SPST pin configuration - Page 1
Forum section: electronics / projects

author: Gowda1
author_rank: Contributor
timestamp: January 11, 2021, 01:52:41 pm
content: Hello fam,I ordered a SPST switch (Through hole type) from distributors and also download the cad file for PCB layout from the same website. I am now confused with the pin configuration, in the footprints it has 3 holes. Usually the SPST switch has only 2 pins right?. I have gone through the datasheet but I did not understand it completely about the pins because they have the same diagram for SPST and SPDT I suppose.My question is, if it has 3 pins then what will be the connection if I want to turn ON and OFF the circuit and If it has only 2 pins then which pin should I ignore(delete) in the footprints. I attach the footprints picture and datasheet link with this. Footprints which I downloaded are perfect with the measurements (for 3 Pins). Please let me know if u have any leads on this. Thank youhttps://eu.mouser.com/datasheet/2/221/MS-100406-1174115.pdf

author: Wimberleytech
author_rank: Super Contributor
timestamp: January 11, 2021, 02:05:11 pm
content: Quote from: Gowda1 on January 11, 2021, 01:52:41 pmYup, that is generally the case.QuoteI suggest that you wait for it to arrive...then measure it. It is very possible that it will arrive with only two pins and the third one cut off.

author: Nominal Animal
author_rank: Super Contributor
timestamp: January 11, 2021, 02:12:44 pm
content: The included PDF has an image of a two-pin switch in the upper left, so I would expect the SPST to have just two pins.The missing pin, or the pin to omit, is on the same side as the toggle is when it is ON, because of the internal construction of toggle switches.If you have a footprint│o o o│and you omit the leftmost pin,│ o o│then the switch will be ON in left position, and OFF in right position. If you omit the rightmost pin,│o o │you would just rotate the switch 180 degrees, so that ON position is on the right, and OFF on the left.

author: Gowda1
author_rank: Contributor
timestamp: January 11, 2021, 02:21:19 pm
content: Thank you for the detailed explanation. Even I was confused after looking into top left side picture in the datasheet. I just not wanted to make the mistake on PCB layout. Can i just delete the pin and the footprint of the one pin and continue with my PCB layout? I mean to ask, Are you sure about this

author: tooki
author_rank: Super Contributor
timestamp: January 11, 2021, 02:22:32 pm
content: If you look at the datasheet, you see the first image shows the SPST switch with only two legs. As would be expected, it's the center leg and one of the outer legs.Generally speaking, toggle switches close the contactsoppositethe side the toggle is set to. So I would expect that if you install it with the two legs on the top, for example, the switch will be closed when the toggle is switched down.Here's my ultra-ghetto ASCII art rendition of the cross-section of a toggle switch. The red is the internal bridge between contacts imagine it as an unbroken bar, not two bars).------|o|\ |--|o|\|\\|o | \\----Assuming the switch comes with only two pins, you could always leave the 3-pin footprint and bridge pads 1 and 3, so that you can install the switch in either orientation, since the middle pin will always be the "common".

author: Gowda1
author_rank: Contributor
timestamp: January 11, 2021, 02:42:33 pm
content: Yea totally understood! I can also remove/delete the 1st or 3rd pin footprint right? I can do either ways.

author: Nominal Animal
author_rank: Super Contributor
timestamp: January 11, 2021, 04:13:49 pm
content: Quote from: Gowda1 on January 11, 2021, 02:21:19 pmDo you have to?If you use the three-pin layout, both SPST and SPDT switches will work. If you remove either outer pin, you can only use an SPST switch (unless you can remove the third pin from a SPDT switch).I would keep the three-pin layout, with the third pin not connected to anything.

author: TimFox
author_rank: Super Contributor
timestamp: January 11, 2021, 04:28:57 pm
content: Quote from: tooki on January 11, 2021, 02:22:32 pmI have used many toggle switches, PCB and wired, traditional and miniature, and almost all have the pinout orientation with respect to lever position described by tooki.However, there have been a few surprises so I now do a quick continuity check before installation.

author: Gowda1
author_rank: Contributor
timestamp: January 12, 2021, 12:13:17 pm
content: Lets say I consider pin 1 and 2, In this case do i need to worry about the configuration? for example pin 1 should be connected to supply and pin 2 should be connected to any device (relay or bulb). Or does it work either way. Please let me know

author: TimFox
author_rank: Super Contributor
timestamp: January 12, 2021, 02:53:59 pm
content: It depends on if you need to know the switch action with respect to the lever position.

author: wizard69
author_rank: Super Contributor
timestamp: January 16, 2021, 07:16:53 am
content: Quote from: Gowda1 on January 11, 2021, 02:42:33 pmIf your switch comes in with three pins you can't delete anything You still will need to have the board drilled for the extra pin and frankly soldering in that extra pin is good for mechanical stability.In fact I wouldn't be surprised if the extra pin is left in place for that mechanical stability. However you could also be misinterpreting the data sheet, seeing a picture for a SPDT switch. In most examples I've seen a SPST switch comes with two pins but that isn't law.