Review on ESP-01 Wireless Transceiver Board 3.3V - 📡 4Pcs ESP8266 Serial WiFi Module for Arduino (Updated) by Dee Parson

The internet is flooded with reports of problems with these modules. In my case I finally found that after grounding GPIO0 during a board reset (to enter software download mode) and exiting the module, it was idle for about 15 seconds code without the " Packet Header Wait Timeout". Error message. The "Blink" code that supports them incorrectly states that the built-in LED (labeled "LED_BUILTIN") is on pin 2. It's actually on pin 1, which is why you modified the code to run the test program. Finally, the download does not work with the default 115,200 baud, but with 74,880 baud, which you can set in AT mode. To run the code you need to reset this module, which isn't mentioned in any of the tips I've read. It's a great little module, but if you're unlucky you could spend days trying to figure out how it works.