Door Sensor Installation
These sensors can be tricky to install because the magnet on the sensor needs to line up with the magnet in the contact. Before deploying to a customer, it's a good idea to apply stickers to the edges that should match up. That way, if the sensor or contact ever needs to be remounted, it will be obvious to the customer which way it goes.
To determine the location of the battery inside the sensor, you may need to open the cover of the sensor and have a look. You should see a little glass tube looking thing. That's the magnetic switch that should be facing the magnet on the door.
The adhesive that comes with the sensor may not hold for more than a year or so. On medication dispensers, a higher grade adhesive, such as super glue, is a better choice. On doors, windows, or cabinets, if it's acceptable to the client, use screws rather than the adhesive.