How Cheap Is Cheap?

The Nordic Semiconductor nRF24L01 is the older sibling of the nRF24L01+ and is not recommended for new designs anymore. Sometimes, if you’re looking for a cheaper bargain, the older chip may the way to go. [necromant] recently got hold of a bunch of cheap nrf24l01 modules. How cheap ? Does $0.55 sound cheap enough?

Someone back east worked out how to cost-optimize cheap modules and make them even cheaper. At that price, the modules would have severe performance limitations, if they worked at all. [necromant] decided to take a look under the hood. First off, there’s no QFN package on the modules. Instead they contain a COB (chip on board) embedded in black epoxy. [necromant] guesses it’s most likely one of those fake ASICs under the epoxy with more power consumption and less sensitivity. But there’s a step further you can go in making it cheaper. He compared the modules to the reference schematics, and found several key components missing. A critical current set resistor is missing (unless it’s hiding under the epoxy). And many of the components on the transmit side are missing – which means signal power would be nowhere near close to the original modules.

The big question is if they work or not ? In one test, the radio did not work at all. In a different setup, it worked, albeit with very low signal quality. If you are in Moscow, and have access to 2.4Ghz RF analysis tools, [necromant] would like to hear from you, so he can look at the guts of these modules.

Thanks to [Andrew] for sending in this tip.

21 thoughts on “How Cheap Is Cheap?

  1. The Semtek SE8R01 are not radio compatible with the nrf, though they are nearly register compatible. Using rf24 or a similar library will work if you have the same modules on both ends. I’ve been intending to do some range testing on them but haven’t got around to it yet. They’re dirt cheap (40-45c on Aliexpress), so they’re not bad value for the money.

  2. Just hope those extra parts are not some kind of impedance matching or RF filters… They were in the design for a reason. I’ll bet that no RF testing for emission etc are done and someone solder parts away and see that it sort of works.

    1. I have no equipment to see how much noise it emits into the channels nearby.
      After testing a little – looks like they can work for hours with no issues, but are not radio-compatible with nrf24l01 (or, better to say, are partially-compatible).

    2. Which means that they are potentially illegal to use in many places. I know that here in the US transmitters are usually required to have output filtering to prevent them from “splashing” signals across other bands. I suspect that in most other places the laws are similar.

      1. The FCC has little time to bother with such a low power source. That frequency band is already flooded with Bluetooth and WiFi signals.
        In 2014 the FCC issued Notices of Violation and Notices of Unlicensed Operation to the following entities-
        The total is 428.
        In 2014 there were more than 33,000 deaths due to car accidents. You have a much better chance of dying in car accident than ever getting in trouble with the FCC.

        1. That’s the same fallacious logic that makes certain people believe that air travel is much safer than automobile travel because so many more people die or are injured in car accidents than aviation incidents. In reality, for a given period of time spent traveling, your chances of being injured in an airliner versus a car are approximately equal (which makes air travel slightly safer per unit distance since in-air travel time is shorter).

          Anyway, the point is, you can’t just compare two statistics measured across different populations. The population of automobile drivers in any given area is certainly much greater than the population of people who design or build their own wireless devices.

  3. The typical way [in China] of cost reducing PCBs:

    1: Remove component, check basic functionality for 10 seconds
    2: If PASS GOTO 1
    3: If FAIL, Replace component GOTO 1 until all components ITERATED THROUGH

    And that boys and girls, is why hardware designed ‘to the margins’ lasts from 12 till noon.

  4. not only would the performance be degraded but they may not be legal as even though the frequency is stable there may be power variation or even noise that could cause problems and even et the fcc after you

      1. Indeed FCC has no power here. The folks that have the authority are called “Радионадзор” here. And the certification is called ‘Ростест’. In reality – you’re unlikely get any problems unless your radio noise will interfere with anything serious. As for these dongles – I doubt they have the guts to ever attract attention, even on maximum PA.

      2. I work at a TV station in the United States. We have this HUGE filter that makes sure that we put ZERO power into the lower (non-TV) RF band. As I understand it from our chief transmitter tech, if we weren’t siting on the end of the RF band regulated to television, we would not need that filter at all.

        So, operation outside of your assigned limits is bad, but operation outside band limits is BAD. Find out who and/or what your signal may “step on”, and plan accordingly. I would be especially careful if the next band up or down is a service assigned for governmental / fire-police / emergency services.

    1. Even in the US the RF power output from these is so low it wouldn’t even draw the attention of the FCC. That part of the ISM frequency band is flooded with Bluetooth and WiFi already. It’s not like the FCC drives around in unmarked vans searching for errant RF signals.

  5. I wonder if these things are used in any products of just for people buying them to do some hobby projects…

    Speaking of how cheap is cheap, I once opened a “from the ISP” type of modem+router. The thing was burning about 15W in the typical situation and was using the cheapest 34063 power supplies. I have calculated that upgrading all the DC/DC converters inside from 70% efficiency to 90% would pay up in about 1 years at local rates.

  6. Managed to get two sets of these – didn’t notice the images were different until I received them (bought them on eBay on my phone – images were really small). Guess I should have checked when the price was so low…

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