{"id":2560,"date":"2016-07-06T00:53:35","date_gmt":"2016-07-05T23:53:35","guid":{"rendered":"https:\/\/www.analysir.com\/blog\/?p=2560"},"modified":"2017-02-03T18:07:41","modified_gmt":"2017-02-03T18:07:41","slug":"esp8266-nodemcu-backdoor-upwm-hack-ir-signals-using-uart","status":"publish","type":"post","link":"https:\/\/www.analysir.com\/blog\/2016\/07\/06\/esp8266-nodemcu-backdoor-upwm-hack-ir-signals-using-uart\/","title":{"rendered":"ESP8266 NodeMCU Backdoor uPWM Hack for IR signals"},"content":{"rendered":"

We received our ESP8266 NodeMCU several months ago it has been difficult to find a working example of Hardware PWM to generate crisp Infrared carrier signals. Most posts we have read suggest that there is no PWM readily available on any of the supported platforms. As we have already shown with Arduinos <\/a><\/strong>and the Particle Photon<\/a><\/strong> we figured it would be possible to use a spare UART pin on the ESP8266 NodeMCU to achieve our goal.\u00a0 After some initial success we encountered some watchdog timeouts\/resets and it seemed like the uPWM hack would not be possible on the ESP8266 Node MCU platform. For a few weeks we ‘parked’\u00a0 the effort and today we decided to try again with all of the latest and updated firmware available from the ESP8266 community via the Arduino IDE. This time we were successful and the ESP8266 NodeMCU Backdoor uPWM Hack for IR signals using works!<\/p>\n

\"ESP8266<\/a>
ESP8266 NodeMCU uPWM Inverted IR Circuit<\/figcaption><\/figure>\n

<\/p>\n

UPDATE: an improved non-inverted version is now available here.<\/a><\/strong><\/p><\/blockquote>\n

The uPWM Circuit diagram above for ESP8266 NodeMCU using UART shows the IR out signal on pin D4 . This\u00a0 signal is inverted and Q1 serves to invert it back to normal. Q2 is the main IR\u00a0 LED driver and the IR current can be adjusted by varying R4 to suit. Note that the IR LED D1 is connected to VUSB(or +5V) and R2 is connected to the 3V3 pin on the ESP8266 NodeMCU device.<\/p>\n

The Hack<\/strong>
\nQuite simple really – just set the baud rate to 10 times the desired Infrared carrier frequency and send a ‘magic’ 8 bit character to achieve the desired duty cycle. Of course we need to take the 1-start bit and 1-stop bit into account plus the 8 bits in each character. Remember that the UART sends the data inverted, so this needs to be taken into account with the characters sent and also in the IR LED driver circuit above, which required 2 transistors instead of the usual one. In summary, sending the following characters will generate the associated duty cycle, as follows:<\/p>\n