Using the 4 pins of the ESP8266-01

esp8266_henhouseThe limited number (4) of GPIO pins on the ESP8266-01 may seem like an obstacle, for any serious application.

Yet if one uses the pins in a smart way it is very well possible to do a lot with only those 4 pins.
In some of my recent postings, I have shown the use of a DHT11 a DS18B20, an OLED, an RTC and a BMP180 with the ESP8266-01.
In this posting I set out to use 4 sensors and a display, while also uploading the acquired data to Thingspeak. It actually is expanding on a project of monitoring the atmosphere in and around my chicken coop. Yes, you could call this a weatherstation, but it is just to illustrate the use of the 4 pins, you could easily make something else this way

I will be using 2 pins for I2C (BMP180 and OLED)
1 pin for 2 DS18B20 sensors via the OneWire protocol
1 pin for the DHT11
Although the ESP8266-01 now has all its pins used, I can still add more sensors (or actuators) through the OneWire protocol and/or via the I2C protocol.

So, what do we need:

  • ESP8266-01
  • 2x DS18B20
  • 1x DHT11
  • 1x BMP180
  • OLED (optional)

and ofcourse a breadboard, a 3.3 V PSU and some breadboard wires and a Thingspeak acount

Just some remarks regarding the BOM:

  • ESP8266-01
    Obviously the project is about utilizing the limited pins of the ESP8266-01, but if you still need to buy one, you could consider an ESP8266-12 that has more pins
  • DHT11
    A cheap all purpose humidity and temperature sensor. It is not hugely accurate but it will do. If you still need to buy one, you could opt for the DHT22 that is supposedly more accurate, but you could also opt for the AMS2321. That is a sort of DHT22 that is suitable for I2C, thus freeing another pin
  • BMP180
    measures temperature and Airpressure. It is the successor of the BMP085, but it also now has some suvessors itself. There is the (cheaper) BMP280, but you could also opt for the BME280 that measures temperature, airpresure AND humidity. That way you can save on the DHT/AMS sensor
  • OLED
    I just used that so I quickly could see whether the sensors were read, but you could just as well check it on Thingspeak. The OLED is too small anyway to print all the read values

The circuit

weerstationThe 4 pins of the ESP8266 are not indicated as such on the PCB, and most images only clearly state GPIO0 and GPIO2.
However the ESP826-01 has a a GPIO1 pin (the Tx pin) and a GPIO3 pin (the Rx pin).
i will be using those pins as follows

  • GPIO0 -> SDA pin of the I2C port
  • GPIO1 ->DHT11
  • GPIO2-> SCL pin of the I2C port
  • GPIO3-> OneWire Bus

As my I2C modules already have pull up resistors, I will not add any I2C pullup resistors there. The DS18B20 still needs a pull up resistor for which I used a 4k7, but it is really not that critical, a 10k is also good. The DHT11 supposedly also needs a pull-up resistor but I found it to work without one as well. adding a 4k7 resistor didnt change any of the readings, so I left it out. Many of the 3 pin DHT11 modules, already have a 10 k soldered onto the module.

I just realized that I didnt draw the connections for the OLED. That is because I only hooked it up for a quick check, but should you want to add it, it is just a matter of connecting SDA to SDA and SCL to SCL… and ofcourse the ground and Vcc pins to their counterparts

The program is quite straightforward. First it sets up the libraries and the sensors.
It attaches the DHT11 to pin 1 (Tx) and the OnWire bus for the DS18B20 to pin 3 (Rx). In order to use more than 1 DS18B20 sensor on the OneWire bus, you need to know their ‘unique adress’. If you do not have that then you need a program to read those addresses. Do that on an arduino for ease.

In the program you still have to provide your WiFi credentials as well as the write API for your Thingspeak Channel


   Field 1 temp roost (DHT11)
   Field 2 humidity roost (DHT11)
   field 3 Coop temperature (DS18B20)
   field 4 soil temperature (DS18B20)
   field 5 Airpressure (bmp180)
   field 6 Outside temperature (bmp180)
 * */
#include <DHT.h>
#include  <OneWire.h>//
#include <DallasTemperature.h> //
#include <Adafruit_BMP085.h>
#include <ESP8266WiFi.h>
#include "SSD1306.h"
SSD1306  display(0x3c, 0, 2);

#define DHTPIN 1  //GPIO1 (Tx)
#define DHTTYPE  DHT11
#define ONE_WIRE_BUS 3 // GPIO3=Rx

const char* ssid     = "YourSSID";
const char* password = "YourPassword";
const char* host = "";
const char* writeAPIKey = "W367812985"; //use YOUR writeApi

//DHT11 stuff
float temperature_buiten;
float temperature_buiten2;

//DS18b20 stuff
OneWire oneWire(ONE_WIRE_BUS); //oneWire instance to communicate with any OneWire devices
DallasTemperature sensors(&oneWire);// Pass address of our oneWire instance to Dallas Temperature.
DeviceAddress Probe01 = { 0x28, 0x0F, 0x2A, 0x28, 0x00, 0x00, 0x80, 0x9F};
DeviceAddress Probe02 = { 0x28, 0x10, 0xA4, 0x57, 0x04, 0x00, 0x00, 0xA9};

// bmp180 stuff
Adafruit_BMP085 bmp;

void setup() {
  //I2C stuff
  Wire.pins(0, 2);
  Wire.begin(0, 2);
  // Initialize sensors
  //dht 11 stuff
  //ds18b20 stuff
  // set the resolution to 10 bit (Can be 9 to 12 bits .. lower is faster)
  sensors.setResolution(Probe01, 10);
  sensors.setResolution(Probe02, 10);
  //bmp180 stuff
  if (!bmp.begin()) {
    //   Serial.println("No BMP180 / BMP085");
    //   while (1) {}

  //OLED stuff

  //  Connect to WiFi network
  WiFi.begin(ssid, password);
  while (WiFi.status() != WL_CONNECTED) {

void loop() {
  sensors.requestTemperatures(); // Send the command to get temperatures
  temperature_buiten = sensors.getTempC(Probe01);//
  temperature_buiten2 = sensors.getTempC(Probe02);//
  //dht11 stuff--------------------
  float humidity = dht.readHumidity();
  float temperature = dht.readTemperature();
  if (isnan(humidity) || isnan(temperature)) {
  //bmp stuff-------------------------
    String t= String(bmp.readTemperature());
    String p=String(bmp.readPressure());
  //OLED stuff--------------------------
  display.drawString(0,10,p);//bmp pressure

  // make TCP connections
  WiFiClient client;
  const int httpPort = 80;
  if (!client.connect(host, httpPort)) {

  String url = "/update?key=";
  url += writeAPIKey;
  url += "&field1=";
  url += String(temperature);// roost (DHT1)
  url += "&field2=";
  url += String(humidity);// roost (DHT11)
  url += "&field3=";
  url += String(temperature_buiten);//coop temperature (DS18B20 nr 1)
  url += "&field4=";
  url += String(temperature_buiten2); //soil temperature (DS18B29 nr 2)
  url +="&field5=";
  url +=String(bmp.readTemperature());Outside temperature (BMP180)
  url +="&field6=";
  url +=String(bmp.readPressure());// Airpressure (BMP180)
  url += "\r\n";

  // Send request to the server
  client.print(String("GET ") + url + " HTTP/1.1\r\n" +
               "Host: " + host + "\r\n" +
               "Connection: close\r\n\r\n");



Currently this program only monitors, but what is to stop you from adding a BH1750 I2C light sensor to measure if it is evening or morning or an RTC to know the time of day and to open and close the door of the coop automatically with aid of a PCF8574 I2C I/O expansion card, or as it is already in the garden, add a PCF8591 or ADS1115 AD converter to measure soil humidity and activate a pump when necessary. Or maybe switching on the water basin heater when the temperature falls below zero
if there is an I2 C chip for it, the ESP8266 can probably use it.

NOTE: the Adafruit DHT library contains an error that may show up in bigger progrms on an 8266. If the majority of readings result in “failed to read”, it is time to comment out two erroneous lines in the DHT.cpp file as in the picture below:

Reset a program from a ‘freeze’

While doing a test run on software for an incubator the software did fine, until suddenly on the ninth day weird things started to happen: the LCD showed some odd data and in spite of a low temperature the heating had not kicked in. I resetted the software, wondering what it could be but only an hour later again something off happened. This time in spite of the temperature being well over the upper limit, the heating was not switched off.
I checked my code, which really wasn’t so complicated: Read DSB1820, read DHT11, compare with high and low limit, switch a pin on or off and write the value to an LCD and I couldn’t find a single mistake and don’t forget, it had run flawlessly for 9 days. I suspected my display. This was a 20×4 LCD with I2C module with 4k7 pull up. Nevertheless there might still have been some ‘static’ on the SDA and SCL lines that supposedly can cause the Arduino to freeze.

Obviously that is not good if you are running an incubator as you dont want to check it and find your eggs boiled.
So, other than maybe straightening out the LCD cables a bit, I decided that I needed some software protection against ‘freezes’
The only (and possibly best) way to do this is with the watchdog timer. I dont want to go into the specifics and the background of the watchdog timer, but just keep it on a practical level.
What we do is to set-up the watchdog timer to initiate a system reset after say 4 seconds. Then in our loop we do a reset of the watchdog timer so it starts counting from zero again. So as long as the program tells the watchdog timer “I am still running” nothing will happen. Should the program freeze up, it will not reset the watchdog timer and then after 4 seconds the watchdog timer will reset the entire system.
It is very well possible to use the watchdog timer by manipulating the various registers yourself, but it is much simpler to use the watchdog libary that is part of the avr libraries.
We do this as follows:

#include <avr/wdt.h>

void setup()
	//wdt_enable(WDTO_1S);// 1 sec
	wdt_enable(WDTO_2S);// 2 sec
	//wdt_enable(WDTO_4S);// 4 sec
	//wdt_enable(WDTO_8S);// 8 sec

void loop()
	//  your program

A capacitive soil humidity sensor: Part 3

Initially I imagined the probe as a sleek device with a wire coming out that one would stick in the soil, if necessary completely. This would mean that the circuit pcb would be an integral part of the PCB that  formed the plates, but as always, things go different in practice. For one thing , the piece of PCB I intended to use just wasnt long enough. Second, my PCB etching possibilites are temporarily impeded. Third, I wanted to add an LDR, meaning that I had to have some sort of  translucent cover.

I constructed the circuit on a small piece of stripboard:

IMG_20160225_163125 i2cprobe

With regard to the LDR, sure that is nonsense. If I want to add an LDR I  could just as well add it to my base station. But I am not adding it because I have to, I am adding it because I can and because I wanted to get some experience in programming an I2C slave. An LDR might not be so usefull but in future I may want another sensor, e.g.a sensor that reads if there is really water flowing from the irrigation tube.

Also I had decided on putting the capacitor plates back to back, but as I did not have double sided PCB I just used two pieces glued and  soldered(!) together.
So My BOM was rather simple:

  • 2 equal size pieces of PCB  Size depends on what you have, but do not make them too small. I used 12×3 cm.
  • 1 piece of 0.5-1.0 cm plastic for a baseboard. I used an old cutting board.
  • 1 clear/translucent cover, I used the lid of a whipped cream spray can.
  • 1 piece of  thin 4 wire cable, length depending on your need

pcbprobe2I glued the two piecesof pcb back to back. drilled a hole in all 4 corners and through soldered a piece of wire through each hole, thus anchoring the plates together. I removed some copper around the solder so it would become an island isolated from the rest of the plate. (See picture).

Soldering the plates together in the corners may not be necessary if you decide to electrically isolate them from the soil with e.g. shrink tube.

Eventually I will place an NTC on the plate as well after it is covered with Shrink tube. Esthetically it might be better to put the NTC under the shrink tube, but that could create an air pocket.


baseplateI made the base plate from a o.5 cm thick piece of soft plastic. Cut out a round shape  with a 5.5 cm diameter to fit the base of my  clear dome and  made a slit 3 x0.3 cm into that in which the PCB  fits snug. made a round hole for the connecting cable.




I soldered two wires on the top of the PCB, one on each side. Soldered wires on an NTC, insulated those, attached the NTC to the bottom of the PCB with the wires leading to the top and then covered the PCB with shrink wrap.

probeEventually the probe looks like this (picture)probe3


A capacitive soil humidity sensor: Part 2

capacitive74HC14_attinyIn a previous article I presented a simple way to read a capacitive moisture sensor with a simple RC generator.
In this post I will present a sensor with some added functionality that can be read through I2C. The circuit (figure) doesnt need much explanation: the RC generator we saw in the previous article and the two variable resistors in a voltage divide read by analogue inputs. The Attiny 45 (NOTE, it is NOT an Attiny25) is the heart, or rather the brains of the sensor. As the Attiny  functions as an I2C slave we will need the TinyWireS library. The library comes with some examples and one example was quite easy to rework to what I needed. The code is as follows.

#define I2C_SLAVE_ADDRES 0x4
#include <TinyWireS.h>//
#define TWI_RX_BUFFER_SIZE ( 16 )

volatile uint8_t i2c_regs[] =
// Tracks the current register pointer position
volatile byte reg_position;
const byte reg_size = sizeof(i2c_regs);

byte LDRvalue;
byte NTCvalue;
int MoistPulse;

 * This is called for each read request we receive, never put more than one byte of data (with TinyWireS.send) to the
 * send-buffer when using this callback
void requestEvent()
	// Increment the reg position on each read, and loop back to zero
	if (reg_position >= reg_size)
		reg_position = 0;

 * The I2C data received -handler
 * This needs to complete before the next incoming transaction (start, data, restart/stop) on the bus does
 * so be quick, set flags for long running tasks to be called from the mainloop instead of running them directly,
void receiveEvent(uint8_t howMany)
	if (howMany < 1) { // Sanity-check return; } if (howMany > TWI_RX_BUFFER_SIZE)
		// Also insane number

	reg_position = TinyWireS.receive();
	if (!howMany)
		// This write was only to set the buffer for next read
		i2c_regs[reg_position] = TinyWireS.receive();
		if (reg_position >= reg_size)
			reg_position = 0;

void setup()
	pinMode(1, INPUT);

void loop()
	 * This is the only way we can detect stop condition (
	 * it needs to be called in a very tight loop in order not to miss any (REMINDER: Do *not* use delay() anywhere, use tws_delay() instead).
	 * It will call the function registered via TinyWireS.onReceive(); if there is data in the buffer on stop.

void readSensors()
	LDRvalue = (analogRead(A2)) >>2; // max value is 1023/4=255 is 1 byte (FF) physical pin3=PB4=A2
        i2c_regs[0] = LDRvalue;
	NTCvalue = (analogRead(A3)) >>2; // max value is 1023/4=255 is 1 byte (FF) pin2=PB3=A3
	i2c_regs[1] = NTCvalue;
	//pulsepin= pin6=PB1
	MoistPulse = pulseIn(1, HIGH);
	i2c_regs[2] = highByte(MoistPulse); // or use = MoistPulse >> 8;
	i2c_regs[3] = lowByte(MoistPulse); // or use = MoistPulse & 0xFF;

I have called the pulseIn function with an integer rather than with a long. This means that you will have to choose a value for R3 that gives a reasonable range for the type of soil that you use. A range of say 0-200 uS is very reasonable. Once you have done that you can also add a timeout to the pulseIn function. This should be abt 2 times the pulselength you expect (but depends on the duty cycle). With regard to the two variable resistors, they are in pull up so their value is Rntc = Rseries/((1023/ADC) – 1));  For the NTC this could be substituted in a Steinhart Hart approximation

The code to call the values (and that is loaded onto the master arduino) is even simpler:

#include <Wire.h>
void setup() {
  Wire.begin();        // join i2c bus (address optional for master)
  Serial.begin(9600);  // start serial for output

void loop() {
  for (byte i=0;i<4;++i){
  Serial.println(readRegister(i),HEX); // print the character
Serial.println(" ");

uint8_t readRegister(uint8_t regaddress)
	Wire.requestFrom(4, 1);


This code only prints out the values, you still need to combine the LSB and MSB from the cycle time and e.g. switch a pump based on the value. You could do that with this function:

int combine (byte lsbv, byte msbv)
	int value = msbv << 8;
	value = value | lsbv;
	//value= msbv<<8 | lsvb;//if you want to do it in one go
	return value;

Don’ t forget that the I2C lines need pull-up resistors of 4.7-10k.

In a next article I will present the construction of the probe itself.

The DS3231 RTC temperature sensor

The DS3231 RTC is a rather accurate RTC that has an internal temperature sensor that is used to calibrate the oscillator. The sensor is however also readable from external software. It has a 10 bit solution and  uses two registers: 0x11 and 0x12. The information in the upper byte is stored in 2-complement notation. The lowerbyte is there for the fractional part and has a solution of 0.25 degrees Celsius.
Two’s complement notation means that with positive numbers  it just follows the regular binary storage, but with negative numbers it does it a bit different. Knowing whether a number is negative or positive is indicate by the MSB in the Upper byte. If that is  a ‘1’, then the number is negative.
ds3231 temp sensorAny reading of the registers therefore needs to include a check to see if the  number is positive or negative.     As the Lower byte only indicates the fraction with an accuracy of 0.25 degreesit only needs to count to 4 (0.0. 0.25, 0.50, 0.75), hence two bits are enough
So suppose we have  retrieved the number:
0b0001100101 => +25.25°C. We can easily see it is 25.25°C because the top 8 bits are 00011001, which is 25, while the lower two bits 0b01, mean 1×0.25.
As the  lower byte, only uses the top 2 bits, it may need to be rightshifted 6 positions for calculations. So how about negative numbers, say -18 degrees.
Well -18 is 0b11101110  (=238 in twos complement notatie).
We can see that the highest bit is a 1, indicating a negative number. In order to make a check, we do the following:
0b11101110 &   0b10000000 => 0b10000000  So we know it is negative
Then we need to convert the 2 complement notation
0b11101110 XOR 0b11111111 => 0b00010001 (=17) // first XOR it
17+1= 18   // and  add a ‘1’
18*-1 = -18 // and then we turn it negative
So, how does that look in a program?

float getTemperature()
    int   temperatureCelsius;
    float fTemperatureCelsius;

     uint8_t UBYTE  = readRegister(REG_TEMPM); //Two's complement form
     uint8_t LRBYTE = readRegister(REG_TEMPL); //Fractional part

	if (UBYTE & 0b10000000 !=0) //check if -ve number
		UBYTE  ^= 0b11111111;
		UBYTE  += 0x1;
		fTemperatureCelsius = UBYTE + ((LRBYTE >> 6) * 0.25);
		fTemperatureCelsius = fTemperatureCelsius * -1;
		fTemperatureCelsius = UBYTE + ((LRBYTE >> 6) * 0.25);

	return (fTemperatureCelsius);


Obvously this isnt a full program but just a function. You still need to define REG_TEMPM (0h11) and REG_TEMPL (0x12), and ‘readRegister’ is another function that just reads the specified registers (using the ‘Wire’library)