Capacitance touch switch using MOSFET

[hcccs]

I'm trying to make a simple capacitance touch switch using a BS170 N-channel MOSFET. The touch plate connects to the gate but I don't know how to choose bias resistor values. It is a 12V circuit and I want the output to drive a SN75441 motor drive. Can anyone give me a helping hand?

 

[(*steve*)]

Are you using a pair of plates that you bridge with your finger?

Do you have a circuit diagram that we can refer to?