Digital Inputs

Mechanical Switches come in a wide range of styles to suit a variety of applications. The metal contacts inside the switch can be described as;

Open - In an open state when there is no electrical connection between the switch contacts as they are apart
Closed - In a closed state there is an electrical connection as the two contacts are touching and therefore connected

In a switch where the contacts remain in one state, the contacts can either be normally open (abbreviated "no") until closed by operation of the switch, or normally closed ("nc") and opened by the switch action

A Push To Break (PTB) is an example of a normally closed switch & a Push To Make (PTM) an example of a Normally Open switch

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The moving mechanical switch contact is defined as the 'Pole' and the contacts at it connects with in the switch is defined as the 'throw'. With this in mind there are 4 main types of switches to be aware of -

SPST - Single Pole Single Throw (commonly power/input switch)
SPDT - Single Pole Double Throw (commonly used to switch modes)
DPST - Double Pole Single Throw(two circuits on one switch)
DPDT - Double Pole Double Throw (controls two separate circuits)

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There are a wide range of different switches for a the many applications they could be used. Importantly they all have the same function of controlling the flow of current around a circuit.

A Tilt Switch is a metal tube filled with an inert gas and a sliding metal contact. As the tube is tilted to a specific angle the contact moves along the tube making an electrical connection. Mercury was used as the conductor prior to it's toxicity being realised. Used primarily for movement sensing in alarms, arcade machines and vending.

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A Membrane Switch, unlike other switches, has no mechanical moving parts. Multiple switches can be placed under the membrane by designing the PCB to have open tracks where required. The plastic membrane is flexible and is layered with a number of small metal domes over the open contacts on the PCB. When the user applies pressure to the membrane the dome flexes and makes contact with the PCB connecting the open contacts. Used primarily for control panels as they are highly reliable, low cost, water proof and resistant to fatigue

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A Reed Switch is a magnetic switch. One side is the magnet and the other switch contacts encased in a glass tube. As the magnet comes near the contacts they are attracted together enabling the switch. Typically used in door and window alarms due to the lack of physical contact required to enable them which prevents mechanical fatigue over time.

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A Mircoswitch has a small pole placed under a metal spring lever. As a small amount of pressure is applied to the end of the lever it applies a larger force to the plastic pole of the switch making contact with the throw inside the case. They are incredibly durable and provide a reliable digital signal. Due to the design, they are used to detect small amount of force in printer copiers, CNC, robotics, safety features, machinery and in the automotive industry.

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A Relay is an electromagnetic switch used to interface between circuits with different voltages. When the coil of wire inside the case becomes engergised a magnetic field is created pulling the switch contacts on the other side together, completing the circuit. Importantly, there is no electrical connection between the circuits. Solid State relays also exist to remove the issue of mechanical fatigue that can occur with traditional relays. They use an opto-isolater, which is an infrared LED and photo transistor that switch using light, so require no moving parts.

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Pressure switches (fluid) have an internal rubber diaphragm that when filled with air or liquid, applies pressure to the sprung contacts connecting the switch. They can be set to switch at different levels of pressure and used widely in Hydraulic/Pneumatic equipment.

Mechanical pressureswitches can also be used to detect physical force for use in pressure-sensitive mats that could automatically open doors or set of an alarm.

Reed Switch

ANALOGUE INPUTS

PTM PTB

Mr BILLINGTON

Systems approach to Designing - Input Switches