Hello, I am not sure if this is the correct category for this post, so if not allowed please move to a different category.

I'm trying to make something pretty straightforward, I just don't know how to approach it, the components needed and what a schematic would look like.

Will just tell you what I'm looking to do and if you can help me in any way it would be greatly appreciated.

Essentially I want a Hub in witch multiple Bluetooth devices or "slaves" can connect to the hub simotaniously.

When one of the Bluetooth "slaves" becomes out of its pairing range with the "Bluetooth Hub" I would like for the "slave" to trigger an audible or vibratory alarm on both the "slave" device as well as the Hub device.

I would like to be able to control the amount of Bluetooth range, 5ft, 10ft, 20ft etc.

I'm assuming the "range adj" could be accomplished by adding some type of variable voltage resistor to the Bluetooth slave or by adjusting the frequency?

Any input and help would be greatly appreciated.

 

Consider that while Bluetooth is an unlicensed radio technology it is very certainly quite regulated.So creating a complete package with all of the RF circuits and using it could bring some issues. BUT interfacing with an approved module would be legal, simpler, and cheaper, and probably work better as well. What is required is Bluetooth modules that provide a Received Signal Strength Indicator output that can be calibrated to indicate distance from the transmitter, and associated electronics to convert that voltage signal to distance measurement. The rest is all software.

 

Some of what you are asking for is going to be tricky, but doable in at least an approximation.

If the slave has moved out of range of the hub, how is the slave going to trigger anything on the hub since it is out of range of it?

I don't know a lot about Bluetooth specifics, so take all of this with a grain of salt, but it might help guide your research.

As I understand things, once two Bluetooth devices are paired, if they move out of range of each other, they simply can't communicate any longer. The only way for either of them to know that the connection has been lost is to detect that an expected message wasn't received from the other side. If the two devices are exchanging data sufficiently rapidly, then they could each reset a timer when a message is received and if the timer expires, it would declare the connection lost. Keep in mind that the distance at which this happens will almost certainly be different for both sides of the link. If they aren't exchanging messages rapidly enough, then you can always send 'heartbeat' messages at a sufficient rate.

Controlling the range is going to be very problematic. Even if you have an RSSI value, calibrating that reliably to a distance is far easier said than done. If you mounted the two devices on the end of a ten fool pole and held it in the middle, you would probably see different values just as you rotate the pole up and down or left and right. You would likely get even more variation if you walk around. Even if you just leave everything stationary, you would see some variation over time, particularly as the batteries drain. Then, the RSSI value may be significantly affected by other Bluetooth devices in the area. Even if the values are consistent enough to be usable, you still have to calibrate them and there isn't going to be a one-size-fits-all calibration. Each device will need to go through a calibration process at different distance. You might get away with doing it at some minimum and some maximum distance and then assuming a profile between those two points.

 

Distance detection via RF signal strength is VERY unreliable.

This system is only used when the acceptable range of detection can vary by large margins, or where the antenna geometry is large. (invisible dog fence etc.)

 

The only simple part is detecting the loss of connection. Connection can be monitored by means of an ongoing "handshake" routine, which could even be implemented in hardware.
Other schemes may be possible, but we will need a more detailed description of the application to be able to provide anything better than random guesses.
Possibly a pulsed magnetic field will be a better choice for range measurement, as magnetic fields are much less affected by the dielectric properties of their surroundings.