Optocoupler schematic/diagram

Using Arduino General Electronics
1

Hello,

I'm working on a Project where I will use a Mega 2560. The Mega will be powered from the USB port, only powered, not for serial Communication other than perhaps monitoring during testing.

I will use 2 HW serial on the board (not USB), one will go to a KNX BTU, and one will go a REGO600 controller card for my Heatpump. The Rego controller uses 5V TTL 19200 baud.

For the KNX-side, I have done Another Project with a Pro Mini board, where the board was directly connected to the BTU with Tx, Rx, GND and Vin (The BTU supplies the Mini with Power). This was also a 19200 baud 5V logic.

So, I will need to have optocouplers to isolate the arduino, but since I'm not good at Electronic engineering I need some advise.
I have found this schematic, that comes from a REGO Project on SF (Same controller I'll have in my heat pump):

Will this schematic work for BOTH of my serial communications?

Thanks!

2

R3 is doing nothing, just bypass it. Make R1 and R4 be 270 ohm and R5 1k, then its symmetrical which
feels much more sensible, and the LEDs get a healthy 15mA drive which is what most are designed for.

If there's any risk of mains level voltages you should not use a 2-channel opto coupler, use 2 separate
opto-couplers so you get the 0.3" board separation you need for flash-over prevention. (Having
said that there are some bidirectional 2-channel opto-couplers which are suitable, but most are
unidirectional).

3

Thanks MartT,

If I already have soldered above schematic, using MCT6

Will it work then, or do I need to rebuild it according to your advise?

I also forgot the biggest question, would it be possible running this setup without any optocouplers?
Or is that a bad idea? thought if it's possible to run only Tx and Rx and no GND or VCC

4

MarkT:
R3 is doing nothing, just bypass it. Make R1 and R4 be 270 ohm and R5 1k, then its symmetrical which
feels much more sensible, and the LEDs get a healthy 15mA drive which is what most are designed for.

If there's any risk of mains level voltages you should not use a 2-channel opto coupler, use 2 separate
opto-couplers so you get the 0.3" board separation you need for flash-over prevention. (Having
said that there are some bidirectional 2-channel opto-couplers which are suitable, but most are
unidirectional).

Rego have one signal 3V3 and second 5V; one signal is inverted and second not.
With this information, would you reconsider your advise or should I keep current setup?

For the KNX uart though, I will need to redo it, because that connection isn't inverted, I've connected a Pro Mini to a KNX BTU before, and all I did was to cross the Rx and Tx, but this time I need to opto isolate.
So I will need to redo the circuit for the KNX UART I guess, because the signals aren't inverted...

5

I've tried to find some schematic for connecting the optocouplers in a non inverted setup, but haven't found anything useful I think.
¨
The only good schematic I've found is this:

1478×1210 17.4 KB

But In my application Rx and Tx should be crossed, while in this diagram Rx seem to be connected to Rx and Tx to Tx. Is it because it is a FDTI adapter?
Also I wont be needing CTS and RTS.

Theese are the wires I need to isolate:

1041×393 68.3 KB

6

That F***ing diagram looks like you've connected power and ground. Then you're not isolated and adding optocouplers to the Tx and Rx won't make any difference.

The common 6-pin "FTDI plug" does label Tx and Rx backwards, so that it matches what's written on the slave device. The FTDI transmits on Rx and receives on Tx. The thing it's plugged in to transmits on Tx and receives on Rx. Connecting two slave devices together will require crossing over Tx and Rx.

7

Is it just me or am I missing to top of the opto? In the start post you tell use nicely about the setup and then, boom, "So, I will need to have optocouplers". Huh, I see no reason why...

8

I also don't see the requirement for RX/TX isolation ... is it a long cable run? The heatpump relay could use an opto (unless SSR type).

Do you have a complete circuit diagram?

9

Hi guys,

First of all, thank you fr taking your time loking into this, it's much apriciated!

@dlloyd;
I'm just following schematics from a project where many people have used this interface with success, the main difference is that they are hooking it up to a laptop and using RS232 in combination with the MAX232 IC.
Unfortunatly, there isn't any circuit diagram what I can found for the REGO6xx controller inside the heatpump.
All I know is that Communication always using 19200 bps, 8 bit, no parity, 1 stop bit (19200/8N1). 5V CMOS 4000 (TTL) signals. Probably thats why the optocoupler is inverted in the schematics?

From what I have read, the cable on the TTL side in conjunction with REGO controller, must not exceed 50cm, then the link will be unstable, it's also in previous project stated that the REGO 5V isn't able to power anything but a couple of optocouplers.

I don't want to risk the REGO controller, without it my heat pump will not work, and the house gets cold. A new REGO is very expensive and is a very vital component to get the whole pump to work.

Pictures of the board can be found here:
http://www.nekropolis.cz/rego6xx/inside/

@septillion;
I tried to give as much info I have on the setup to make it more easy to get correct help, as stated I'm not good at electrical designs, hence I assumed I need optocouplers since people who have done this before have used optocouplers (and MAX232, because of RS232 interface to laptop)
I'm curious to know, to learn, Why I won't be needing any optocouplers?

@Morgan;
Thanks for the explanation, I know understand the Rx/Tx dilemma! So probably, this schematic from softsolder would work between the Mega --> KNX BTU after all, just revise a bit and connect Rx to Tx, and Tx to Rx instead.
If you are referring to the illustrated diagram with the Pro Mini, then this is not the one I want to opto isolate, because in that case the BCU powers the pro mini, and the only thing connected to the pro mini is temp/humidity sensor, the are on the same ground plane.
It's for this project, where I'm going to use a Mega 2560 with external power supply I want to avoid ground loops and magic smoke :slight_smile:

10

sx3000:
Theese are the wires I need to isolate:
[F***ing diagram removed to protect your eyes]

That's why F***ing is bad. It somehow seems to generate nice pictures of equipment you don't have, don't plan to have or just simply don't know.

dlloyd:
I also don't see the requirement for RX/TX isolation ... is it a long cable run? The heatpump relay could use an opto (unless SSR type).

Do you have a complete circuit diagram?