RF12B module interfacing question?

Submitted by Vster on Wed, 14/08/2013 - 21:37

Hi, I'm new to the OEM scene, but so far I've got an emonTX with 2 x CT and the voltage measuring set up. This reports to a bare bones emonTX shield (just the radio module plugged into an Arduino) which I've got plugged into my home server acting as an RF to USB gateway. I don't need any more servers/computers/boxes powered up!

First let me state that I'm no electronics expert, if I have plans I can copy/build them just fine and cobble stuff together, but the low level stuff just eludes me.

My hardware question is this, I built an Arduino powered "Ambient Light" a long while back, and it struck me this really could do with joining to the emon network, so I ordered an extra RF12B radio module. I've been looking at the diagrams on Solderpad and I'm somewhat confused. In the emonTX shield diagrams there are several resistors connecting the Atmel mcu to the RF12B, but these aren't on the normal emonTX and the lower power temperature node. Are they important/do I need them/what do they do?

Many thanks,

Mark

Re: RF12B module interfacing question?

Submitted by Robert Wall on Thu, 15/08/2013 - 18:09.

The RFM12B module only works up to a little over 3.3 V, which is the normal working voltage of the emonTx. On the other hand, the Arduino (and of course the shield) runs off 5 V, so the extra resistors etc lower the 5 V signals inbound to the RFM12B so that it is not subjected to over-voltage. Going the other way, there's no problem.

Re: RF12B module interfacing question?

Submitted by Vster on Thu, 15/08/2013 - 23:56.

Robert,

Thanks for that, I'd missed the fact the emonTX runs on 3.3V.

So essentially, I'll need the bank of resistors (voltage dividers?) to lower the voltage of the connections to my Arduino that runs 5V. I'll also need to add the 3.3V regulator and its auxillary components to get a smooth 3.3V to run the rfm12b. I don't suppose I can just use another 2 resistor to get 3.3v? I'd guess not...

Many thanks,

Mark

Re: RF12B module interfacing question?

Submitted by Robert Wall on Fri, 16/08/2013 - 14:25.

So essentially, I'll need the bank of resistors (voltage dividers?) to lower the voltage of the connections to my Arduino that runs 5V.

Correct.

I'll also need to add the 3.3V regulator and its auxillary components to get a smooth 3.3V to run the rfm12b. I don't suppose I can just use another 2 resistor to get 3.3v? I'd guess not...

Also correct. The current drawn by the RFM12B varies greatly so you couldn't get a stable voltage with resistors only.

Re: RF12B module interfacing question?

Submitted by Tinbum on Fri, 16/08/2013 - 15:22.

Have a look at

http://jeelabs.com/products/rfm12b-board

Does what you need, I think, in a nice neat package.

Re: RF12B module interfacing question?

Submitted by WindyBoy on Wed, 28/08/2013 - 17:02.

I was wondering about this also, as I've bought a RF12B and was planning to add it to my arduino MEGA which has a 3.3V regulated supply on it, I'm assuming I can use that to power the RF12B with no problems ? And don't need the extra board and components like the jeelabs rfm12b-board ?

Thanks

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