What is the large center hole for in this sensor?  - Page 1
Forum section: electronics / beginners

author: Steve Wayne
author_rank: Contributor
timestamp: November 24, 2015, 05:18:45 am
content: What is the large center hole for in this sensor? Is it just for a fastener? Thanks.

author: Paul Moir
author_rank: Frequent Contributor
timestamp: November 24, 2015, 05:29:24 am
content: Yes.

author: albert22
author_rank: Regular Contributor
timestamp: November 24, 2015, 01:37:19 pm
content: Actually the sensor is the optocoupler, (the black component mounted on the circuit board). The circuit board is just convenient to mount the sensor and a connector.

author: Steve Wayne
author_rank: Contributor
timestamp: November 24, 2015, 07:20:56 pm
content: Quote from: albert22 on November 24, 2015, 01:37:19 pmThe device is a Photo Interrupter sensor. My perplexity is the hole in the center, of what seems to be a rather intricate fashion with the eight holes around the circumference of what appears as a metallic ring or bushing. This would be a pretty fancy screw hole would it not, rather than just a bare hole cut in the center?http://www.ebay.com/itm/2015-New-Keyestudio-Photo-Interrupter-Module-for-Arduino-UNO-R3-MEGA-2560-R3-/252033470243

author: Monkeh
author_rank: Super Contributor
timestamp: November 24, 2015, 07:29:39 pm
content: It's probably just whatever mounting hole footprint the guy had. That's a very normal pattern when you expect the screw to make an electrical connection.

author: Steve Wayne
author_rank: Contributor
timestamp: November 24, 2015, 08:00:09 pm
content: Quote from: Monkeh on November 24, 2015, 07:29:39 pmWhat I'm really looking for is facts, rather than conjecture. There appears (upon personal visual inspection of other similar boards, with a similar hole, from various manufactures) to be no circuit connectivity to the hole, such as with a "via".

author: Steve Wayne
author_rank: Contributor
timestamp: November 24, 2015, 08:05:18 pm
content: Quote from: Paul Moir on November 24, 2015, 05:29:24 amMaybe a longer than one word response is all that's needed here. With your categorical "yes", could you please elaborate a bit on the intricacies of this screw hole? Thanks.

author: Monkeh
author_rank: Super Contributor
timestamp: November 24, 2015, 08:05:33 pm
content: Quote from: Steve Wayne on November 24, 2015, 08:00:09 pmWhy are you asking people who have nothing to do with the design of the board if you're not interested in conjecture? We can't give you facts.It's just a mounting hole.

author: Steve Wayne
author_rank: Contributor
timestamp: November 24, 2015, 08:15:21 pm
content: Now that's a definitive answer. That's all I was looking for. The reason for asking is on the chance that maybe there were some" facts" about this hole that I (as a beginner) wasn't aware of. %^#

author: bitslice
author_rank: Frequent Contributor
timestamp: November 24, 2015, 10:01:50 pm
content: Quote from: Steve Wayne on November 24, 2015, 07:20:56 pmThe eight holes are tin plated through holes, the idea is to connect the ground plane of the top of the PCB to the ground plane of the bottom of the PCB. Think of it as sewing on a button.You could just use one plated hole, but using eight makes for a better contact. And the mounting holes are generally the point where the PCB is connected to the metalwork of a case, thus ensuring a good ground contact throughout the equipment.Sometimes there is no plating, and it relies on the bolt to make the electrical contact from the PCB ground to the case, but such an arrangement is less than adequate.This board probably doesn't need (or even have) a ground plane, but generally when you see mounting holes on PCB's, they all look like this.

author: Steve Wayne
author_rank: Contributor
timestamp: November 24, 2015, 10:23:36 pm
content: Thank you bitslice.

author: SL4P
author_rank: Super Contributor
timestamp: November 25, 2015, 03:39:23 am
content: Don't use that hole for mounting, it will melt your Arduino!Better to use hot-snot, or some 20mm panel-pins for permanent mounting (on a wooden baseboard).If you must use that hole for mounting, use nylon nuts and bolts (with washers) to avoid shorting the top and bottom layers of the PCB.Edit: Sorry, I was feeling mischievous! Youcanuse steel nuts & bolts

author: Ton
author_rank: Regular Contributor
timestamp: November 25, 2015, 10:37:36 am
content: Welljust going with my gut feeling hereFR4 is not a structural strong material, if clamped by a screw it will over time deform i.e. thickness will become smaller and you need to tighten the screw to keep your sensor in place.By adding the extra smaller plated holes around the bigger screw hole you add strength to the hole structure, by adding several small copper bushes.These additional copper bushes you have to expose to a significantly bigger force for them to deform, there by you have a more reliable fastening point, than with a simple non plated hole (or even a simple plated hole).Maybe some of the more mechanically minded can confirm.Ton

author: Maxlor
author_rank: Frequent Contributor
timestamp: November 25, 2015, 01:20:24 pm
content: Quote from: Ton on November 25, 2015, 10:37:36 amPlain holes, sometimes plated, sometimes not, are usedeverywhere. If deformation because of pressure by the screw was an issue, even a rare one, we'd encounter loose screws all the time. But whe don't.

author: PA0PBZ
author_rank: Super Contributor
timestamp: November 25, 2015, 01:42:38 pm
content: Quote from: Steve Wayne on November 24, 2015, 08:05:18 pmOne thing you could have done is google for 'pcb mounting hole' and then click the pictures button. Here is the first result I see:

author: bigsky
author_rank: Regular Contributor
timestamp: November 25, 2015, 04:34:40 pm
content: Quote from: Ton on November 25, 2015, 10:37:36 amSorry but I would totally disagree. If you are correct then virtually every designer has been doing it wrong. Over the years, I've seen hundreds of products, from both professional/industrial and consumer markets, that have PCBs screwed down through plain holes. They don't deform at all. If mounting screws ever work loose it's because of thermal cycling or vibration. FR4 is actually a very strong material (ever tried to break it?).

author: Ton
author_rank: Regular Contributor
timestamp: November 25, 2015, 08:10:11 pm
content: Ok, I stand corrected, as I wrote it was just a gut feeling.Ton

author: Paul Moir
author_rank: Frequent Contributor
timestamp: November 26, 2015, 06:44:02 am
content: Quote from: Steve Wayne on November 24, 2015, 08:05:18 pmThe funny thing is I wrote a long winded reply. Then I decided it wasn't really well written and I should review the whole thing. I reread your question and thought "You dummy, this guy isn't looking for some boring long winded answer, he just wants to know if it's a screw hole as he has already guessed so he can get on with life." So I deleted it all and wrote "Yes, it's just a screw hole." Then I edited it down to "Yes." since the other business was redundant on such a clear question.I dunno: maybe saying something like "Long winded replies from geezers are actually appreciated!"