Compact and low cost MySensors + NRF24L01 Board Project

[frenchclem]

@m26872 the most expensive is the Atmega I got a quote at $1.2, but I hope I can get it cheaper. then PCB and NRF chip will cost between $0.6-0.7. The other small SMC don't cost much. Depend now how much I'll get it manufacture for in quite small batch? But let's face it, if we can buy an arduino pro mini and a NRF24L01 module for less than $3-4 in China, then it should be easy to make an all in one board for abour $7-8. Depending on the quotation I'll get, I hope I'll be able to manufacture it in France.

@hek I'm doing this because I just love your project, and I think the cheaper you will give out people ready board, the more the mysensors communitty will grow.
Of course everything will be open! It's already all over the web, the arduino, the NRF, even PCB antenna design. I really don't mean to compete with the Sensbender, I'm really meanning to contribute somehow to the Mysensors Projet.

[tbowmo]

The real price hog on the sensebender is the si7021,prices are around 4-5$ alone. That is what's keeping the prices up

[m26872]

@frenchclem I thought the manufacturing (pcb, assemble, test...) was a high cost, unless you buy in huge quantities.
But great. I really hope you make it!

[frenchclem]

Hello,
Here is draft 1 of the schematics.

I still haven't decided whether the temp sensors will be the famous SI7021 or for cost reason a more basic DS18b20 (about half the price of the SI7021)

For powering the board, I'm planning on having 2 solutions : direct 3.3V (bypass by solder jumper of the regulator) or 5v power supply with a Micro-B

The Antenna I found 2 design :

• http://www.ti.com/lit/an/swru120b/swru120b.pdf : the one used in the schematics because I found a lbr for it)
• http://www.ti.com/lit/an/swra117d/swra117d.pdf : I like this one more, the gain is better, but I've got to create the part in eagle.

I'm still missing the breakout of the main pins.

But I wanted to have a first feedback / point of view : Is EEPROM critical ? Is the temp sensor absolutely necessary ? Same with the Micro USB and Voltage Reg ?

Edit : The Oscillator won't be monted but the solder pad will be here (If I have room for it), I also will remove the capa

[tbowmo]

@frenchclem

Please note that the DS18s20 doesn't work at supply voltages below 3V

also external flash is a really good idea, as are added atsha204a as well..

[frenchclem]

@tbowmo Thanks for shooting out some idea !

Indeed, I read the thread about the birth of the Sensebender which is actually a mine of information and thought about minimal design.

As you mention it while working on the sensebender, the RF will be the hardest part. I can add all the cool feature on the board, if you can't connect it, it's useless.

I'm still waiting for cost estimation from china, and french supplier specialized in IOT, prototyping, and small batch production.
I'm in a design to cost process, so first things first, I want to validate the cost objective from various quotation. Then I will see if I have to make hardware design choice, or if I just can't make it, ...

[Nuubi]

*The *board that should be implemented is just arduino nano connections to NRF module. Very simple, yet, haven't gotten to do it.. anyone?

[frenchclem]

@Nuubi can you explain why you need the :
FTDI USB-to-TTL Serial chip ?
mini B connector is not so used anymore (samsung cellphone use micro USB)

Because beside that, it's what I plan on doing. Oh I might add a temp sensor, but not sure yet.

Then I think it's worth having a thought about it :
EEPROM can allow OTA FW update. which is quite cool, when you what to update your node without taking everything down.
I'm not really convinced with the ATSHA204 tho

[Anticimex]

@frenchclem do you have another security solution planned or do you intend to skip security and completely rely on sw implementations only for security?
I do not recommend that as it makes the board sensitive for memory dumping which would reveal the secret shared keys which in turn would compromise the network. The atsha204 protects from that type of attack.

[korttoma]

What I would like is basically a Sensebender Micro without the Si7021 but with the NRF24L01+ on the same tiny board.

[frenchclem]

@Anticimex not really. My first idea about this board was to say : lot of us by chinese arduino pro, NRF24L01, spend some time soldering everything together, because we are looking for the cheapest solution even if its not the most sexy one, the most secured one, ... So a lot of us, end up designing their own little board, with DIP Atmega, to have something smaller, more sexy, but not secured.

So I guess on my first thought, this board is more for all these people. Now when you start digging, and thinking about it, of course you want to add stuff to it, temp sensors, EEPROM, security.
My ultimate goal is the price, on that I won't deviate.
Then the secondary goal is size, functionality, and evolution : on that I'm open.

• The more I think about it, the more I thinking that the temp sensor is expensive and maybe not mandatory
• EEPROM : I really like the Idea of OTA FW update, plus you can get red of the ISP programming pin. But I'm not sure whether it's working already on mysensors or not (I have not look to be honest)
• atsha204 : As I said, I lot of us don't use HW protection with our DIY solution. But I like the idea of secured communication, so if it's not a cost driver (I have not quoted the chip yet) then it could be part of the board.

By Evolution, I mean that I could throw in some PCB foot print, for SI7021, or other component (on the bottom side for example) so people that want to add more functionality would be able to.

@korttoma I agree, SI7021 might not be absolutely necessary. About the size of the board, there is no way it will be the same size as sensebender ! @tbowmo did an incredible job on that board, plus he used 0402 chip which can be a cost driver for some production. I'll keep it small, but not as small at the sensebender !

[frenchclem]

Small Update : Got the first quote for PCB and PCB assembly in china. That validates that the $7 objective of the board is feasible for 400 boards batch.

I ask for quotation the atsha204, some packaging are obsolete, which one is used on the Sensebender ? Sot23 or SOIC, or TSSOP ?

Also for size reduction, I thing I'll use the ATMEGA QFN32

Now lets do a dummy proto board for antenna benchmarking

[Anticimex]

@frenchclem then why don't just use the existing SenseBender board design files and just order the raw PCB and then solder only the components you want? It's open hardware. You get the footprints to add components later on if you need. To me it sounds like you would like to build a SenseBender but skip a few components to keep BOM cost down.

[frenchclem]

@Anticimex I'm thinking about that option too, as a back up solution if I fail with RF.
Also I think Sensebender is missing voltage regulator and connector so one can choose battery operation or other power supply

[frenchclem]

@Anticimex still thinking about the ATSHA204 and the HW security vs SW security.
If you have a node with the ATSHA204, does the GW needs to have it on board to ?
BTW do you have the link to the security thread so I can update my knowledge on that side ? please

[mfalkvidd]

@frenchclem said:

@Anticimex still thinking about the ATSHA204 and the HW security vs SW security.
If you have a node with the ATSHA204, does the GW needs to have it on board to ?
BTW do you have the link to the security thread so I can update my knowledge on that side ? please

Signing thread http://forum.mysensors.org/topic/1021/security-introducing-signing-support-to-mysensors/1
Software and hardware signing can be mixed, no problem. The biggest difference is that software adds a few kB to the sketch size and that the key can be extracted if someone "borrows" a software-based node.

[Anticimex]

@frenchclem my post is pinned in the development category so you should have no problem finding it. SW and HW signing is compatible so no, you don't have to have hw support in your gw. But sw signing also take more memory. And the quality of the random numbers is much poorer.

[frenchclem]

Thanks I'll read that carefully.

[tbowmo]

@frenchclem

I would suggest to at least add the footprint for ATSHA204A (sot23 housing) on the board. Also with external flash / eeprom, add the footprint..

Another thing, does the price quotation include raw pcb's, all components, and smd assembly? What type of PCB materials have you specified for them to give a quote (RF performance depends on this!). Component quality (specially around the RF parts).

Please remember to give the end users as many options to use the board, as possible.. A feature that you think is not necessary (due to pricing etc.) is the one thing that users want to have on the board. And thus find something else instead.

Also please have a look at mysx connector description https://github.com/mysensors-kicad/documentation/blob/master/TheMYSXconnector.pdf this will be used by more projects in the future..

[ahmedadelhosni]

@frenchclem I have a question please. Why do most people add temp and hum sensors in every node ? I really can't get the benefit of that.
Thanks.