Cheap cell modem / design considerations?

[dcrookston]

I'm (trying to...) design a product that can send data over the cell network.  It only needs to send a few bits of data, and it only needs to send them once.  The big constraints are cost and size: the final product needs to be thin (less than 1/16th of an inch, or 1.5mm) and about 6 inches by 4 inches (150mm x 100mm), and it might get thrown away afterward so it needs to be as cheap as possible.  The product will be used exclusively in the United States.

Yes, the idea of people throwing away a perfectly good cellular modem pains me, too.

I'm thinking my best option will be to send the data as an SMS, since (as far as I know) SMS is part of the most basic cell service, and therefore the simplest possible cell modem will still get the job done. I've never developed anything that does any kind of radio transmission so I'm completely in the dark here.  What specs do I need to look for other than physical dimensions?  What blindingly obvious things am I overlooking?

Thank you!

[neilhao]

Are those kind of cheap cell modem you are looking for?
https://www.fibocom.com/en/products/list_lcid_124.html

I never used any products from this company, just heard that this listed company provides almost all communication modules for POS machines in mainland China. The POS machine is not often broken, so the quality of the communication module should be acceptable.

[dcrookston]

The problem is that I don't know what I'm looking for.  Those are apparently 4G LTE modules.  I doubt I need 4G capability if I'm only sending a single SMS.  Then again, maybe there are no more "1G" towers.  I've been reading everything I can find about this topic but none of it actually tells me what I need to know.  Maybe the entire cell phone industry actually is an ugly jumbled mess of different technologies, acronyms, frequencies, and standards and that's why I can't make head nor tail of it.

I guess my first question should be, what is required in a cell modem in order to send an SMS?  That, and only that, is what I will need.

[SmallCog]

Not 100% sure what you're trying to make, but sending an SMS is relatively easy

Get your module, look up it's AT command set, send a suitable set of commands to the modem

Quick example I found on google to save typing it up myself:

Below shows a simple example that demonstrates how to use AT commands and the HyperTerminal program of Microsoft Windows to send an SMS text message. The lines in bold type are the command lines that should be entered in HyperTerminal. The other lines are responses returned from the GSM / GPRS modem or mobile phone.


AT
OK
AT+CMGF=1
OK
AT+CMGW="+85291234567"
> A simple demo of SMS text messaging.
+CMGW: 1


Here is a description of what is done in the above example:

Line 1: "AT" is sent to the GSM / GPRS modem to test the connection. The GSM / GPRS modem sends back the result code "OK" (line 2), which means the connection between the HyperTerminal program and the GSM / GPRS modem works fine.

Line 3: The AT command +CMGF is used to instruct the GSM / GPRS modem to operate in SMS text mode. The result code "OK" is returned (line 4), which indicates the command line "AT+CMGF=1" has been executed successfully. If the result code "ERROR" is returned, it is likely that the GSM / GPRS modem does not support the SMS text mode. To confirm, type "AT+CMGF=?" in the HyperTerminal program. If the response is "+CMGF: (0,1)" (0=PDU mode and 1=text mode), then SMS text mode is supported. If the response is "+CMGF: (0)", then SMS text mode is not supported.

Line 5 and 6: The AT command +CMGW is used to write an SMS text message to the message storage of the GSM / GPRS modem. "+85291234567" is the recipient mobile phone number. After typing the recipient mobile phone number, you should press the Enter button of the keyboard. The GSM / GPRS modem will then return a prompt "> " and you can start typing the SMS text message "A simple demo of SMS text messaging.". When finished, press Ctrl+z of the keyboard.

Source: https://www.developershome.com/sms/howToSendSMSFromPC.asp

I know nothing of the available cellular technologies in the US, in my part of the world 3G cellular has a an expiry date and we'll only have 4G and 5G available. You may find that the modules you see are compatible with newer standards than you require as the older tech has gone by the wayside.

[dcrookston]

I agree completely, evb149.  These things are going to be mailed out to recipients, who will open the package they're in, activate them, and then be encouraged to put them back in the mail afterward.  Part of the package will tear off to reveal a return-post label so literally all they'll have to do is read the part that says "Please tear here for return postage," actually tear it off, and then put it in a mailbox.  I have very little faith that more than 20% of recipients will go to that much effort.  I also expect that hackers will discover what's in these things, word will get around, and a handful will "disappear" into people's own projects, which I personally look forward to.  I doubt my customers will share my perspective.

As far as coverage, they need to work basically anywhere that people live inside the United States.  I can't rely on the recipient owning a cell phone at all, much less having it nearby when my product is activated, and I definitely can't rely on anything with less coverage than the cell phone network.  It looks like NB-IoT has pretty solid coverage so I'll definitely look into that.  Ultimately it depends on who the customer wants to mail these to.

SmallCog: I've got a NimbeLink Nano (documentation linked in my first post) and I've built a proof of concept using that.  Unfortunately they're about $70 each, and their mounting requirements means the final product will be about 1/4 inch thick, which is huge compared to our requirements.  A lot of other options are surface mount, which is much better.

But as I'm looking through those options I see things like this:

> Cellular GSM, GPRS Transceiver Module 850MHz, 900MHz, 1.8GHz, 1.9GHz Antenna Not Included Surface Mount

> Cellular GPRS, GSM, LTE Transceiver Module - Antenna Not Included Surface Mount

and I know what these things mean in isolation but not how they translate to the capacity of the device to do what I need.  It feels like I'm trying to jump into the middle of a topic, so maybe I just need to get my HAM radio license so I can start from the beginning.

[dcrookston]

I'd encourage you to re-think this product target, but I feel like a product this absurd shouldn't be encouraged.

I come here for advice on electronics, not business decisions.  But your response has been noted, thank you.

[neilhao]

Always try to use the module with the newest standard since cell network are operated by ISP, they will retire old standards at some given time point. Those life span schedule could be found on the ISP's website.

On the on the other hand, the cost also need to be considered. I think this is why 4G modules are widely used right now. I checked the 4G module from fibocom, the price is around 7.11 bucks/PCS on the retail market. I personally expected at least 30% to 50% off for the bulk purchases.

[tszaboo]

I hope you're aware that the FCC certification process for a product like this runs $60k-200k.

Fascinating. Do you have any reference about these costs, or is this personal experience?

[dcrookston]

I hope you're aware that the FCC certification process for a product like this runs $60k-200k.

Fascinating. Do you have any reference about these costs, or is this personal experience?

I'm not sure where he got those exact numbers, but the process certainly isn't cheap.  It's also already completed by the manufacturer of the component -- you can't sell a cellular modem, or any RF device, in the US without getting the FCC's approval.  I am buying the cellular component, not manufacturing it, so this work has already been done for me.  The expense is spread out across every component sold.

[Bicurico]

Just get a SIM900 module. They cost like under 5 bucks. Your can then control them using some basic microcontroller or Arduino.

If your customer mails the devices within the next month's, using 2G or 3G should not be a problem, but check that out first.

The main problem you have is the hight. I don't think you will meetvthat requirement. Also, consider how you will power the device.

It would help if you could state your budget.

Finally, did you consider how long the envelope needs from opening to booting to logging into the network and sending the SMS?

And have you thought about regulations regarding shipping batteries?

Cheers,
Vítor

[neilhao]

I'm not sure where he got those exact numbers, but the process certainly isn't cheap.  It's also already completed by the manufacturer of the component -- you can't sell a cellular modem, or any RF device, in the US without getting the FCC's approval.  I am buying the cellular component, not manufacturing it, so this work has already been done for me.  The expense is spread out across every component sold.

You are totally correct about FCC certification. I was in charge for a hardware startup as GM, responsible for the FCC certification of dozens of products. Generally, FCC certification and UL certification are very different. FCC certification is usually done by an authorized laboratory, but UL certification is usually done by UL itself, and of course UL certification is very expensive.

FCC certification fees usually includes documentation fee and testing fee (conduction test, chamber), the laboratory determines the test items according to the product category.For IOT products, if we know EMC very well, the product can pass the test at the first time. Then the FCC certification fee usually only costs thousands of dollars. Of course, most companies' products simply cannot pass the test easily, they need to constantly modify the design, sometimes authorized labs provide consulting services, and it is common to spend $60k-200k in this case.Authorized labs prefer this kind of client.

Usually the major problems of FCC certification are ACDC and RF parts. If the product is connected with a network cable, it will also be very troublesome. The network cable often becomes an antenna, or the conduction test cannot be passed.

Both FCC and UL have module certification. When you purchase a module that has certification. The module does not have to be tested again, which usually speeds up the certification process.