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ESP01 is the smallest available package for the popular Espressif ESP8266 WiFi System on a Chip. It is often used to provide WiFi connectivity to other Arduino boards, but is quite capable in its own right. Its small form factor makes it especially handy if space is at a premium.
The ESP01 is somewhat finicky to work with in developing new circuits. The pins are in a 2x4 header format, so it can’t be plugged into a standard breadboard. Four GPIO pins are exposed, but three have dual function. If GPIO0 is grounded at power on, the chip goes into the programming mode. GPIO1 is TX for the serial line, while GPIO3 is RX. Only GPIO2 is always available.
Unless the circuit only needs only one pin, pins have to be switched between programming and run modes. It’s easy to misconnect a jumper cable and, as I’ve found to my chagrin, connecting your 8266 to anything other than 3.3V will fry it.
This Instructable shows how to make a development board for the ESP8266-ESP01 package. All the pictures show my prototype, but an actual PCB can easily be made from the Fritzing sketch.
The goal for the board was to have a complete package, so that whatever circuit is being developed won’t need any additional power supply. It can be powered via 12V (CUI VXO78-1000) or USB (AMS1117-3.3V), selected by a switch. Assuming only one of the two power cables is connected, this switch can also be used as an on-off switch for the whole board.
Serial to USB is provided by the FTDI FT232RL board. To make the package more compact and less snaggable, I mounted the FTDI on a daughterboard to be parallel to the main PCB. It can be mounted vertically to simplify the work.
Three SPDT micro switches connect GPIO0, GPIO1 and GPIO3 either to the output header or respectively ground, FTDI RX and FTDI TX.
12V and 5V output headers are provided to power any circuit, wired in parallel to the board. They are not affected by the voltage selector switch. Board also provides 3.3V power output as well as 3.3V 15mA positive output header in case the circuit being developed needs a low power current for any digital logic.


CUI Inc VXO7803-1000, 6V-36V to 3.3V, 1A:
Power supply note: power supplies that plug into a 1x3 header have three possible output pin configurations. Different 3.3V power supplies can be used, provided their output pins match one of the supplies on this list.
spdt micro switches
10K ohm resistor
220 ohm resistor
two wire terminals
70x50mm double sided PCB or 16x24 perfboard
Optional: 1x6 90degree header and 4x6 perfboard to mount the FTDI board horizontally”

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