A reliable unit which complies with EN 14604.certiified 

 Fitted with test button and flashing red LED. Low Battery warning. 

If it's your first detector, site it on the ceiling away from walls and obstructions, possibly at the top of the stairs, but in any case, where it will wake you when you are asleep.

Test it weekly by briefly pressing the test button.

Always replace a failing battery immediately.

9v DC battery

flashing red alarm indicator

test button

supplied with wall plugs and screws