Question

We would like to connect sixteen vibrators to an Android phone using Bluetooth, and control the vibrators individually.

As far as I know, you can only have eight devices in a piconet, so that would place a limit of seven vibrators (the phone itself being the eighth device). First of all: Is that correct?

And do up to seven connected devices work well and reliably in Android? Or is there some additional limit or problems from Android’s Bluetooth implementation or APIs?

For our sixteen vibrators, will we have to build a scatternet with additional devices that bridge between the phone’s piconet and additional piconets with some of the vibrators? Does anyone have experience with this, and does it work well?

(And no, it’s not a sex toy!)

Answer 1

As far as I know, you can only have eight devices in a piconet, so
that would place a limit of seven vibrators (the phone itself being
the eighth device). First of all: Is that correct?

Ok to be technically precise – Bluetooth Classic can connect and be in active connection with upto 7 devices
at a time. But then an active device can then be put in park mode and it can have a large number of device in park modes, so device can be moved to park from the connected – active state and vice versa.
But again at any one point you can have only 7 active devices So the master device should manage a large number of devices by keeping (unto 7 ) active and rest parked and keep switching them between active and parked modes.

And do up to seven connected devices work well and reliably in
Android? Or is there some additional limit or problems from Android’s
Bluetooth implementation or APIs?

Well in Android the problem is – There is no one implementation and many different bluetooth Radio hardware gets used by different manufacturers. So the answer is it depends. Some are pretty reliable Some are really bad.
But there are no public APIs to control / use the Park mode that I described above – But if you can operate on the internals or have access to it from your app you could do what you are asking for,

On Scatternet :

Again Android does not have any API for you to control it, It will be complicated – but your could force it into a scatternet configuration, but again there are limits – the best I have seen in commercial devices is for a device to be in 2 or 3 piconets at the same time, Which means you can be connected to (7+2) 9 devices at a time (it does not meet your requirement of 16).

Bridging / Mesh configuration may be feasible – Where 2 of your devices form their own piconetcs i.e with 8 devices in each group then the leader of the group (Master) connects to Android deevice – and you manage the data relay at the application.

Now having said all this – have you looked at Bluetooth Low Energy – A perfect candidate to conenct a bunch of sensor devices – Ther is no theoritical limit on the number of devices that can be connected at a time – But practically 16 or even larger is very feasible.
Android currently does not have public APIs for it . (As of Today)
But most (almost all) latest adroid devices comes with Bluetooth Hardware that is Version 4.0 meaning it is capable of Bluetooth Low Energy.
And iOS devices – Mac, iPhone , iPad has great support and developer access / apis for it.
So it will be the way to go, and I am pretty Sure Android will come with developer APIs soon for BLE (atleast I hope so)