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 😉