The original video was for a Brother HL-2270DW printer, I have tried it with an HL-2250DN printer and it works as well.
To reset the toner sensor:-
- Open the front cover and leave open while completing the following steps.
- Turn the printer off.
- Hold the ‘go’ button while turning the printer on. All lights should be on.
- Release the ‘go’ button (or “start’ button).
- Press the ‘go’ button (or “start’ button) 2 times.
- Pause. All panel lights should be on.
- Press the ‘go’ button (or “start’ button) 5 times.
The toner light should be off. (error may be flashing)
The paper light should be on or flashing.
- Close cover. The ready light should be the only light on.
Further information here and here