I bought a Bluetooth connected Relay and controlled it via an Android App. The app was created from an example provided as part of the Anywhere Software Basic4Android (B4A and B4X) IDE.

The tricky part is knowing what and how to send the byte string to the relay once a bluetooth connection has been established. Once you have had success using this example you can write your own app and control the relay how you want.

The Basic4Android package from Anywhere Software is excellent and it supports iOS programming too.