Technical specifications:

It has doorbell function, can remind user to set alarm. Both sides have magnets. Easy to install and change batteries. 

Power supplied 12V(inside battery12V)
Transmission frequency:433MHz±0.5 MHz
Sending distance: No obstacle 80m
static current: ≤10uA
Alarm current:≤30mA(door bell not work) ≤80mA(door bell work)
Alarm distance: 20mm
Work condition: temperature -10~40°C  
Humidity ≤90%