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Ttelmah
Joined: 11 Mar 2010
Posts: 19538
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 Posted: Mon Mar 06, 2017 9:11 am |
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I was just about to add a self resetting fuse to my suggestion. 
Even nicer, these now can be tiny. Little 1206 SMD parts, rated to 'fuse' at say 200mA. |
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starfire151
Joined: 01 Apr 2007
Posts: 195
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| Posted: Mon Mar 06, 2017 9:14 am |
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For I2C isolation, Analog Devices makes the ADuM1250/ADuM1251 Hot Swappable Dual I2C Isolator chip.
As before, this is just for your information... |
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newguy
Joined: 24 Jun 2004
Posts: 1909
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| Posted: Mon Mar 06, 2017 9:28 am |
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| Ttelmah wrote: | I was just about to add a self resetting fuse to my suggestion.
Even nicer, these now can be tiny. Little 1206 SMD parts, rated to 'fuse' at say 200mA. |
And if you require a smaller current limit, these fuses get tinier yet. I've used a variation of this protection network (plus resettable fuses) to protect a pair of RS485 networks bundled with 24V and 55V dc supplies. I remember getting field support requests basically saying the comm link dropped but cycling the power brought it back. The field tech suspected a certain board being the issue.
Me: "The cable is bad - one or both of the supplies shorted to a comm line."
Them: "No, it's not bad."
Me: "Yes it is - replace it soon."
Them: "Well a bulldozer ran over it but it's fine."
Me: "(sigh) No it's not, replace it."
Then, a day later....
Them: "I had trouble with comm dropping twice again yesterday so this morning I swapped that cable and so far it's been good."
Gee, I wonder why..... |
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temtronic
Joined: 01 Jul 2010
Posts: 9243
Location: Greensville,Ontario
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| Posted: Mon Mar 06, 2017 12:55 pm |
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re: "sacrificial" op-amp... good giggle over that ! I socketed 1488s and 1489s for the same purpose.
Every once in awhile some guy with a backhoe would 'oospy' the Belltell cables. Funny thing is Bell didn't KNOW about it until I told them someone took out a pole between adrs A and adrs B. My single wire system was far better than their computer...
re: protection. Be sure to consult specs for PCB track spacing.....
Jay |
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ELCouz
Joined: 18 Jul 2007
Posts: 427
Location: Montreal,Quebec
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| Posted: Tue Mar 07, 2017 5:36 pm |
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| Quote: | | Have you looked at the Tindie I2C soil moisture sensor (www.tindie.com)? |
Most likely going this route. But even simpler than that, he left the AVR ICSP accessible on the PCB. Firmware is open-source, in fact the whole thing is 
I'm gonna switch from the I2C to Serial on TTL instead (low baud as suggested)
Regarding the protection, can I apply the same suggestions (for ADC,analog) to a TTL signal?
Thanks again!!!
_________________
Regards,
Laurent
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Here's my first visual theme for the CCS C Compiler. Enjoy! |
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newguy
Joined: 24 Jun 2004
Posts: 1909
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| Posted: Tue Mar 07, 2017 5:58 pm |
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Yes, the same principles apply. But design the LPF to have a cutoff freq perhaps at least 20x your baud rate so that the digital bits aren't rounded too badly by the filter. That said, you should be able to find a suitable buffer IC rated for ESD (for your receiver/master board of course). Your transmitters (the soil moisture sensors) will still be susceptible to ESD unless the manufacturer "hardened" them in some way. Given the picture and their cost, I'd rather doubt they have any protection. It should be relatively simple to solder transient voltage suppressors (TVS) across all relevant lines to GND. All relevant lines = any lines leaving the sensor and running back to your master. Use bidirectional TVS.
Be sure to include a resettable fuse on each power line going to each sensor (on your master).
One more thing: try to solder a diode across the power input to the soil sensors to protect against reverse polarity. |
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temtronic
Joined: 01 Jul 2010
Posts: 9243
Location: Greensville,Ontario
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| Posted: Tue Mar 07, 2017 8:11 pm |
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You can probably buy some 'hardened' RS485 transceivers these days. Could be the cheapest, easiest solution. Add a MOV and NE-2 .....
Jay |
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newguy
Joined: 24 Jun 2004
Posts: 1909
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| Posted: Wed Mar 08, 2017 7:00 am |
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| The issue with something like a RS485 transceiver is that one needs to be added to the commercial board in the picture. Not easy and not exactly "ready for the garden" kind of quality. |
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temtronic
Joined: 01 Jul 2010
Posts: 9243
Location: Greensville,Ontario
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| Posted: Wed Mar 08, 2017 7:15 am |
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Well, he does have to somehow connect the big, clunky XLR cables !
Details...it's always those pesky 'little things' that have to be worked out.
For me, I'd build a cable to sensor 'interface' PCB so that a bad sensor could be replaced or a fried 'interface' PCB.
500ml water bottles are great rain shedders btw, cut bottom out, slide onto device.
Jay |
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ELCouz
Joined: 18 Jul 2007
Posts: 427
Location: Montreal,Quebec
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| Posted: Wed Mar 08, 2017 4:18 pm |
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| temtronic wrote: | Well, he does have to somehow connect the big, clunky XLR cables !
Details...it's always those pesky 'little things' that have to be worked out.
For me, I'd build a cable to sensor 'interface' PCB so that a bad sensor could be replaced or a fried 'interface' PCB.
500ml water bottles are great rain shedders btw, cut bottom out, slide onto device.
Jay |
Everything will be potted (pcb epoxy potting of course) because the sensor will be underground not on the surface. I need to read deeper than the surface.
This will be one shot deal if it goes bad...down the trash the sensor will go (they are cheap, and I will have extras)
Well tough luck the XLR is not rated for UV, this will give me trouble. I found some nice instrumentation cable spool 4 conductors - twisted pair/18awg/shielded... IP67 and UV resistant.
I know this is getting out of topic (I do feel bad ) but while we are at it...does the twisted pair will affect the data if the single DATA wire is twisted with GND or 5V? I know it is used mainly for differential signaling which will be NOT the case.
_________________
Regards,
Laurent
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Here's my first visual theme for the CCS C Compiler. Enjoy! |
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newguy
Joined: 24 Jun 2004
Posts: 1909
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| Posted: Wed Mar 08, 2017 5:35 pm |
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| Not for the speeds you'll be utilizing. |
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ELCouz
Joined: 18 Jul 2007
Posts: 427
Location: Montreal,Quebec
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temtronic
Joined: 01 Jul 2010
Posts: 9243
Location: Greensville,Ontario
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| Posted: Wed Mar 08, 2017 5:36 pm |
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With 4C, 2 twisted pairs..
I think the best configuration could be GND and +5 as 1 twisted pair, GND and data as the other twisted pair.
This config does allow for future RS485 use; Gnd and +5, A and B
You might also consider CAT5 cables.If long enough they already have gold plated connectors! 4 pairs give several 'options' in network design.
Design is like chess, trying to think 3-4,20 steps ahead....
Jay |
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newguy
Joined: 24 Jun 2004
Posts: 1909
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| Posted: Wed Mar 08, 2017 5:53 pm |
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| temtronic wrote: | You might also consider CAT5 cables.If long enough they already have gold plated connectors! 4 pairs give several 'options' in network design.
Design is like chess, trying to think 3-4,20 steps ahead....
Jay |
I second this. If you design a PCB with a RJ45 connector on one side and a header to solder directly to your sensor on the other, things get simpler. No soldering wires directly to the board. You could even put two RJ45 connectors on each board to allow for daisy-chaining. Makes field (garden) deployment much easier and neater.
OSH Park is a quality supplier of cheap PCBs: https://oshpark.com/ |
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ELCouz
Joined: 18 Jul 2007
Posts: 427
Location: Montreal,Quebec
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| Posted: Thu Mar 09, 2017 2:19 am |
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I understand clearly guys but in-soil application 1 ft deep underground (don't forget soil is corrosive) makes it harder to find a liquid-tight 8PC8 connector. Also CAT5E for direct burial cable with gel is expensive and less flexible than "rubberized" instrumentation cable (SJOOW type).
Trust me, I've done research on the cable and connectors the cheapest yet resistant for a garden solution would be soldered to potted pcb sensor on one side and using M12-4pins "aviator type" connector on the waterproof junction box where the DsPIC board will be.
These connector are 10 sets for 10$, splash proof but not waterproof so I will need to protect & orient the connector so water don't seep in these.
_________________
Regards,
Laurent
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Here's my first visual theme for the CCS C Compiler. Enjoy! |
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