Here is a device to locate the mobile phone. It emits intermittent flashes and beeps to indicate the presence of an active mobile phone. The circuit becomes active even if the mobile phone is in silent mode. It can be used to detect mobile phone call in noisy environments. Range of the circuit is 15 cm.
The circuit is basically an RF detector. During the activation of mobile phone, strong RF field will be generated. The sensor coil L detects the RF signals and T1 amplifies the signals. The amplified signals are given to the clock input of IC1. CD 4017 is a Johnson decade counter IC with 10 outputs. Its clock input pin 14 is highly sensitive to RF pulses so that it is a very good choice for RF detection. Only two outputs (Q1 andQ2) of the IC are used while the Q3output is tied to the reset pin 15 so that IC will reset on every third pulse.
This will repeat the activation of LED and Buzzer. When the sensor detects the RF signal, clock input of IC1 gets pulses and its output pins 2 and 4 becomes high and low alternately giving flashing lights and beeps. The coil used in the circuit is a ready made inductor.Buzzer used is a small piezo buzzer.