Where the TOP value is he maximum Timer/Counter value.

The following table shows the frequencies generated by Timer# 1 of an Arduino Uno (Atmega 328) on pins 9 and 10,  with a 16 MHz clock and in “phase­correct PWM” mode. In this mode, the timer counts from 0 to 255 and then back down to 0. 

Prescaler divider (Np)	Prescaler code	PWM frequency
1	B001	31372.549
8	B010	3921.569
64	B011	490.196
256	B100	122.549
1024	B101	30.637

The prescaler code must be put in the three least significant bits of the register TCCR1B – Timer/Counter1 Control Register B. For example, to generate a PWM of 3921 Hz, the following instruction must be inserted in the setup function:

TCCR1B = TCCR1B & B11111000 | B00000010;// set timer 1 prescaler to 8

Using a common optocoupler with a phototransistor, as 4N25, the frequency is limited because of the high transition times, so I used a faster optocoupler with photodiode and with an open collector output, such as the 6N136.

To eliminate the output noise I utilized a second order active low-pass filter, Sallen-key type, with a cut-off frequency of about 11.2 Hz. The isolation is achieved with an optocoupler, of course you must use for this circuit a power supply different from the one used for Arduino. If the insulation is not required, things become even simpler and connect the filter to the PWM output, in this case not even need the reference source U2.

The circuit diagram, shown in Figure 1, is quite simple. I recommend using for U1 a double operational amplifier suitable for single-rail power supply, such as LM358. 

The LM358 chip must be powered with a voltage higher than 7 V (and lower than 32) to have in output a maximum voltage of 5V and also the regulator has a 2 V dropout.

The advantage of the open collector of the optocoupler is that you can easily obtain a different output range, for example, using a 10V reference voltage and R2=10 kohm the output range became 0-10V. In this case the LM78L05 must be replaced with a LM317 with an appropriate circuitry.

In figure 2 you can see the arrangement of the components of my prototype.

Hardware components

1x Arduino board,

Components list

R1= 330 ohm ±5%	R2= 5.1 kohm ±5%	R3= 100kohm ±5%	R4= 100 kohm ±1% metal film
C1= 100nF ceramic	C2 = 10 MF,50V Electrolytic	C3= 200 nF Mylar ±2%	C4 = 100 nF Mylar ±2%
	U1= LM358 dual op amp	U2= LM78L05 regulator	OPT1= 6N136

The capacitors used for the filter must be measured with a capacimeter, for my prototype I selected for C3 some 220 nF capacitors to search for a value that approached 200nF and C4 have selected a value half of C3. 

The test on the circuit

The Figure 3 shows the results of the linear regression on the 14 measurements points made on my prototype. The test conditions are:

·        PWM frequency = 490.196 Hz;

·        Vin = 12V;

·        Vref = 5.00 V

The standard error is about 6.1 mV, so the results are very good at the default PWM frequency.

I also tested the system with a frequency of 3921.569 Hz, but with a standard error of 39 mV. The largest errors are found for high duty cycle values, in this area the pulses are narrow and the rise time is high and this phenomenon creates non-linearity. The period is: T = 1/3921.569 = 255 µs. The more narrow pulse has a duration of about 1 µs, approximately the same value as the rise time of the pulses, the cause of non-linearity is due just to this phenomenon. Using the default frequency of 490.196 Hz, the minimum pulse has a duration eight times larger, so it greatly improves the linearity.

The program list

To test the system I used an Arduino Uno with a LCD display and the analog input A0 connected to a potentiometer to vary the duty cycle of the PWM.

// program to test Arduino Uno PWM at 3.9 kHz

// G. Carrera 30 sett 2016

#include <LiquidCrystal.h>

int PWMpin = 9;      // PWM out on digital pin 9

int analogPin = 0;   // potentiometer connected to A0

int val = 0;         // variable to store the read value

char spacestring[17] ="                ";

// initialize the library with the numbers of the interface pins

LiquidCrystal lcd(7, 6, 5, 4, 3, 2);

void setup() {

  pinMode(PWMpin, OUTPUT); // sets the pin as output

  lcd.begin(16, 2);// set up number of columns and rows

  lcd.setCursor(0, 0);// set the cursor to column 0, line 0

  lcd.print("Stalker PWM");// Print a message to the LCD

  // set timer 1 prescaler to 8 for PWM frequency of 3921.57 Hz

  //TCCR1B = TCCR1B & B11111000 | B00000010;

}

void loop() {

  val = analogRead(analogPin) >> 2;// read the potentiometer as 8 bit

  analogWrite(PWMpin, val);

  val = 255-val;// complement

  lcd.setCursor(0, 1);

  lcd.print(spacestring);

  lcd.setCursor(0, 1);

  lcd.print(val);

  delay(500);

}

References

1.      “Secrets of Arduino PWM”, Ken Shirriff, https://www.arduino.cc/en/Tutorial/SecretsOfArduinoPWM

2.      “Atmel 8-bit Microcontroller with 4/8/16/32KBytes In-System Programmable Flash”, 8271G–AVR–02/2013

4-20 mA current output for Arduino Uno

The purpose of this project is to provide a 4-20 mA output from a PWM signal generated by a microcontroller ATmega328 and numerous other chips, such as the PIC. One of the more interesting applications of this circuit would be to replace or to realize a smart sensor with Arduino.

Last year I had designed a circuit suitable only for Arduino Due, this new work makes use of a common Arduino Uno or similar to create a standard 4-20 mA analog output.

Arduino Uno, or systems based on the ATmega328 chip has no a true analog output. The easiest way is to use one of the PWM outputs and filter the signal with a passive RC filter to obtain an analog signal proportional to the duration of the pulses. This expedient creates a considerable noise due to the frequency of the PWM itself. To eliminate the noise I used a second order active low-pass filter, Sallen-key type. The frequency of the Arduino PWM (with 16 MHz clock) on pin 9 is about 490 Hz, so I used a very low cutoff frequency (11 Hz) but with a bandwidth sufficient for the majority of industrial controls.

By connecting the filter directly to the PWM output is obtained a signal which varies from 0 to 5 V which would give an output current of 0 to 20 mA. The pulses duration is programmed with a word of 8 bits, losing 1/5 of the full scale. To improve the current resolution from 20/255 to 16/255, I modified the minimum amplitude of pulses from 0 to 1 volts, giving at the output a 4 to 20mA current. The block diagram is shown in figure 1.

Figure 2 shows the complete diagram of the circuit. To obtain pulses from 1 to 5 volts I had to use a 1 V source realized with U1A and the transistor Q1 that works as a switch. The operational U1B operates as a separator; the filter uses U1C and the voltage / current converter uses U1D and Q2.

The transistor Q1 inverts the PWM signal, so the software must complement the number of PWM duty cycle.

The trimmer pot Rp1 is used to adjust the minimum output current (4 mA) and the Rp2 to adjust the maximum (20 mA). The theoretical value of the emitter resistor is Re = 5/0.02 = 250 W, but that does not take into account the tolerances of the voltage supply of Arduino and of the resistors.

The resistor R8 is used as U1D output current limiter in the situation of absence of load.

A step down converter is a good solution for powering the system because of the 24 V, this value can be varied from 12 to 30 V, depending of the load circuit.

Arduino Uno has a +5 V output pin, It does not recommend using it as a power input          inasmuch this would be in parallel with the internal regulator but it can be powered at +5V using the USB connector, other boards as Arduino Pro Mini, have a +5 V input.

Hardware components

1x Arduino board,

1x Step-down switching converter,

Components list

R1= 27 kW ±5%	R2= 47 kW ±5%	R3= 10 kW ±5%	R4= 27 kW ±1% metal film
R5= 6.2 kW ±1% metal film	R6= 100 kW ±1% metal film	R7= 100 kW ±1% metal film	R8= 1 kW ±5%
R9= 270 W ±1% metal film	R10= 1.8 kW ±1% metal film	Rp1= 1 kW trimmer	Rp2= 10 kW trimmer
C1= 100nF Mylar	C2= 100 nF Mylar	C3= 200 nF Mylar	C4 = 10 MF,50V Electrolytic
C5 = 100 nF Mylar	U1= LM324 quad op amp	Q1 = 2N3904 or eq.	Q2= 2N2219A or eq.

The capacitors used for the filter must be measured with a capacimeter, for my prototype I selected for C3 some 220 nF capacitors to search for a value that approached 200 nF and C2 have selected a value half of C3.  Q1 is a transistor that must have a low Vce(sat.) and Q2 must have a current gain of at least 100 and a Vceo of at least 40V with a minimum power of 500mW.

The operational amplifier U1 must be suitable also for single-rail power supply, such as LM324.

The components layout of my prototype is shown in Figure 3, the resistor on the top is a precision load used for calibration of the system. Q2 has a small heat sink because, with at 20 mA and a low voltage load,  as in this case,  dissipates: (24-3-5) *0.02 = 320 mW. In these circumstances is better to reduce the 24 v.

The test program

To test the system I used an Arduino with an LCD display and a potentiometer connected to analog input A0, as pin PWM I used D9. The program is very simple: read the potentiometer, converts 10 to 8-bit Analog reading and produces the PWM.

// program to test Arduino Uno PWM

// G. Carrera 2 ago 2016

#include <LiquidCrystal.h>

int PWMpin = 9;      // PWM out on digital pin 9

int analogPin = 0;   // potentiometer connected toA0

int val = 0;         // variable to store the read value

char spacestring[17] ="                ";

// initialize the library with the numbers of the interface pins

LiquidCrystal lcd(7, 6, 5, 4, 3, 2);

void setup() {

  pinMode(PWMpin, OUTPUT); // sets the pin as output

  lcd.begin(16, 2);// set up number of columns and rows

  lcd.setCursor(0, 0);// set the cursor to column 0, line 0

  lcd.print("Stalker PWM test");// Print a message to the LCD

}

void loop() {

  val = analogRead(analogPin) >> 2;// 10 to 8 bit conversion 

  analogWrite(PWMpin, val);

  lcd.setCursor(0, 1);

  lcd.print(spacestring);

  lcd.setCursor(0, 1);

  lcd.print(val);

  delay(500);

}

I reported on the spreadsheet the PWM values and the measurements in volts made on a precision resistor (150 W ±0.5%) that worked as a load,. The PWM / output current diagram is shown in Figure 4.

The linearity is very good as confirmed by the coefficient of determination R² = 0.999992.

If you want a positive slope, the value must be complemented to 255 in this mode:

val = 255-val;

In my program, you could generate a new value every 500 milliseconds (2 Hz), but you could reduce this period of up to 100 ms (10 Hz).