Bad by design SMJ RFC2SC remote controlled mains sockets – fix

Maplin flog these SMJ remote controlled mains sockets.

SMJ Electrical remote and socket

The first thing you need to know about remote controlled mains sockets is make sure you get one which has separate on/off buttons. These things are ratty by nature, and some are a toggle – press the button to switch on, press again to switch off. That’s fine in a light switch, where you get to see if the light is on or off, but dire in a remote switch. The whole point of a remote controlled switch is you can’t necessatily see the remote item. A toggle is worse that useless in that case 😉 That’s why this is my second purchase of this sort of thing, these are made by SMJ electrical. I was using this to switch on the coffee machine in the kitchen in the morning. As the expensice 12V battery ran down the reliability of this got worse and worse.

I measured the battery and it was down to 10V. Putting the remote on a bench power supply showed it drew 5mA on transmit, and getting a scanner onto the job showed the frequency was 433.855 MHz.

This  was a little bit off the advertised frequency of 433.92 MHz on the  SMJ RFC2SC label, but a 65kHz discrepancy isn’t too bad. This falls within the 433.050-434.790 MHz ISM band

SMJ RFC2SC label says 433.92 MHz

So the first thing is to pop the lid of the remote and see what gives

Internal view showing PCB

The RF sub-board is a separate assembly, presumably to deal with various different regions’ RF allocations. It’s a pretty nasty piece of work compared to the rest of the remote, hand-soldered by the looks of it.

SMJ Electrical remote RF board showing really shoddy workmanship

Of note was that the hot side of the antenna is at the top. I had allocated button 1 to the coffee machine socket, and to press this you tend to put your hand round the top of the remote. I determined that this detuned the signal – it was easy enough to hear on the scanner that putting a hand round the back to press button 1 shifted the tone of the signal, and this was confirmed with a scope that the waveform was distorted. So a good tip for using these is to use the 4 buttons rather than the 1 buttons!

The next step was to test the range with the socket, which was poor initially. It wasn’t particularly sensitive to supply voltage, but it wasn’t particularly sensitive full stop. The claimed range of 50m was ridiculous, I fitted a lamp to the socket and moved to the room next door, and communication was lost. I adjusted the trimmer capacitor of the remote (on the other side of the board) and range improved dramatically.

Frequency 433.845 MHz

This now measured 433.845 MHz, which is still acceptably in the ISM band. It’s now 75kHz off. In an ideal world I’d have moved both the socket and the remote closer to the original frequency, but this will do and it’s still in band. It’s still on the original battery – the transmitter frequency isn’t particularly sensitive to voltage down to about 8V, but it was so marginal originally that the lower power must have pushed it over the edge. And I get coffee in the morning 😉

10 thoughts on “Bad by design SMJ RFC2SC remote controlled mains sockets – fix”


    This comment a little late, but I’m responding to Richard’s comment about using the remote switch for his coffee machine.
    I used ours, (they ARE different to the ones illustrated. Mine are Part Reference: RFE1TC) and destroyed two of them as a consequence, to switch the kettle on remotely while getting dressed upstairs, so its ready to make tea in the morning when I arrive downstairs!
    I plugged the kettle into it and came down to make Tea and it had worked, but never again as the light was off. It had “Burnt out”.
    I thought it was just a fault, never mind I thought, just send it back and get a new one but, I plugged in a replacement and that only boiled once and was done for!
    I then looked at the small “warning”, TOO indistinct to BE an actual WARNING, “Max. 1000W”. Yes that’s all it said!
    Not exactly very helpful. The WARNING should have been in BOLD RED letters and stating “WARNING. IF USED ON AN APPLIANCE OVER 1000W, THIS UNIT WILL SELF DESTRUCT!!!
    Needless to say I phoned SMJ several times to complain, and they of course are saying and doing NOTHING!
    I imagine this must be a UK Manufacturer, as that’s how most UK Manufacturers deal with faulty things, IGNORE THE CUSTOMER!
    They state on their web site that one should go back to the Retailer (Amazon) and complain, but this is clearly a manufacturing/packaging FAULT, it has nothing to do with the Retailer?

    1. Sorry to hear of your experience! I hope Amazon at least refunded you, they’re usually good about that sort of thing.

      I dissected the spare receiver that I trashed in the original exercise because I couldn’t work out how to get it apart. Inside there is a relay HKE V6-S-DC24V rated ’10A 250VAC’. That’s within spec for the coffee machine (~1.5kW) but may not be man enough for a kettle 🙁

      On the back of the Rx it says quite clearly 240V~ 50Hz 13A Max 3120W which doesn’t quite square with that. In all fairness, it’s been in service for over a year and a half on my coffee machine load, and is now reliable – there’s no sign of heating which I checked for after your comment!

    2. “They state on their web site that one should go back to the Retailer (Amazon) and complain, but this is clearly a manufacturing/packaging FAULT, it has nothing to do with the Retailer?

      Your legal contract is with the retailer. That’s why you should always contact them first. The manufacturer has no relationship with you.

  2. Hi,
    You wouldn’t happen to have a copy of the pdf for these units would you?

    I have 2 sockets, but no remote and am wondering whether I can get them to work with a different manufacturers kit, but on the same frequency.



    1. Yes, but it’s not particularly useful on the protocol. Here you go. The RX uses a SPMC802 OTP microcontroller. I guess you could bit-bang the codes for a transmitter from a recording if you had one using a PIC or suchlike and I think 433 MHz TXs are to be had. But you have ot ask yourself why you’re putting work into making this sort of junk work when the going rate on ebay seems to be about £12 😉

  3. I’m currently looking for replacements for what we have at home (here in southern Germany). I discovered some ELRO products and interesting to note the receiver casings look identical to what we bought some 6, 7 years ago now. Devices I put outside (e.g. movement detectors and ‘build-in’ switching modules) died like flies within one or two years . I opened them up and discovered the ‘chips’ where ‘black blobs of plastic’ direct upon the small p.c.b. with fine copper wires radiating outwards. Such miserable quality production and construction would mean these components would have been slowly ‘poisoned’ from contaminants and their substrates super-stressed from temperature extremes. In house our devices have lasted but one failed recently. It ‘seems’ okay cranking the channel switch around on the receiver. Our ‘older’ sender has ordinary AAA or AA batteries in. The receivers have an expensive 12v (VR22).
    We switch nothing other than a few watts of lighting – filament and/or saver-lamps.
    I’m looking for something better quality or to build myself (when I have the time I am an electronics hobbyist)
    PS. bye-bye ELRO! maybe next time?

    1. I eventually bought this when the time came to chase this back down the band as it gets colder, because coffee is more important on cold mornings 😉 Looks similar to the ELRO range

      If you’re looking to DIY the Ciseco XRF range can be used as straight button push actuators without a PIC by uploading suitable firmware. But you must have mains power at the RX site because the XRF draws 20mA @ 3.3V even on receive only!

      I found using acrylic conformal coating helps service life outside, in the UK it seems condensation is the problem, I guess you have more temperature extremes in Germany!

  4. I had a set of these units. Typically the remote failed just out of warranty. I hadn’t quite thrown them away but forgotten them. I remembered them the other day and took the remote apart to see if there was an obvious problem and cleaned some soldering residue but no joy. I googled 433.92 Mhz switch faulty remote and found your site. It confirmed my view on the soldering quality and I persevered on the transmitter sub board. Bingo, it sprang into life. So thank you very much for the website. It’s always a nice feeling when you fix something.

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