Relay guidance

Hello forum ! I need some help with choosing my relay boards, i need 70-100 CH in my relayboard but what kind of arduino bord should i use ? could relly use som help ! :slight_smile:

Hi, you will have to be a bit more detailed in what you are trying to do.
What do you need 70 to 100 channels for, if that is what CH means.
What is the application?

Tom....... :slight_smile:

yeha it's channels :slight_smile: its for a large home automation system :smiley:

Which relayboard did you have in mind?

I think i will use theese relay boards


You want a set of I2C expander boards using the MCP23016 chip which provides sixteen (input/)outputs per chip and you can chain (gang) up to 8 by setting different addresses, for a total of 128 outputs. And within a half metre or so, you can have them on different boards with the 4 wire (two and power) I2C bus connecting them.

Alternatively, you can use PCF8574-based expanders which have only eight (input/)outputs per chip so you then can chain (gang) up to 8 by setting different addresses, for a total of 64 outputs. However, you can have two separate chains of 64 by using alternate software-defined I2C ports (two pins each). These are available relatively cheaply as modules, but even more cheaply for somewhat perverse reasons, as LCD drivers where the eighth line is buffered through a transistor which may or may not be suitable for your purposes. (In fact, it is for driving a bright LED up to about 1W, so it may be quite useful.)

This means you can use any Arduino as you need only four port lines.

Something like this?

Looks good.

Price is OK.

Pity about that.

As it happens, I am interested in the "home automation" matter - have been for some years in fact, subject of considerable domestic disharmony - due to lack of progress. Mind you, seventy to a hundred devices is more than I have bargained for, I expect to settle for 32 or so.

I think i will use these relay boards

Those are probably quite suitable. The main problem relates to certification for the local electricity supply authorities, whether such an assembly will be acceptable in your permanent wiring. That is what you need to determine first.

My own approach is a DIN rail array of socketed relays with standard 24V DC coils, in a standard enclosed service box. I do have (somewhere) the 74HC595 shift register boards, my problem is the intermediate driver FETs (FETs because they have zero input current whilst the system would be running backup battery during a power failure) in a nice assembly. My best bet so far, and not entirely ridiculously expensive, is this board.

The board Nick indicates would - if available - be fine for driving the other relay arrays as you cite. I can't seem to find any readily available alternative to this. There is of course, the Freetronics 8 relay shield, you can use eight of them with different addresses set to drive 64 (external) relays but they are pricey.

Why not use tpic6b595 shift registers instead of 74hc595? They can drive your 24v relays directly.

Why not use tpic6b595 shift registers instead of 74hc595? They can drive your 24v relays directly.

Silly me - you are dead right!

Just ordered ten of them from G&C.

I did look into them some years ago - they are not exactly new. I was hoping to find a glass PCB (rather than having to make one) to match; that was the cause of my procrastination. Unless I find something, I will simply chase up some glass perfboard or "Vero" and put 4 (or 5) of them on that. Then get my licenced electrician friend to build and install the array (and get paid for it).

Could improve domestic harmony dramatically!