I just want to turn on a led if the tank is close to be empty.e.g.full -> 60Kg / empty -> 38kgif the weight of the tank is about 10% close to empty, something about 40kg, i'll turn on the led.is pretty simple.
I think I'd go low tech with a limit switch. On my old gas grill the tank was set a little offset on an edge under the end of the grill with a little wire arm that stuck out the front (in an empty/full slot). A spring was attached to the top of the tank pulling it opposite to the natural full tank tilt to the offset side. As the tank empied the spring would gradually pull the tank back from the heavier offset side, moving the fuel indicater wire back toward the empty side of the slot it was in. The tall tank could be set on a piece of wood with a slightly offset ~ 1/8" dowel under it. The full heavy tank would slightly tilt to the offset side. Instead of a spring on the top of the tank pulling it in the opposite direction, an arm with a weight attached to the top of the tank could probably do the same. When the tank got empty the weighted arm would cause the tank to tilt to the weighted arm side, actuating a limit switch. teeter-totter tank.