How to Use the ESP8266-01 Pins and Leds




Updated 1st July 2018 -- added note on reprogramming when GPIO0 is used as an output.

Updated 2nd April 2018 to show ESP8266-01 Leds. You can control these leds from the pins they are connected to.


ESP8266-01 is a very low cost WiFi enabled chip. But it has very limited I/O. At first glance, once you configure it for programming all the pins are used.

This instructiable builds on Using ESP8266 GPIO0/GPIO2/GPIO15 pins to show you how you can get four (4) usable inputs/outputs for you next ESP8266-01 project and how to use IC2 to get even more inputs.

The code here assumes you are programming the module using the Arduino IDE setup as described on under Installing With Boards Manager. When opening the Boards Manager from the Tools → Board menu and select Type Contributed and install the esp8266 platform.

These instructions are also available from at ESP8266-01 Pin Magic

Step 1: ESP8266-01 Pins

The ESP8266-01 is the smallest ESP8266 module and only has 8 pins. Of these VCC, GND, RST (reset) and CH_PD (chip select) are not I/O pins but are needed the operation of the module. This leaves GPIO0, GPIO2, TX and RX available as possible I/O pins, but even these have pre-assigned functions. The GPIO0 and GPIO2 determine what mode the module starts up in and the TX/RX pins are used to program the module and for Serial I/O, commonly used for debugging. GPIO0 and GPIO2 need to have pull-up resistors connected to ensure the module starts up correctly.

Step 2: ESP8266 Programming Tips (espcomm Failed)

When programming the ESP8266 using the Arduino IDE (see ESP8266-01 Wifi Shield) you sometimes (often) get an error messages in the Arduino IDE like:-
esp_com open failed
error: Failed to open COM33
error: espcomm_open failed
error: espcomm_upload_mem failed

In that case follow these steps to get it working:-

  1. Check you have ESP8266 board selected in the Arduino Tools menu
  2. Check you have selected a COM port in the Arduino Tools menu
  3. Power cycle the ESP8266 with GPIO0 grounded (clean power application, see below)
  4. If 3) does not fix it, unplug the USB cable from the computer wait few secs and plug it back in
  5. If 4) does not fix it, uplug USB cable from PC, close Arduino IDE, open Arduino IDE, plug USB cable back in.

When you apply power to the ESP8266, after grounding GPIO0, make sure it is applied cleanly. Don't jiggle the connection. The ESP8266 led should just come on and stay on without any flashes.

Step 3: Best Trick – Use I2C

The best trick to get extra inputs into the ESP8266-01 is to use an I2C interface.

One choice is to use GPIO0 and GPIO2 as the I2C bus.

The pullup resistors needed to get the module to start up correctly can double as the I2C bus pull-up resistors and the other, slave, components on the bus are open collector and so should not pull the bus down on power-up. However in some cases slaves, particularly those with battery backup, can become stuck and hold the bus down. In those cases you will need to isolate the bus until the ESP8266 gets through its boot stage.

You can avoid this issue by using TX and RX for the I2C bus

A few things to note:

  1. GPIO1 (TX) is used as the Data line, because you will always get some debug output on GPIO1 on power up. There is no way to suppress this output, but the Clock line (RX) will be held high so none of this data will be clocked to the slaves
  2. When programming the ESP8266, the RX line is connected to the programmer's output. At the end of the programming the ESP8266 reboots and 330 Protection resistor prevents RX shorting the programmer's output drive.
  3. The I2C series resistors provide similar protection for the TX, RX from shorts on the I2C bus

The ESP8266 is 3.3V device so preferably use 3.3V I2C slaves. Many, but not all, I2C devices are are 3.3V these days. “In general, in a system where one device is at a higher voltage than another, it may be possible to connect the two devices via I2C without any level shifting circuitry in between them. The trick is to connect the pull-up resistors to the lower of the two voltages.” (SparkFun I2C tutorial) For a mixture of 5V and 3.3V devices connect the pullup resistors to the 3.3V line, as shown above.

Using I2C is a great way to add a multi-channel A-to-D converter to the ESP8266-01 which does not expose the single ADC input of the underlying module. For example using Adafruit 12bit I2C 4 channel ADC or for analog output SparkFun's I2C DAC Breakout – MCP4725 board. Many other types of sensors are also available with I2C buses.

See for more information of overcoming I2C problems. Also see Reliable Startup for I2C Battery Backed RTC for a short method to help clear the bus

Step 4: Using the GPIO0 /GPIO2 for OUTPUT and RX for INPUT

While can send debug messages over the WiFi connection it is often convenient to use the TX connection. The next example show how to use GPIO0 and GPIO2 as outputs and RX as an input.

allows you to use RX as a general purpose input (or another output), while still writing debug messages to Serial. Again the 330 ohm resistor in the RX lead to the Flash programmer protects against shorting out the programmer's driver. NOTE: S1 will have to be open in order to program the ESP8266.

The TX pin can be accessed from the sketch as GPIO1 and RX is GPIO3

How to reprogram when using GPIO0 as an output

Note: GPIO0 is needs to be grounded to get into programming mode. If you sketch is driving it high, grounding it can damage you ESP8266 chip. The safe way to reprogram the ESP8266 when your code drives the GPIO0 output is to :-
a) Power down the board
b) short GPIO0 to gnd
c) power up the board which goes into program mode due to the short on GPIO0
d) remove the short from GPIO0 so you don't short out the output when the program runs
e) reprogram the board
f) power cycle the board if necessary.

Step 5: Another Trick – Driving a Relay and Reading a Push Button Using GPIO0 / GPIO2

Here is another way of configuring the pins. Note: This trick only works if you have a relay module with an isolated input (N1 and N1-com). Because of this limitation and the complexity of the supporting code, the previous example, using RX as an input, is preferable.

Using ESP8266 GPIO0/GPIO2/GPIO15 pins has already covered how to use GPIO0/GPIO2 together to get an extra input. Here that example will be extended to use GPIO0 as an relay driver output and GPIO0/GPIO2 as an input.

Here is the schematic as a pdf.

Here GPIO0 is used as an output to drive the relay and GPIO0/GPIO2 is used as an input to read the momentary push button which is used as a manual override to turn the relay on and off, in addition to the remote control over the WiFi connection. The momentary push button is also used to enable the config mode if it is press when power is applied.

The trick here is to do all this while still keeping GPIO0 and GPIO2 high when the ESP8266 module is initializing.

The pull-up resistors, R1 and R3, provide the necessary High for these two pins, but you have to ensure that any extra circuitry attached to GPIO0 and GPIO2 cannot not pull pins low. The optically isolated relay is connected between +3.3V and GPIO0. This keeps GPIO0 high on start up but allows GPIO0 to be made an output, after startup, and ground the relay input to operate the relay. It does not matter if the momentary push button is operated while the module is initializing, as that just connects GPIO0 to GPIO2 and connect both of these to their pullup resistors.

Detecting Config Mode

Using the ESP8266 as a temporary access point you can configure it via a web page as described here. Part of that process is to use a push button, or shorting link, on power-up to indicate to the software that you want to enter config mode.

Once the ESP8266 module has initialized, it runs the setup() code. In that code, to detect if the momentary push button is pressed, you need not make GPIO0 low to supply a GND to push button and then check the GPIO2 input to see if it low. A side affect of this check is that the relay will always be operated when the unit is being put into config mode. Once you see the relay operate you can can release the push button, because its input will have been detected by then. Here is some sample code to do this in the setup()

boolean configMode = false;  // not in config mode normally
void setup() {
  pinMode(0, OUTPUT);
  digitalWrite(0, LOW); // make GPIO0 output low
  // check GPIO2 input to see if push button pressed connecting it to GPIO0
  configMode = (digitalRead(2) == LOW);
  if (configMode) {
    // start AP and get ready to serve config web page
    // leave relay on to indicate in config mode
  } else {
    // normal usage
    // make GPIO0 HIGH to turn off the relay
    digitalWrite(0, HIGH);
  // rest of setup()

Detecting the Manual Override Push Button

The previous section covered detecting when the push button was pressed on power up to enable config mode. We also want to use that push button as a manual override for turning the relay on and off in addition to being able to control the relay via the WiFi link.

The WiFi control of the relay is not covered here, but can easily be done using pfodApp. See OLIMEX Menu Generator for how to generate Arduino code with pfodDesigner for ESP8266 modules.

This section will deal with how to detect when the push button is pushed, indicating the user wants to toggle the relay, i.e. turn it OFF is it on or turn it ON if it is off. The schematic is the same as above, all the tricks are in the code. There are two case to consider:-

  1. The relay is OFF and the user wants to turn it on using the push button,
  2. The relay is ON and the user wants to turn it off using the push button.

The relay is OFF and the user wants to turn it on using the push button.

In this case the output of GPIO0 is HIGH. Actually GPIO0 can be an input in this case as the pull up resistor R1 will ensure the relay does not turn on. That is the trick. In this case make GPIO0 an Input and make GPIO2 Output LOW and then when the user presses the push button, two things will happen:- a) the relay will turn on due to the ground provided by GPIO2 via the push button and b) the Input GPIO0 will go low. The code checks the state of the Input GPIO0 and when it goes LOW the code knows the use has pressed the push button and wants the relay to be on. The code then makes GPIO0 an Output LOW to keep the relay on when the push button is released.

The relay is ON and the user wants to turn it off using the push button.

In this case, following on from the case above, GPIO0 is an Output LOW holding the relay ON. Now for this case make GPIO2 an Input (pulled up by R3) and then when the user presses the push button the Input GPIO2 is pulled LOW by the LOW Output on GPIO0. When the use releases the push button the code detect the LOW to HIGH transition and then makes GPIO0 an Input, which releases the relay due to the pull up resistor, R1, and makes GPIO2 an Output LOW to set up for case i) above.

One more trick. For case ii) we need GPIO2 as an Input which detects a LOW to HIGH transition to turn the relay OFF. But if we make GPIO2 and input at the end of case i) then we will get a LOW to HIGH transition as the user releases the push button they just pressed to turn the relay ON. To avoid turning the relay off again immediately, the first LOW to HIGH transition after turning the relay will be ignored as it is just the user releasing the push button they pressed to turn the relay ON.

Sample loop() Code for Relay Manual Over-ride

In this code I am ignoring switch debounce for simplicity. The inputs should be debounced in any real application.

The sample code is here, ESP8266_01pinMagic_1.ino

Again this leaves the TX / RX pins available for Serial debugging or use as other I/O


This page shows how to get the most out the limited pins available on the ESP8266-01. Using GPIO0 / GPIO2 as an I2C bus gives the biggest expansion, but if you project does not use I2C, you can still drive a relay and detect a push button input using GPIO0 / GPIO2. In either case TX / RX are also available for Serial debugging or if you send debug print statements over the WiFi link, these pins are available for general I/O as well..



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54 Discussions


1 year ago on Step 5

Added ESP8266-01 on-board leds and resistors to the circuits. You can control these leds from the pins they are attached to.


Question 1 year ago on Step 2

error: espcomm_upload_mem failed
how to fix it
i will try all above steps for errors they are not use for this error
plz tell how to fix it

1 answer

Answer 1 year ago

Other things to try,
Try another USB cable, some USB cables are power only,
Make sure you have a good power supply at 3.3V. Good for >250mA. Add a 100uF capacitor directly across VCC Gnd pins. Keep the pullup resistor leads short and well connected. Solder them to the pins.
Try another ESP8266 board. (Is your board broken?)
Try opening the Arduino IDE monitor at 74880 baud and then power cycle the ESP8266, If the ESP is running you should see some start up messages. If not then try the two suggestions above.


Answer 1 year ago

There is one there but it is not bought out to the edge of the board. Use one of the other esp boards that have more edge pins, like the ESP8266-12


Question 1 year ago

I am following your instructions, and have pull-up resistors connected to GPIO 0 and 2, RST, and CH_PD, and yet when the ESP-01 boots up, it doesn't run the code. However, when I program it using an Arduino shield, it runs right away after being programmed. I'm having it send a value to Adafruit IO feed as a way of testing the setup() function. I transfer the ESP-01 after I program it in the shield. Is there something I'm missing as to why it won't run normally? Thanks!

3 answers

Answer 1 year ago

It is a bit difficult to tell just what is going wrong.
How is the Arduino shield wired up to program the ESP? Which Arduino shield are you using (URL?)
When not using the Arduino shield, make sure you remove the ground on GPIO0 and power cycle after programming.
Other things to check are that you have a good 3.3V supply. Try adding a 10uF capacitor directly across the ESP VCC and Gnd pins
See this project for a complete circuit -- ESP2866_01_WiFi_Shield


Reply 1 year ago

Thanks for the response. The Arduino shield is just a proto-shield I made, but I followed this tutorial to program via Arduino Uno, first on a breadboard, then made a shield for it:
The shield seems to work just fine -- it will run the code immediately after being programmed, but with it being a shield just for programming, it doesn't have anything extra connected. It's when I take the ESP out of the shield and put it on a separate breadboard that it never seems to be able to run. The only thing connected at this point is a DHT11 sensor connected to the UTXD pin. Neither GPIO0 nor GPIO2 have anything connected except the pull-up resistors (10k). Added the capacitor (tried 10uF and 100uF), but had no luck there either.. Also, the 3.3V regulator I'm using is supposedly able to provide 800 mA of current, which is substantially more than the 300 mA requirement for this ESP-01.


Reply 1 year ago

I would suggest using 3K3 resistors instead on 10K and if you are on a breadboard keep all the leads with the 3k3 resistors as short as possible to the VCC.
Also try removing the DHT11 module and just program the blink example and get that working first.
Also since the ESP is 3V3 device I would be wary about connecting its TX / RX to the 5V Uno. It is usual to have some protection for the ESP8266.
shows a diode and resistor protecting the ESP RX and TX respectively.
I usually use a USB to 3.3V TTL cable for programming the ESP, e.g.


1 year ago

How many devices can I do on/off with an ESP8266 module and Arduino Uno?

1 reply

Reply 1 year ago

Doing it simply, it is limited by the number of free I/O pins.

Uno == 18, using Analog pins as digital (20 if you use TX/RX).

ESP8266-12, ignoring the Flash pins GPIO6-11 (e.g. Adafruit Feather ESP8266) == 7 (9 if you use TX/RX)

If you want more use a Mega2560 or a decoder (e.g. 74LVC138) or port expander (I2C/SPI)


1 year ago

Hi MRezaN, sorry for the delay in replying, but for some reason Instructables is not sending me notifications of comments :-(

Anyway checkout the reply to lotekjunky below. Looks like you need to add a Fet between GPIO0 and IN to fix the problem.


1 year ago

can the giop consider as a digital pins by defualt ?

3 replies

Reply 1 year ago

GPIO-0 and GPIO-2 default to inputs. You need to use the mode command to change their mode to something else.


Reply 1 year ago

So can i connect my dht22 sensor to the gpio 2


Reply 1 year ago

Sorry I have not used dht22 sensor, but provided you can keep GPIO-2 pulled high while powering up the ESP8266, you can use GPIO-2 as you like.


2 years ago

I've learned a lot from your instructables, but I'd like to make a request. Would you mind making an instruction on how to use GPIO-0 or GPIO-2 as an output via a transistor or mosfet? i'm trying to drive an IR LED off of an ESP-01 and it is giving me all sorts of problems. I'm using a 2n2222a with c @ 3v3, base is coming from GPIO-0 with r3k3. Emitter is going to anode of IR LED, IR LED Cathode is going through R100 to ground. This seems to work... sometimes. Other times, the ESP just hangs with rst 2, boot 3,7 after turning the circuit on, or boot 3,6 after I hit the reset button.

3 replies

Reply 2 years ago

A 3k3 resistor on GPIO-0 to a transistor base is problematic. A P-Fet would be much better. Here is a circuit you can use a relay (or with a suitable limiting resistor an IR LED). The SI2333DS fet is specified down to 1.8v and works well at 3V. The relay needs to have a 3V coil. The second circuit uses the 3V fet to drive an opto-isolated 5V relay module. See the instructable above for how to drive a 5V relay module from 3V.


Reply 2 years ago

Thank you for the response, I truly appreciate it. I've only chosen GPIO-0 as I'm a newb. I'm not using GPIO-2 for anything. Would my original ckt work better if I switch from 0 to 2? Or, do you think the best path forward is to follow your instructions with the coil / relay solution?


Reply 2 years ago

You can use either GPIO-0 or GPIO-2, both have similar limiation, i.e. they must be in pulled to +3V3 with a <10K resistor when booting normally.

Here is revised circuit that drives an IR led as your original question asked.

(Also the instructable referred to in my previous reply as was my Home Automation - How to Add Relays to Arduino )