Limit Switches Limit Switches are the most numerous input devices in an elevator. They tell the system the position of gates, doors, turnheads, or distributors, and the alignment of belts. A switch can only tell whether an object is present or absent at a certain location. A pair of switches can tell if a gate is completely closed or fully open, but not where the gate is if it is somewhere between those two limits. A limit switch can be operated by mechanical contact, by breaking a beam of light, or by detecting the disturbance of a magnetic field caused by a metal object. The following diagram shows a mechanical limit switch wired to a pair of indicator lights. It has an actuator arm which operates the switch contacts when it is moved slightly by contact with a piece of equipment such as a slide gate. The arm is spring-loaded, so it returns from its actuated position to its normal position when the equipment moves away. In the diagram, you can operate the limit switch by moving the arm with a mouse. A limit switch has at least two sets of contacts: "Normally Closed" (NC) and "Normally Open" (NO) . When the switch is in the normal position (nothing touching the arm), the Normally Closed contacts pass electricity, so the diagram shows the Normally Closed indicator light as lit. When the switch is in the actuated position, the Normally Open (NO) contacts pass electricity, so the Normally Open indicator light is lit. The "Common" wire carries electricity to both sets of contacts. The terminology of switches is the reverse of the terminology of gates: Switch contacts let electricity pass when they are closed. A gate lets grain pass when it is open. If you have a browser for Virtual Reality Modeling Language (VRML), such as Cosmo Player from Silicon Graphics, you can operate an interactive 3-D model of the gates and limit switches under a shipping bin. VRML browsers normally require Windows 95 or Windows NT. ----- Original Message ----- From: VK4UTE To: seqpg@xxxxxxxxxxxxx Sent: Tuesday, April 28, 2009 8:55 PM Subject: [seqpg] Thoughts for a mates project Hi All Here's one for the think tank..... One of the guys a work is building a house [and on a very steep hill by the sounds of it.] He wants to install a 3 stop dumb waiter from the kitchen (Top floor) via the laundry (gnd floor) to the Garage. (basement) for shopping. I believe the original plan had the void as a laundry chute and he has just reversed the access from the linen press to the pantry so it can be used for groceries as well as clothes. He is thinking about a gate controller and motor (The sliding kind) as it has already got stops for full open, full closed and partial open option for pedestrian access. I was thinking about the cheap winches from the automotive shops and a PLC (programmable logic controller) as some year ago I saw a developer board that had reversible high amperage mosfet motor control plus switch and sensor inputs. Plus this has some software so it could be programmed by drawing a flow chart. NOTE: A real electric dumb waiter set-up would cost more than hiring a teenager to carry the shopping upstairs weekly for about 7 years. Cheers Rod