Good afternoon,
Cellphones emit a frequency that can be picked up by a stereo receiver, sometimes if you get your cellphone close to a speaker you can hear the feedback in the form of beeps and buzzing. I can use this effect to create a device to find a cellphone using a receiver and an amplifier, small cheap and effective. As for if a cell phone is off and you are trying to find it that will be different as it would be emitting nothing to pick up on and would be rather difficult for the price range. I can make the previously mentioned design ( receiver and amplifier ) in budget no problem. Please let me know if you have any questions or comments or would like to get to work
Thank you,
Baslr Engineering