Probably a couple of metres. I will look at Xbees.

I've seen wireless serial RF devices or WiFi modules on eBay. Wouldn't mind which type, just need to send a signal from one to the other. Could WiFi ones connect together or would it need to go through router?

Only used Xbees for simple serial point to point stuff, I don't really know the differences between the umpteen different versions. They're meant for mesh networking so I imagine it could do it peer to peer.

Have a look at nRF24L01+ adapters. Really cheap on ebay. You can get ones with a tiny onboard antenna (99p) or slightly more expensive ones with a bigger screw-on antenna (~£3).

Edit : If you want to buy some on ebay the cheap ones come from China/HK. Also, since you want more than one search for something like "nRF24L01 -1pcs pcs" to get some good prices for multiple adapters.

Those are the ones I saw on eBay earlier. I think I will look into how they work. Thanks.

Apparently the screw on aerial types can transmit up to 1km. Long far.

Yeah, they're supposed to get some pretty impressive transmission distances.

I've got one with a massive antenna connected to a Raspberry Pi and a couple of arduinos with the tiny antenna adapters around the house transmitting temperatures. It might be because mine are indoor with decent thick walls but I've definitely seen some range issues. It could also be 2.4 GHz interference. I imagine in an open space you'd get great range.

It does say unobstructed. The ones with the small antennas don't seem to mention range. Our house is a mix of plasterboard upstairs and brick downstairs.

Are they easy to use?

I haven't had 2 arduinos communicating with them (because that wasn't my aim) so I can't say if that's easy.

Getting them to communicate with the RPi wasn't too easy. IIRC the arduino side of things was fairly easy but the RPi wasn't straight forward, I had to email the person who wrote some examples for advice, I was clueless with what I needed to do.

The arduino side of my setup is pretty "dumb", all it does is transmit a payload of the temperature and another number every 10 seconds or so. It was getting the RPi to receive them that was harder.

You're probably best getting a couple, using some example sketches and getting something working, then adapt it to what you want to actually do.

Ah, Arduino to Pi, I see. Hopefully it will be easier Uno to Uno. Which cheap Arduinos did you buy? Was it the little mini ones from eBay?

Had a play with the 2 line LCD in the kit last night. Made a scrolling display (autoScroll only seemed to scroll both lines) and realised the pot for the contrast is definitely needed which may explain why my 4 line didn't show anything with the Pi, it may actually have been working.

Also any idea about debugging sketches? I put too many for loops inside my scrolling display code. Took a while to work out what I'd done.

There's two types of the cheap small arduino clones about. There's the Nano and the Pro Mini.

I've got some of both but I much prefer the Nano type. They come with USB serial built in and possibly more pinouts, slightly larger and marginally more expensive I think. You can power over the USB connection too which is really useful because I've got a load of mini-USB phone chargers from old phones lying around. I believe you'll need a Nano for the nRF24 adapters because the Pro Minis I have don't have a 3.3V pin which is required.

If I've still got a spare Nano I might make another wireless temperature sensor today for a different room.

Cheers Ant. Helpul info there.

There doesn't seem to be a huge difference in price between the cheapest minis and nanos. The USB socket does sound useful though. Some of the mini boards say switchable between 3.3V and 5V.

How many Arduinos do you have?

The minis require you to have a serial adapter of some sort to load sketches to it. Once loaded you don't need the serial adapter connected anymore but I found this to be a pain to get working first time every time. The USB nanos work first time, much less hassle.

I have 4 in use and I think I bought 5 nanos and 5 pro minis a while back.

That does sound easier. I'm sure I have a few mini USB leads around too. Just over £2 for an unsoldered board. I might order a couple of them and wireless modules and maybe some temp sensors too. If I had more time I could get carried away with these.

Not sure if all mine came with USB cables but some I have bought did, I was fairly impressed since I was just expecting board and pins.

About £2.80 for a nano with a cable. Cheap. People probably pay about 5 or 10 times that just for a USB cable in some shops.

Things I have ordered so far:
2 nanos
4 wireless transceivers
1 real time clock
2 temp sensors

One more question (for now): I can probably get my hands on quite a high capacity 18V Li Ion battery from an old cordless drill.

How could I use that with an Arduino. External voltage regulator or sommat?

Most Arduinos can take 6-20V on the Vin so you shouldn't need a regulator (there's one onboard).

Both my SainSmart and the nanos say up to 12V.

Had a quick look on Google. Depends how much current the board will be consuming. If it's not doing too much it should be ok with the 18V, otherwise it would need another regulator that could handle tthe current.

A quick question about the mini boards: what cable is required to program the board? Is it just a cable or is there a circuit board between the two?

I might buy an 8MHz mini to make a custom transmitter to use my Taranis controller with my Hubsan X4.