Electronic – detecting passing vehicles

Depending on the type of your door and key, you might have some success with inductive measurement from the inside. If your door, lock, and cylinder are mostly diamagnetic, and the key is ferromagnetic , you can measure a change in inductance of an inductor mounted on the inside of the lock when a ferromagnetic key is inserted. This should work for paramagnetic keys as well.

If your key is ferromagnetic, you might successfully magnetise the key and measure the inducted electromagnetic field when the key is inserted.

If the door cylinder is useable from both sides, you might insert a pin into the lock from the inside that will be moved when a key is inserted on the outside. Not a good idea because it might possibly damage your lock, though.

A totally different way would be to force people to put their keys on a specific hook, and alert if the key is not there.

Or make the door keyless.

A few ideas come to mind:

• Your tank is polyethylene. You should be able to sense liquid level through the wall of the tank using your capacitive sensing. If that doesn't work it won't work in a PVC pipe. Advantages: no wireless, no battery, no retrieving the unit out of the tank ...
• I'd consider an ultrasonic level sensor. These are used in all sorts of industrial applications, quite reliable and can go in the top of the tank where it shouldn't get dirty. If it does get dirty you may have problems. Advantages: simple, analog output for level display, easily retrieved for adjustment, etc.

I've never considered the matter, but is there any risk of methane buildup making the airspace an explosive atmosphere? If so you need to take appropriate measures with any electrical devices inside the tank. From memory ATEX equipment is limited to 10 V or so at 8 mA. (Someone will jump in with the correct numbers.) This is low enough to prevent a spark in any circumstances.