Review on 💡 Adafruit Trinket - Mini Microcontroller with 5V Logic [ADA1501]: Enhance Your Project with Compact Power! by Dave Tapia

These are great little Arduino boards. I was able to very easily create my own countdown clock with an attached piezo buzzer and power it off of a cell phone backup battery. Beware of people saying they don't work, they take about 10 minutes of special setup. They do not have a built in serial programmer and many can be programmed directly via USB. You need to install custom boards via a URL in your settings and also install a dedicated custom programmer. then you have to press the button on the board before you write on it. You should visit the adafruit website for instructions. This is the 5V version of the board (there is also a 3.3V version). It can run at 8MHz or 16MHz depending on which board you select in the Arduino IDE.