I have built the proximity switch circuit in extreme circuits http://www.extremecircuits.net/2011/06/proximity-switch.html but the circuit doesnt make noise only when the sensor terminal is grounded and when I connect the sensor end to a metal object it makes sound but when I connect the ground to the the metal object it doesn’t sound and also when I touch the metal when the ground terminal is connected to it.. It doesn’t make a sound.. My question is how to make the metal sensor for this circuit???

 

for this to work you need to connect negative terminal to ground, sensor input should be connected to sensing plate and P1 and P2 need to be adjusted.
sensing plate can be something like 4"x4" piece of metal for example. as mentioned in article, P2 need to be at the middle (~50k). P1 need to be adjusted carefully to get to sweet spot of the circuit. the trimpots used in article seem to be precision type (probably 10 turn). be careful when adjusting, they should have slip clutch at the end of the travel.

 

Please tell me how I can arrive at the sweet spot for the potentiometer..

 

that is the point where circuit responds to proximity of other objects