Please find a way to show clearly that the front door is locked when inside the house.
At first the request was for a lamp to illuminate when the door was locked. The plan was to fit a sensor onto one of the bolts which inters the door frame when the lock knob is turned to deadlock the door.
However the type of door was found not to have such a bolt, and the only way to check that the locking knob has been turned was to instrument it in some way. I ruled out placing a sensor within the door mechanism. The knob inside the door rotates more than one full revolution, and also is rotated by the key action outside, and this must not be compromised.
In the end we tried a ribbon, looped round the shank of the existing lock knob, with an elastic chord to hold it in pace across the door. As you turn the knob a marker on the ribbon moves quite a few centimetres, and this is used to reliably show that the door is locked or unlocked and allows multiple rotations of the knob.
It can be removed easily as the ribbon and elastic are held in place by good quality self adhesive hooks. The client is also free to make adjustments or repair.
Our client and her daughter are now free from worry that the door might be left unlocked.