i would like to construct a circuit that will switch on the circuit when a touch by a finger. how do i go about doing this/?

 

Depends on the circuit. Here's a link to several - http://www.discovercircuits.com/C/capacitance-sw.htm

 

If I were going to do this I would probably use a Darlington transistor and the base of the transistor would be my touch input. A capacitor could possibly be tied to the transistors base to keep the circuit on for a short period of time or used to turn on a latching relay. Just look up darlington transistor on wiki and that may give you a head start. Keep in mind this probably isn't the best way of accomplishing this task as a micro-controller would be much more sophisticated and probably work much better as well.

 

Capacitive touch sensors are available, sparkfun has some single input ones, and Freescale Semiconductors have other more sophisticated IC's for touch sensing such as the MPR084Q Capacitive Touch Controller IC... Analog Devices has the AD7746 capacitive touch sensor IC also available at sparkfun with breakout board..... but all these sensors still require a microcontroller to control them.

My .02

 

I have a friend who is using a pair with an arduino for a Ghostbusters prop...