SMT battery pull tap switch - Page 1
Forum section: electronics / general

author: Psi
author_rank: Super Contributor
timestamp: April 26, 2016, 06:11:58 am
content: Has anyone come across a very basic cheap switch intended for battery pull taps (to keep the power disconnected until the user pulls some plastic tab out).Since this only needs to support 1 cycle all that's needed is a two layer metal plate 0.5mm apart with gold plated spring metal contacts built in.Needs to be available in reels for SMT placement.If you're not entirely sure what I mean think about a singing birthday card where a paper insert is pulled in/out of some contacts to activate the card.

author: Zbig
author_rank: Frequent Contributor
timestamp: April 26, 2016, 12:19:37 pm
content: From my experience, every product that needed "arming" like that I encountered, didn't use any switch at all. The pull tab was just stuck between the battery contact and the battery, isolating them electrically.

author: rrinker
author_rank: Super Contributor
timestamp: April 26, 2016, 12:23:18 pm
content: I've pretty much only seen that in toys, but yes, the 'arming strip' is always just a piece of plastic stuck between the battery and the contact, or if the device uses 2 button cells, sometimes between the two cells. I've seen some on devices that use a pair of AA or AAA cells, usually stuck between the positive of one battery and the contact in the battery box. I doubt any of this is machine placed.

author: Psi
author_rank: Super Contributor
timestamp: April 26, 2016, 12:27:28 pm
content: Unfortunately we need to use tabbed coin cells that are TH soldered. So there is no place to insert the tab.

author: Kjelt
author_rank: Super Contributor
timestamp: April 26, 2016, 01:29:25 pm
content: never saw those.You could (not sure about the price but very common) use a small jack with switchable inlet (like headphone jacks but smaller) place it further down the product so no user can insert a normal jack in it and place a long plastick rod inside the jack sticking out of the product, user pulls the rod and the jackswitch is made. Something like that. Other option would be to place another coincell holder with a piece of metal in it and the plastick strip isolating that one. You actually pay double the bom price, why not use a coincell holder instead of a fixed th battery? No pun but this is part of the requirements for the design and the BOM, so if this requirement was there the designer should already have found a solution or would have to demand another battery solution.

author: ajb
author_rank: Super Contributor
timestamp: April 26, 2016, 08:09:20 pm
content: Maybe a normally-closed pushbutton, with the tab between the plunger and a housing feature?

author: ConKbot
author_rank: Super Contributor
timestamp: April 27, 2016, 03:54:44 am
content: https://e2e.ti.com/blogs_/b/powerhouse/archive/2014/06/05/maximize-shelf-life-with-a-one-time-push-button-switchWould either of the solutions shown there work for you?

author: Psi
author_rank: Super Contributor
timestamp: April 27, 2016, 08:54:08 am
content: Unfortunately no, it has to be a full disconnect.Ideally we want a tiny 3x3 part that can be SMT placed onto the board. Then a 0.2mm plastic shim can be inserted into a gap in the part to break the contacts.

author: ajb
author_rank: Super Contributor
timestamp: April 27, 2016, 11:32:37 am
content: Quote from: blueskull on April 27, 2016, 11:07:45 amThis would have to be hand-soldered in order to provide a useful amount of spring force at the contact point. If you just plop the part on the board it's going to sit at whatever angle gravity dictates and the spring force at the contact is going to be only a fraction of the part's weight. That may be adequate while it's sitting on the bench, but provides no margin for vibration or shock. With clever design and careful process control you might get some additional force from the solder pulling the solderable end down to the board when it flows, but probably not much. Really what you need is to hold the part down in a stressed position during soldering so that you get enough spring force at the contact.

author: thm_w
author_rank: Super Contributor
timestamp: April 27, 2016, 08:41:43 pm
content: QuoteWhy?QuoteAny reasons why a SMD dip switch or miniature slide switch could not be used?A reed switch and magnet is a classic solution, but is a bit larger than 3x3.A NC SMD switch can be used on the side of the board, and a clip could be placed on it to hold it open.