rat tech page (2)
although springs can be used to close doors automatically, it is easier to
control the speed and force of closure by using the pulley principle.
the mass of the counterweight can be adjusted to produce just the right
touch for cabinet doors, entry doors, or fence gates
nylon string, small eyescrews, and any sort of weight may be used to construct
this simple yet useful device.
in addition, it is easy to add indicators if you need to know whether the
door or gate is being opened in a remote location. a chime can be made from
two pieces of metal (many old car parts make a pleasing yet substantial tone)
an indicator lamp actuated by a mercury switch is another option.
When the weight is raised by the opening door, the mercury switch is tilted to
the on position, actuating an indicator light or buzzer.