Simple general use plug and play locator beacon for lost R/C model, transmitter hunting, etc... I know it looks odd because I used an old empty medicine container as a water resistant project box to encase the beacon.
Step 1: Watch the Video.
The video shows a demo of how to locate the beacon, and also the circuit diagram. It's pretty self explanatory.
In case you are on mobile and cannot see the video embedded: Video
Step 2: Acquire the ESP-201 Module
You can buy a ESP-201 on sites such as eBay.com, Gearbest.com, or Aliexpress.com. The shipping can take up to 40 days. Consider using PayPal to stay slightly more secure.
Step 3: Acquire the Battery and Wires.
I just used an old toy quadcopter battery and spliced jumper wires from eBay, but you can buy these on the sites I mentioned in the last step, or salvage them.
Step 4: Assemble!
Follow the wiring diagram and assembly photos from the video!
The wiring is fairly simple, the ESP-201 pins GPIO15 and any GND pin both go to battery negative (ground wire), and any of the 3.3V pins and the CH_PD aka CHIP_EN pin both go to the battery positive wire.
Then just snap the external antenna on.
Step 5: Build the Reflector Can
Wrap a can such as a Pringles can or in my case a corn meal can with aluminum foil using tape. Make sure your phone fits in it snugly.
If you want to try something to maybe, theoretically speaking, increase range significantly, a cheap USB OTG Host Power Splitter like this one: USB OTG SPLITTER and USB power pack like this one: USB POWER BANK, you might be able to use an external WiFi antenna like this one: USB WIFI ANTENNA to locate the beacon from 3 kilometers away or so. I know that the OTG splitter + USB power bank + USB WiFi antenna + Android phone setup does not work with Motorola phones because of driver issues, and the app may not be compatible either, but you could try it if you want. I saw on YouTube a USB WiFi antenna working with the Nexus 5 1st gen. The purpose of the USB power bank is to supply power to the USB Antenna, because the phone cannot supply that much current.