I have a "base station" device that I want to communicate wirelessly with a few sensor stations. I've been trying to find a good type of wireless module to use for this.
I know that bluetooth is supposed to be able to have a master with up to 7 slave devices, but the hc-05 modules seem to only allow one master/slave pair.
The nRF24L01+ modules are cheap and effective and easy to use. Have a look at this Simple nRF24L01+ Tutorial
If your sensor stations (slaves) are awake all the time I suggest that your master unit should poll each of them in turn using the technique in my second example.
If your slaves will need to spend most of the time asleep to save energy then they will need to initiate the messages so the first or third examples would be more suitable. However you will have to plan for what should happen if messages from two (or more) slaves overlap and cause garbage.
It would be possible to operate a system in which the master polls the slaves if the required frequency is not high (say not more than once every 10 seconds) and provided each slave is awake for long enough to hear a message from the master. That should still allow the slaves to sleep for 90% of the time.
WiFi using NodeMCU or WeMOS boards is another option. Your master can set up a network, and the others connect to that specific network and use it to transmit heir data. They can go low power as well - build your own boards based around the ESP8266 processor and you get true deep sleep options.
This will work best if the master can be awake all the time, ready for slaves to connect to and to receive their data.