RF transceiver recommendation, please

Jeremy Harris

Senior Member
With the clocks shifting back recently, I was reminded of what a pain it is going around setting the time on several time switches we have around the house. Years ago I thought of automating this (no one seems to make an affordable radio time code time switch) but have never got around to it. So, having dabbled with RF data links and learned some of the plus and minus points for each, I'm looking for a recommendation from all the many modules out there to suit this task. The intention is to build a master timer unit, incorporating a real time clock that is periodically reset by a radio time code receiver. I already have this working, as I use the same technique to keep a couple of data loggers synced up to the time from an MSF receiver, so would like to stick to the same protocol (mainly because it's already built and just squirts out serial time and date data at 2400 baud). The master timer unit would have the programmed times in for each slave unit and I'm confident I can do this bit OK, with a 4 line display use for programming and status indication of each controlled switch. The idea is that the master unit will transmit data to each slave unit, receive back confirmation that the slave unit has actually switched to the commanded state, and display it, so I can be sure that the slave units are all operating correctly (as least so far as having received a command OK).

Ideally I'd like to use a 5V supply, as that would make life simpler.

The units need to work over a maximum range of around 20m indoors, and I'm not bothered much by the frequency, as long as it's a legal band for low power devices.

I'd very much prefer units that use simple serial comms, with just supply, data in and data out connections, as I don't really wan't to have to faff around using SPI and setting up modules if I can help it.

Price is important, so the cheaper the better to some extent. Each slave module will probably use an 08M2, the smallest 5V supply I can find and a solid state relay, as well as the transceiver.

I need to be able to run at least 4 slave modules, but may well add more in future. I can't see a problem with just sequentially transmitting data as required, with a unique header to identify each slave module in the preamble and the same header used by the slave module when transmitting its response, which would mean I could have a net of lots of units, given the slow nature of the individual command timing. Half duplex would be fine, as slaves will only transmit after they've received a command from the master unit.

I'd rather not use pairs of the cheap and nasty ASK modules, but would prefer a transceiver that has a modicum of error correction built in, so is reasonably immune from interference.

Baud rate needs to be modest, within the range of a Picaxe software serial port, so the cheap 115k baud units that are around aren't really suitable.

I'm not averse to buying from China or that well known auction site, IF I know pretty much what I'm buying. I've looked at some UK suppliers but the prices add up when you're looking to buy half a dozen of the things, plus there aren't many 5V offerings. I could, at a pinch, level shift everything, but it will be a pain having to do it in every slave, as optoisolated SSRs usually need at least 4V to operate reliably.

So, If anyone here has some recommendations for transceivers on either the 433, 868 or 2400 MHz licence-exempt UK bands (and power levels) then I'd be very grateful to hear them. I've spent hours hunting around and getting ever more frustrated at the lack of information available from the majority of sellers!

Jeremy
 

hippy

Technical Support
Staff member
I would choose ERF modules. The main advantage of those over any other is that you can add a URF or SRF to a PC to easily see what's happening on the network and extremely useful while developing it.
 

Jeremy Harris

Senior Member
Thanks hippy, I have a pair of them running the data link for my excess PV power diverter, from our external meter to the power switch upstairs and they do work very well, and are easy to use. They are on my shortlist (in fact they are the only modules on my shortlist!) but I was wondering if there were some cheaper devices around, as I'll need at least five, possibly more if I expand the network of switches.
 

veewee77

New Member
Using an ERF module, do you know how to set the frequency? The ones I have defaulted to the 868 frequency but to be legal in the US, I need to set them to 915. The datasheet says to do it with AT command, but says absolutely nothing about AT and how to use it with the ERF modules.

Doug
 

Jeremy Harris

Senior Member
To be honest, I've only used the ERF modules for one simple application (so far) and have just used the default settings, so I haven't explored their full capability. What I have found is that they are solidly reliable, and dead easy to interface to a Picaxe. I've not explored it, but there is a Picaxe wizard for configuring these modules: http://www.picaxe.com/downloads/urf.zip

It looks (from a quick scan of it) that you can use it to reconfigure these modules to the USA compliant 915 MHz frequency.
 

Circuit

Senior Member
Using an ERF module, do you know how to set the frequency? The ones I have defaulted to the 868 frequency but to be legal in the US, I need to set them to 915. The datasheet says to do it with AT command, but says absolutely nothing about AT and how to use it with the ERF modules.

Doug
See my simple programmer here; http://www.picaxeforum.co.uk/showthread.php?26188-Configuring-ERF-with-Wizards-and-Ciseco-Explorer-Plus&p=267371&viewfull=1#post267371
and use the wizard for URF/SRF/XRF/ERF Configuration in the PICAXE/wizards menu in Program Editor 6.
 
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