AK-DS2482S - not seeing some devices

Support
#1

Hi, we’ve purchased two AK-DS2482S modules and are having difficulty getting them to work with all of our sensors.

We have 7 DS18B20 sensors buried in the concrete floor of our new house. The lengths of the sensor wires range between 6m and 30m. Initially we connected them directly to a Raspberry Pi 3B using the 1-wire interface but found that we could only connect 3 at once; when we connected more than 3, all the sensors disappeared. We also connected them one at a time to check that they all worked, and they do.

Researching the problem suggested that the lengths of the wires could be the problem, and various links led to your AK-DS2482S-100 board, so we bought a couple. However, we are still finding that we cannot get more than 3 connected and working.

We have connected all the solder jumpers on the board, including SPU. We have both I2C and 1-Wire enabled on the Pi.

Please can you help?

#2

Hello @mirandapoth,

Please refer to the following guide for a general grasp on one-wire typical issues: https://www.maximintegrated.com/en/design/technical-documents/tutorials/1/148.html

How are you powering the sensors?

How are you powering the AK-DS2482S board?

Standing to the above link, which topology are you using?

Are you able to do some measurements with an oscilloscope?

Thanks.

#3

Hi Ivan,

Thanks for your quick reply. The board is being powered with 5V from the Pi, the sensors are not being powered. We no longer have an oscilloscope.

Sadly we have the star topology up to where the wires join the AK-DS2482S board, so I appreciate that it’s “the most difficult to make reliable”, and some of our wires are pretty long. They are coming through solid concrete up to about a metre before they reach the board.

Can you think of any way we can make this work? Do you think we could impedance-match the individual wires? We know the lengths of them all.

If not I guess we will need to make it a switched network. Do you think we could we feed all 7 into the Pi directly, then write a bit of code to output one at a time back into the Pi, or are the electrical signals involved not suitable for this approach?

Alternatively, does Artekit sell anything to do this sort of thing?

#4

Or - another thought - could we just extend the shorter wires so that all 7 are the same length? Would that help?

#5

Hi @mirandapoth,

Of course you can try to figure out the impedance and adapt the circuits/cables.

But based on what you wrote (“we could only connect 3 at once”) and that your topology is star (I imagine that the wires for all the sensors come to a single point) it comes to my mind that you can try with three AK-DS2482S-100 boards, and connect up to 3 sensors each, and the 3 AK-DS2482S-100 board connected together to the same I2C bus, using different I2C addresses (using the A0, A1 pins to configure the addresses).

Unfortunately we don’t have a product that adapts itself to every situation/topology.

#6

Hi @Ivan,

Thanks. So am I correct in understanding that A0 and A1 are by default pulled to GND via 4K7 resistor, i.e. address 00? So to set the binary address for each board we connect one/both of them directly to VCC as appropriate to get different addresses?

#7

Hello @mirandapoth,

Exactly.

Also: now that you are/will be using several board on the same I2C bus, please remember to use just only one pull-up resistor on both I2C lines. That is, join the solder jumpers (labeled as “I2C PU” in the board) on only one of the boards you plan to use.

#8

Thanks @Ivan for all your help with this. In case anyone else has a similar problem/question and is interested in a solution, we’ve got it working as follows.

We’re using a DS2482S-800 chip, which is the 8-channel version of the DS2482S-100 chip. Because we have only 7 sensors we can wire this up with one sensor per channel. In the end it was pretty simple, the only components we needed was the DS2482S-800 and a 4K7 pull-up resistor for each sensor. The software side on the Pi is pretty straightforward too and we now have all our slab sensors showing up in Home Assistant :smile:

#9

Hello @mirandapoth

Thank you for the feedback. The DS2482S-800 seems an interesting solution for a case like yours. We will think about making a DS2482S-800 board in the future (perhaps when the silicon crisis is over :slight_smile:).