Copyright © A.F.Billington 2018. All rights reserved.

Reed Switch 

PTM                          PTB

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)


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. 


A Relay is an electromagnetic switch used to interface between circuits with different voltages. When the coil of wire inside the case becomes engergise a magnetic field is created pulling the switch contacts on the other side together. There is no electrical connection between the circuits.









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Switch Bounce occurs when the metal contacts inside a mechanical switchvibrate against each other as they come together creating a 'dirty pulse'. All switches ideally produce a completely Digital square wave when in operation and if not then a IC or PIC (genie) could confuse the signal. 


Typically there are two ways to eliminate switch bounce and 'clean' the pulse, this is know as de-bouncing. Firstly a 555 timer in monostable mode can be used with a very short time period.


Alternatively a Schmitt Trigger, which is a special time of logic gate, can be used with a capacitor which discharges to de-bounce the signal. 


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An Opto-Isolator is a component that allow the switching between circuits with differing voltages much like a Relay. Encased in a 4/6/8 Pin DIL is a infrared LED and an infrared phototransistor separated by a small gap. When the LED is on the phototransistor conducts allowing the attached circuit to become active. As with a relay, no electrical connection is made between the two


Advantages over a electromagnetic relay -

  • Incredibly fast switching (speed of light) compared to relay
  • Low voltage/current required - higher efficiency than relay
  • No fatigue (wear) as there are no moving parts 


Disadvantage is that they are not suitable for high current switching ​so are only suitable for low voltage electronics such as computing




Slide Switch             Rotary Switch       Membrane switch            Key Switch                    Microswitch

​​​​​​Mr BILLINGTON

 Systems approach to Designing - Input Switches

Tilt Switch