robott:
Hi!I made a prototype of a allarm with 3 PIR sensors and one buzz with Arduino One.
It works fine!
Atmel Attiny85 http://www.forkrobotics.com/wp-content/uploads/2012/04/attiny85pinout1.png
You can program it with Arduino UNO as isp, you have to download Attiny85 core and add it to boards on Arduino IDE.
Attiny85 core for Arduino 1.0.X Google Code Archive - Long-term storage for Google Code Project Hosting.
If you add more stuff then change to Arduino mini pro (Atmega328P) http://www.apcc.tk/images/phocagallery/tech/Arduino-mini-pro/thumbs/phoca_thumb_l_Arduino-mini-pro-size-long-33mm.jpg
