Creating a 240V vibration sensitive circuit

Hi there,

I am new to arduino so please forgive my limited knowledge.

I am looking to use a chip to control a 240V string of fairy lights ( http://www.fairylightuk.com/Products/100-06169 ) in a sculpture.

I want the lights to be activated by vibration in a guitar string. I was going to use a piezo electric disc as the sensor. I would like the lights to rest at half brightness when there is no vibration but when the strings are plucked I would like the light levels to rise and fall with the amplitude of the vibration of the string. I have tried to build this circuit with a piezo disc and a transistor. It nearly worked but I need to be able to calibrate the sensitivity of the whole system.

As a first step I was hoping that somebody might advise on which board to choose. I would like it to be as small and discrete as possible to hide within the sculpture.

Please do take a look at my sculpture work at http://jacksawbridge.com/ .

Many thanks Jack