Switch pain - Page 1
Forum section: electronics / projects

author: PerranOak
author_rank: Frequent Contributor
timestamp: June 10, 2019, 04:46:45 pm
content: Switches and buttons give me more pain than any other component!I have a microcontroller pin that has a pull-up resistor. All I need is a simple slide switch (PCB mounted, through-hole, about 8mm x 4mm ish) for the user to take this pin low while other things are selected then to return it to the pull-up situation.So is this an SPDT, SPST, this, that or what!?!? Argh!Any help, gratefully received.

author: Buriedcode
author_rank: Super Contributor
timestamp: June 10, 2019, 04:56:26 pm
content: The P is the number of poles - how many actual switches are inside the switch either a number of S for single, D for dual, and T is toggle - how many ways each individual switch can be switched. Eg DP4T has two separate switch, each common can connect to 4 nodes.If you just need to pull a pin low/high then you only need one toggle, and one pole - SPST. But you can of course use SPDT as you just need the NC (normally closed) unconnected. And you can use DPDT and just leave one whole switch unconnected.In my experience, you narrow down the search first by type (slide, toggle, rocker etc..) then by termination (PCB, solder lug, tab, screw terminal).

author: MarkF
author_rank: Super Contributor
timestamp: June 10, 2019, 05:07:56 pm
content: Maybe this will help:

author: soldar
author_rank: Super Contributor
timestamp: June 10, 2019, 05:16:23 pm
content: Quote from: PerranOak on June 10, 2019, 04:46:45 pmNot being American I always found the pole-throw nomenclature kind of silly.If you are using the switch with a microcontroller you want to be very careful with debouncing.

author: Gyro
author_rank: Super Contributor
timestamp: June 10, 2019, 05:56:38 pm
content: Quote from: Buriedcode on June 10, 2019, 04:56:26 pmThe T actually stands for throw, not toggle, as in 'throw the switch'.

author: KL27x
author_rank: Super Contributor
timestamp: June 10, 2019, 08:02:18 pm
content: QuoteIn my personal experience, being American doesn't help. I have never been able to remember pole from throw. It seems backwards, if anything. Do you want to throw 1 switch at a time or 2 or 3? How many poles do you want to connect with each throw? Totally backwards, if I try to make it sensible.QuoteWell, I might actually remember it like that. I like toggle.Humm. I like P for pins (like ribbon cables; 10P is 10 parallel "connections"), T for toggles.

author: ajb
author_rank: Super Contributor
timestamp: June 10, 2019, 08:27:18 pm
content: Quote from: soldar on June 10, 2019, 05:16:23 pmAs opposed to what, the Form A/B/C/D naming?

author: soldar
author_rank: Super Contributor
timestamp: June 10, 2019, 08:58:50 pm
content: In Spanish:SPST is just a breaker or interruptorSPDT is a "conmutator" or switchAnd the "poles" are "circuits".A "3 circuit breaker" breaks 3 circuits, like for a three phase motor.A 3 circuit switch would be a 3 pole double throwIf you have more than two "throws" then you talk about "positions".A "one circuit 10 position" switch allows one input to be connected to any one of 10 outputs.A "2 circuit, 10 position" is the same but duplicated, with two circuits.In other words, "circuits" are the number of inputs and "positions" are the number of outputs for each input.

author: ArthurDent
author_rank: Super Contributor
timestamp: June 10, 2019, 10:14:42 pm
content: It helps if you know the evolution of switches. From around 1900 on, the ‘knife’ switch was very common and the name came from the moveable arm of the switch resembling a knife blade. To connect this contact to complete the circuit you had to move or ‘throw’ the blade so it slid between the two halves of the fixed contact. Anyone who has watched an old Frankenstein horror movie or a gangster movie from the 1930s where the villain gets the electric chair has seen them ‘throw the switch’. The photo below is an example of a knife switch. Some of the terms from these days carried over so that is why you when you activate a modern small switch it is still referred to as ‘throwing the switch’ or with switches getting smaller and smaller, ‘flipping’ the switch.Each separate circuit is referred to as a ‘pole’ and SPST or 1P1T are used interchangeably but when you start talking about center off double throw switches it is easier to refer to them as ON-OFF-ON. As mentioned above, once you get to multi contact rotary switches they have a certain number of of decks, and poles, and a number of positions, not throws.Conventional terms can be confusing as when you record a video and use terms like ‘film’ or ‘tape’, when neither is involved, or for that matter, there’s no record!

author: soldar
author_rank: Super Contributor
timestamp: June 10, 2019, 10:34:50 pm
content: Quote from: ArthurDent on June 10, 2019, 10:14:42 pmTo "record" means to register some information on any permanent media so it can be reproduced and consulted at a later time. Records of laws or other information were kept on clay tablets, papyrus, parchment, paper and now electronic media.Etymology: from Old French record ‘remembrance’, from recorder ‘bring to remembrance’, from Latin recordari ‘remember’, based on cor, cord- ‘heart’. The noun was earliest used in law to denote the fact of being written down as evidence.So yes, "record" is correct.

author: ArthurDent
author_rank: Super Contributor
timestamp: June 10, 2019, 11:34:53 pm
content: QuoteSorry that my humor doesn't translate well. The verb 'record' is correct but my reference was to the noun, 'record', a physical disk with music or whatever recorded on the record.

author: PerranOak
author_rank: Frequent Contributor
timestamp: June 11, 2019, 03:34:44 pm
content: Thank you all very much.So, I need an SPST.I've got the de-bouncing in software: I love doing that kind of programming.Out of interest, in the diagram I get why SPST has "on-off" but why does the SPDT have "on-off-on" and "on-on"?

author: soldar
author_rank: Super Contributor
timestamp: June 11, 2019, 04:00:30 pm
content: Quote from: PerranOak on June 11, 2019, 03:34:44 pmSome do, some don't. Some have two positions, On-On, and other have three with a neutral stop in the center, On-Off-On.