ATtiny85 is a 8-bit AVR microcontroller based on AVR enhanced RISC architecture. It has an 8-pin interface (PDIP) and comes in the category of low-power microcontrollers. This microcontroller is designed and manufactured by Microchip. Know More
So that the Arduino can act as a device to upload code to ATtiny85.
File - Examples - Arduino ISP - ArduinoISP
Add this line #define USE_OLD_STYLE_WIRING to the code before setup()
UPLOAD!
The pins are not labelled so you will have to refer to the pinout.

Arduino --> ATtiny85
- 5V --> Vcc (8)
- GND --> GND (4)
- Pin 13 --> Pin 2 (7)
- Pin 12 --> Pin 1 (6)
- Pin 11 --> Pin 0 (5)
- Pin 10 --> Reset (1)
Only when you are uploading code to ATtiny85
Put a 10uF capacitor between GND and RESET on Arduino
We have to make ATtiny compatible with Arduino IDE first, so that we can choose ATting85 from Tools -> Board
Go to Arduino Preference
Copy the below code and paste it into Additional Boards Manager URLs, if you already have a board manager URL just add a comma before pasting. Click OK and restart Arduino IDE.
https://raw.githubusercontent.com/damellis/attiny/ide-1.6.x-boards-manager/package_damellis_attiny_index.json
Go to Tools - Board - Boards Manager, search for ATtiny, then install!

Before uploading the code, we have to change some settings.
- Tools -> Board scroll to the bottom select ATtiny25/45/85
- Tools -> Processor--> 8 MHz (internal)
- Tools-->Programmer-->Arduino as ISP
- Check that all wiring, capacitor, and board selections are correct.
Open up a basic code and upload as usual!
If it doesn't work, try Tools - Burn Bootloader


