This circuit can be used to detect the RF signal and electromagnetic noise signal. These RF interference signal may produced by several electrical and induction appliances, if you are designing RF based circuit means you need to protect your design from these noise.
Circuit diagram for RF signal detector
To identify the presence of RF signal noise this circuit helps a lot.
Construction and Working
This circuit constructed with LM386 operational amplifier with transistor pre-amplifier using BC550, if you want to detect high frequency RF noise means you should pre-amplify the noise signal. The speaker connected to the output of op-amp gives sound indication while the circuit detects the RF noise.