Detecting PCB edge connector - Page 1
Forum section: electronics / projects

author: tdarlic
author_rank: Contributor
timestamp: January 13, 2017, 07:32:15 pm
content: Hi guysI'm developing a small sensor (cca 80x40mm) that is installed on site to collect data. The board is enclosed in casing and should have a connector for charging and downloading data, reading status etc. On the previous spins we had a power switch, connector for power and connector for data/programming. I'm developing a new version and I want to get rid of this and have only one connector exposed. My idea is to have a PCB edge connector with 12 contacts (6 on each side). The connector would be symmetrical so it doesn't matter which way it is plugged. The MCU would detect that connector is plugged and stop collecting data and do it's thing. Also the PCB edge connectors would be mounted on a big PCB where you could plug like 50 of them and charge/program at the same time which is a hassle now. The device would be idle until unplugged at which point it would start working until plugged again or until battery runs out. On design we have now the problem is dirt getting into the connectors, guys on the field forgetting to turn them on and such.Now, I'm concerned that touching the connector pads or getting them wet would somehow trigger the connection mode on the MCU and stop data connection. To prevent that I would connect several pins together on a connector as on schematic below. The MCU would periodically switch on the voltage on one pin and check is that voltage appearing on other pins that are shorted by connector (represented as a blue line between connectors). I was also thinking about pulsing the pin and detecting that frequency on other pins to be sure.This would reduce the BOM cost significantly, reduce the complexity of CNC for the case, solve the problem of charging multiple devices and probably cure cancer and bring world peace. But...I'd like to have your input, is this viable thing to do? Would detect connector differently or abandon this alltogether?

author: Gribo
author_rank: Frequent Contributor
timestamp: January 16, 2017, 12:45:55 am
content: That is a good solution. One more thing you might wish to consider for an edge connector is to make the GND pads longer than the rest of the pads.

author: texaspyro
author_rank: Super Contributor
timestamp: January 16, 2017, 02:30:40 am
content: Edge connector are horrible, unreliable things for applications that require more than a few insertion cycles... use a real connector... even a DB-9 is a LOT better of an idea.When I had my TM500 and HP-5370 extender cards built, I checked what it would cost for "hard gold" edge tabs (instead of ENIG gold flash). Setup charges were well over $500 and the quoted costs of the boards went from a few bucks to over $60 per board.

author: janekm
author_rank: Unknown
timestamp: January 16, 2017, 05:26:19 am
content: Quote from: Gribo on January 16, 2017, 12:45:55 amThat, and perhaps making the pads used for detecting card presenceshorterso that the board only tries to use the edge connector once it is fully plugged in. Though I do agree with texaspyro about the reliability and cost considerations (the cost may not be quite as extreme when getting quotes from a factory that regularly processes hard gold edge tabs, rather than having to set up for it specifically, but it won't be as cheap as regular PCBs for sure).Finding a good connector for this kind of application isn't quite easy though, especially if DB-9 and the like would be considered heavy.Abusing USB connectors can be a least-bad solution if there's no likelihood of confusion by the end-user.

author: @rt
author_rank: Super Contributor
timestamp: January 16, 2017, 06:30:30 am
content: The device I’m looking at now (not my design) has a pin normally tied high with a resistor, and the inserted card will hold it low.That looks to all for the hardware side. But they could then go ahead and verify some 0xFFFF values to see if the data bus is ok.

author: MarkF
author_rank: Super Contributor
timestamp: January 16, 2017, 06:46:44 am
content: What about a RJ11 modular phone jack or a RJ45 network jack? Depending on your environmental requirements.

author: tdarlic
author_rank: Contributor
timestamp: January 16, 2017, 08:50:48 am
content: Thanks guys for valuable input.This connector would have approximately 10 to 20 mating in his lifetime. I'll spin 10 test boards and and see the results. I'd like the edge connector because this will eliminate the need to CNC the case. I have misused the USB connectors before but the place they are deployed is full of dust so they also get dirty and you need to blow them with compressed air before connecting.I'll make the contacts for detecting the presence of connector shorter and the GND a bit longer and I'll also test the rest of the pins for voltage just to make sure something is not shorting the connector before I start communicating.