The Pi Hut Adafruit Trinket - Mini Microcontroller - 5V Logic Item Sku: SKUB00K9THV04 Click / hover / scroll for larger images
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ATtiny85 on-board, 8K of flash, 512 byte of SRAM, 512 bytes of EEPROM
USB bootloader with a nice LED indicator looks just like a USBtinyISP so you can program it with AVRdude (with a simple config modification)and/or the Arduino IDE (with a few simple config modifications)
Mini-USB jack for power and/or USB uploading, you can put it in a box or tape it up and use any USB cable for when you want to reprogram.
Power with either USB or external output (such as a battery) - it'll automatically switch over
5 GPIO - 2 shared with the USB interface. The 3 independent IO pins have 1 analog input and 2 PWM output as well. The 2 shared IO pins have 2 more analog inputs and one more PWM output.
Trinket may be small, but do not be fooled by its size! It's a tiny micro-controller board, built around the Atmel ATtiny85, a little chip with a lot of power.
We wanted to design a micro-controller board that was small enough to fit into any project, and low cost enough to use without hesitation. Perfect for when you don't want to give up your expensive dev-board and you aren't willing to take apart the project you worked so hard to design. It's our lowest-cost arduino-IDE programmable board!
The Attiny85 is a fun processor because despite being so small, it has 8K of flash, and 5 I/O pins, including analog inputs and PWM 'analog' outputs. We designed a USB boot-loader so you can plug it into any computer and reprogram it over a USB port just like an Arduino. In fact we even made some simple modifications to the Arduino IDE so that it works like a mini-Arduino board. You can't stack a big shield on it but for many small & simple projects the Trinket will be your go-to platform.