The ATTiny85 is an Atmel AVR board that is a small in form. It is like the little brother to that ATmega32p that is found in the Arduino boards. It has all the same kind of functions but with fewer GPIO pins.

• Official product page: https://www.microchip.com/wwwproducts/en/ATtiny85

SparkFun has a nice pinout diagram: https://learn.sparkfun.com/tutorials/tiny-avr-programmer-hookup-guide/attiny85-use-hints

SparkFun also has a nice one-page PDF reference guide for ATtiny85: https://cdn.sparkfun.com/assets/2/8/b/a/a/Tiny_QuickRef_v2_2.pdf

The `o` represents the circle indentation in the board.
Align the `o` to the top-left where pin 1 is.

 Reset  1 -o==- 8  5V
 A3/D3  2 -   - 7  D2/A1 (SCK)
 A2/D4  3 -   - 6  D1 (PWM, MISO)
 GND    4 -===- 5  D0 (PWM, MOSI, AREF)

Since the ATtiny85 has no USB plug for serial over USB, you can use an Arduino UNO as an inline serial programmer (ISP) to flash the ATtiny85 with a sketch.

• Nice YouTube video by Mitch Davis on programming Attiny85 https://www.youtube.com/watch?v=N591sLGYWnM

Wire up the Arduino UNO to the ATtiny85 over GPIO pins. Also add a 10uF capacitor between RESET and GND in the Uno (anode to ground) to prevent the upload causing the board to reboot.

    Uno:ATtiny85
=======:========
     5V:Gcc
    GND:GND
 Pin 13:Pin 7 (SCK)
 Pin 12:Pin 6 (MISO)
 Pin 11:Pin 5 (MOSI)
 Pin 10:RESET

The MISO, MOSI, and SCK pings are used for SPI communication.

In the Arduino IDE, set the target board as the ATtiny85 1MHz speed. And select for the Programmer: Arduino as ISP. If you need to add the board, add this URL to your board manager: https://raw.githubusercontent.com/damellis/attiny/ide-1.6.x-boards-manager/package_damellis_attiny_index.json