Two Great Radios Taste Great Together

[Johan Kanflo] sent us his latest recipe: a blend of one part RFM69 sub-gigahertz radio transceiver with one part ESP8266 module. The resulting dish looks absolutely delicious!

We’re all charmed with the ease of use that the ESP8266 brings to the table — plug it in and you’re talking to your existing WiFi network — but we hate the power consumption for battery-powered applications. WiFi is a power hog. And although ISM-band radio modules make point-to-point communications cheap and power-saving, getting them to talk with your computer takes an adapter.

So [Johan] combined the two radios and made a sweet ISM-radio-to-WiFi bridge. His demo application takes whatever data is sent over the ISM band and pushes it to an MQTT broker on his WiFi network. Hardware and firmware are up on GitHub.

We’ve been wanting a device like this for our home network for a while now. Kudos, [Johan] for making it so easy!

25 thoughts on “Two Great Radios Taste Great Together

  1. Arguably pretty useless – why not just have a RFM module on a USB stick, and do the processing on the machine it’s plugged into rather than wifi it off somewhere else? (RFM range is way better than wifi, so any argument about locality doesn’t hold)

    1. Apparently you’re one of those people who use USB for something other than its two convenient power pins. :) I would jam one of these into those wall-wart style USB power plugs that make a tremendously useful 5V source.

      Or you could plug this into a router that has non-jailbroken firmware and a free USB port.

      Or maybe he just doesn’t feel like hassling USB programming, on either client or host side.

      You gotta admit, this solution is _simple_. And for the extra cost of an ESP8266, you can’t even really say “overkill”.

  2. The future scope is good actually its point to point transmission makes it more useful. It has an extended range as compared to wifi. imagine internet for a village through RF transmission!!.Kudos to you all the best . Requires little more perfections.My friend & I discussed on this topic day before yesterday.Open for discussions on this topic may the best outcome results

    1. That’s exactly why everybody complains. Battery powered stuff falls broadly into two acceptable categories: 1) Have to charge it frequently enough that it’s routine (ie phones every night), and 2) Pretty much forget about it (ie watches, sensors – once a year or less)

      Anything between the two gets seen as an irritation/inconvenience.

  3. Put your MQTT broker on your OpenWRT router/*nix appliance, deploy two or three of these as gateway nodes, and then attach an RFM69HW to whatever endpoint application you have.

    Instant industrial automation. No fuss, no muss. (Sure, we can do direct SPI with a Pi, wireless router, or elsewise but the elimination of multiple wall warts is worth the effort in the ESP8266; esp. where space is at a premium.)

  4. I hope this isn’t going to be used in an actual product….colocation of radio modules within 20 cm = voided FCC part 15 certification. Unless…it gets re-tested and certified once it passes….

    1. Really? How did the old Wifi/Bluetooth (with shared antenna) modules work then? Or was that a (wifi+bluetooth) module. IE, if the esp8266 and rfm were both on the same pcb, would this become a single module?

      1. That was a wifi and bluetooth module combined. The way round this issue is to perform testing with an FCC accredited Lab on both the modules and get proof that the radios are within the limits. If they pass the test and a report is generated then a new FCC number is provided. In Europe this is covered by the Radio Equipment Directive and is covered under CE marking. The cost of testing is prohibitive. If someone wanted to self certify a device though with two radio modules without performing extra testing then the modules have to be 20 cm apart and reports are required proving that testing has been performed on both modules. It’s gets incredibly complicated pretty quickly. My advice – don’t design a product with two distinct radio modules unless you are prepared to do lots of testing (and pay for it) or place the modules 20 cm apart.; There is a sort of get out clause…if you can ensure that the radios are not transmitting or receiving at the same time then its possible to co-locate….difficult to do mind! – page four, third line

        1. Collocated radio modules require you to (re)certify the whole assembly regardless of the modular-approval-type certifications that each module might come with (if any).

          At most, existing modular certifications would just save you *some* money during the re-certification by not having to perform all the tests again.

          Alex – I couldn’t find the paragraph in the FCC regulations about the “20 cm” rule you mentioned above. Please share a link.

          My guess is that your “20 cm” rule is actually Separation Distance related, more specifically:

          “if an end host device can be operated close to the human head or body, i.e., transmitting within 20 cm of a person, SAR testing may be required, if the transmitter frequency and power output deem it applicable. If a device will always be used at distances of greater than 20 cm from a human body, a maximum permissible exposure (MPE) calculation is required to determine the safe operating distance for users, and actual SAR measurements are not required.”

          A recertification is MANDATORY. There is no workaround, not even if the modules are talking on different frequency bands and time slots, or are pre-certified. Seriously.

          Now, regarding the “fantastic” idea of having a “village-wide Internet connection via the RFM module”… keep dreaming. You might get better communication distance, but the communication speed and bandwidth can’t be compared (wifi vs sub-GHz protocol). Your led-blinking projects although would work. ;-)


          1. I’m afraid I have to disagree…a re certification is not mandatory at all….anyone can self certify a product just without proof that it meets the requirements they leave themselves open to assessment by the authorities who can impose fines and force product withdrawals in the event the device is found to not meet the requirements of the appropriate directives or standards.

            The website below mainly covers CE marking but the same is mostly true for FCC providing prior testing has been performed. A product can be self certified for a CE mark and placed on the market as long as the declaration is made correctly and the product has a complete technical construction file with technical rationale or proof as to how the product meets the requirements of the directives. This includes products which fall under the radio equipment directive. As long as the product is electrically safe, fit for purpose and meets generally understood manufacturing guidelines and a complete technical construction file is available upon request a responsible person can CE mark a product and place it on the market for sale with no issue.


            The FCC has a similar scheme in place. It becomes much more difficult to self declare a product with a radio module as that is an intentional radiator. There is no simple technical rationale that can be applied to show something meets all of the requirements by design…it is still possible to self certify a device even with two radio modules…it just isn’t a good idea….without thorough documentation and proof that the emissions from the product are within the limits.

            IF a complete technical construction file is present with reports proving that the modules are compliant then self declaration is an option. For instance…take the example of the product in the article above. If the designer has test reports proving that both modules (ESP8266 and RFM69 used meet the requirements of appropriate test standards and the designer has followed the manufacturer guidelines explicitly…and they have performed emissions testing verifying that the unit does not radiate at any other frequencies apart from those used by the modules. The cost of a quick emissions test compared a full re-certification of a product with two radio modules is significant…seriously!

            This is a direction quotation taken the from the UL PDF link above from page four, second paragraph from the bottom

            “Per DA-00-1407 (June 2006), the FCC
            does not encourage recertifying host
            products containing pre-approved
            wireless modules unless the rules make
            it essential to do so. Therefore, the
            existing modular approvals should be
            used without a new certification of a
            host product whenever possible.”

            The 20 cm rule does apply to SAR testing. if RF modules are co-located then in order to meet FCC part 15 requirements they cannot be transmitting simultaneously…that is possible and combined with a technical rationale that SAR testing does not apply because the device is not body worn or used in an area where human exposure is less than 20 cm along with proof that the emissions have not been compromised then in my opinion it is possible to self certify…however it’s difficult. All the paperwork would have to be in good order!

      2. I suspect that the old bluetooth/wifi modules were combined and therefore counted as one module when being assessed. If FCC certification or CE certification is needed then unfortunately no self certification allowed using the manufacturer’s testing as proof. If the modules are spaced 20 cm apart then things are good. If not re-testing required. Most labs charge 10,000 USD for intentional radiator testing. My advice…don’t co-locate modules in products.

        1. Wifi/bluetooth modules also support coexistence to avoid them transmitting at the same time. This makes it possible to ignore the 20cm rule according to the FCC (makes sense, a transmitter module collocated with a receiver module also gives no problem). IIRC it are the antennas that can’t be within 20cm, the modules doesn’t matter.

          Here I also think only the ESP8266 is transmitting, the RFM radio is just receiving, so maybe there is no problem.

          In EU module certification cannot be used, so you have to test again anyway. Fortunately you are allowed to do it yourself, having the equipment in the company quickly pays off (certainly for EMI, intentional radiator depends on whether you make a lot of wireless things)

          1. In my understanding of the rukes for the EU you can self certify to the according to the radio equipment directive using the module manufactuerers certification you just need technical rationale as to why it meets the requirements of the directive. I work for an EMC test lab and have done this for customers…the issue is that’s it is very difficult to self certify without proof that both modules would meet the requirements without further testing. Nearly all modules these days have antenna’s which are close to the modules to keep the antenna matching correct and not lose transmission efficiency.

            Most designs cannot locate the antennas 20 cm apart….it is not practical hence my original comment. I find the regulations irritating…I sincerely doubt co-locating radio modules would ever cause so much of an issue that a pre-approved module would affect the operation of another module that much that it would cause an issue…the problem is that it canot be proven easily…regulation is always a double edged sword.

      1. The many wireless technologies (especially in the ISM bands) and commonplace electric devices (your microwave?) already in use hinder wireless performance. Always!

        RF interference is definitely at work in his house, is intermittent in nature, and it can create major problems to wireless performance, creating security vulnerabilities, wireless network instability, battery depleting through hidden retries, etc.

        I bet Johan’s approach was “network looks to be working fine, so interference must not be a problem”…

        Some of those RF wireless protocols (including 802.11*) have a little built-in resilience to interference, and the packets generally get through, however the result of retransmissions is that the throughput and capacity of your wireless network are significantly impacted, and so is your device’s expected battery life (if battery operated).

        Anyhow, two freq-different radios glued together.. it works as it works, and it pleases the man. A more scientific approach using spectrum and network analyzers, and the labs at a wireless certification facility, might show a different story.


  5. Although data rates are modest compared with 2.4GHz WiFi, 433/900 MHz ISM band modules indeed usually have greater range. However it’s NOT that great – typically you’ll be lucky to get 100 metres thru’ urban clutter, & less in built up environmnets. LoS (Line of Sight) It can extend to more however, especially if the antenna are elevated.

    For greater coverage consider Semtech’s SX12xx based LoRa™ modules, which astoundingly have ranges an orderb of magnitude greater for the same power. Yes – km instead of 100s of metres! See =>

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