Bluetooth on/off?

Good morning,
I tried to put a regular check (at the same time that I ask the value of an ldr every 10s) but the text does not change ... any idea?
(the result for the ldr value works)

The DisconnectOnError block does not return whether bt is connected or not. Read the BluetoothClient component documentation.

Dear @olivier_tran,
there are really too few details to tell you a meaningful answer.
Anyway, at a first glance, if you expect an answer immediately after having sent on the BT comm's the character "L", most probably you'll never succeed.
In other words, once having sent the "L" you shall wait for the character to be transmitted, received, processed by the receiving counterpart (an Arduino ? or whatever ? ), then the answer could arrive. This can take some hundreths of milliseconds, so attempting to check whether a BT answer is ready immediately after having sent the "L", could be unsuccessful..
To solve this you can start an additional clock after having sent the "L", to allow 100 milliseconds (hypotesis), in that clock service procedure you can attempt to receive. If successful, you disable the additional clock.
Meilleures salutations.

this might be useful