Touch Sensitive Aluminium Block

[spadez8]

Hi,

I initially posted this question on another forum, but I wanted to get a second opinion. I think I have bugged them too much ;)

I have an aluminium block the size of a house brick that will have the inside milled out to leave around 5mm-10mm wall thickness. I want to design a circuit that will sense when the block has been touched. I want the user to be able to put their hand anywhere on the block and have it picked up by the circuit. The brick will have a 12V input via a mains transformer.

What kind of system would be most effective at allowing me to do this? The current suggestions I have so far is:

• IC555 based "hum" based system, picking up the body's mains hum
• A high frequency capacitor charge and discharge circuit - sensing the body's capacitance
  

Please can I have your input on this, as I find it extremely confusing.

[Alex Tsekenis]

I think a hum based system will be more appropriate here. The surface is quite big, it will probably act as a proximity detector if you go with capacitance sensing.

[spadez8]

Thank you for the reply. Two follow up questions. Would it not be possible to fine tune the sensitivity so it only triggered when being touched?

How reliable would either one of these systems be?