This app allows you to check remote control radio transmitters- car key fobs, garage door openers and others. You can save time and money by checking it yourself prior to battery replacement or visiting the locksmith.

please follow these 3 simple steps:

install an app;

install a rtl-sdr driver app (free):
https://play.google.com/store/apps/details?id=marto.rtl_tcp_andro

connect your rtl-sdr dongle to the phone and enable OTG-host mode in phone settings menu


Some notes:

rtl-sdr dongle is available at ebay for $8 to $30, please check if it comes with OTG cable.
Cheapest "mini" dongles will not detect 868MHz band. If signal level is false high (urban area and good dongle), use it without antenna.