what type of sensor should be used???

Actually im using a pnp type of metal sensor, so that whenever the sensor senses a metal object,a digital timer circuit connected to it starts running.

But what i want is that, the digital timer should be started whenever the sensor is not sensing.even i connected a npn type of metal sensor to achieve the above,but im not successful.......

is there any other sensor/method to solve the above problem????

insufficient data ....
What voltage level is required to initiate the timer?

If it is just +5 VDC, you might be able to run the PNP sensor to a TTL type inverter gate, with the output of the gate going to the timer. Also, it may be advisable to add a pull down resistor to the input of the gate, in order to insure a positive gate level, one that switches distinctly.

If you could include an annotated sketch or circuit diagram of your problem and idea, it might be helpful.