Serial optoisolation
- Thread starter Iain_C
- Start date
If this connects directly to a PC serial cable, and assuming I'm correct that opto isolators have low R when LED on, high R when LED off ...
For RX/Serial In - When the RX line is high, Pin 2 must also be high. RX high, LED lights, transistor becomes a low R, Pin 2 pulled high - Looks like that is okay. With +12V on the RX line you may want a larger than 200R current limiting resistor.
For TX - When Pin 7 is high, TX line must also be high. When Pin 7 is high, LED off, transistor becomes high R, TX floats - That's also okay but you'll probably need a pull up resistor to +5V/+12V for the PC to register a high.
For RX/Serial In - When the RX line is high, Pin 2 must also be high. RX high, LED lights, transistor becomes a low R, Pin 2 pulled high - Looks like that is okay. With +12V on the RX line you may want a larger than 200R current limiting resistor.
For TX - When Pin 7 is high, TX line must also be high. When Pin 7 is high, LED off, transistor becomes high R, TX floats - That's also okay but you'll probably need a pull up resistor to +5V/+12V for the PC to register a high.
Thanks both.
Indeed, it doesn't currently work for me on breadboard.
I expected the Tx side to be fine as it's taken from Wilf's Universal IO schematic - Wilf, care to comment on lack of pull-up in that circuit?
Edit: Aha - perhaps you never intended that circuit to communicate with a PC
Is there another way I can get galvanic isolation?
Indeed, it doesn't currently work for me on breadboard.
I expected the Tx side to be fine as it's taken from Wilf's Universal IO schematic - Wilf, care to comment on lack of pull-up in that circuit?
Edit: Aha - perhaps you never intended that circuit to communicate with a PC
Is there another way I can get galvanic isolation?
Last edited:
andrew_qld
Senior Member
Talking of programming through opto's, this Reminds me of a commercial SMS alarm sender we use at work. It has 6 opto inputs and one of them is set up for programming the micro (an 8051) over 2 wires using proprietry software.
The "special" opto is a high speed one.
I have thought about using this idea in some Picaxe projects. I always liked the idea of protecting the poor innocent Picaxe from the nasty outside world with Opto's on all its inputs, even the programming interface.
Mind you, unless the opto's are socketted (or otherwise easilly replaceable), if you kill one opto you disable the whole board regardless of weather the processor survives.
The "special" opto is a high speed one.
I have thought about using this idea in some Picaxe projects. I always liked the idea of protecting the poor innocent Picaxe from the nasty outside world with Opto's on all its inputs, even the programming interface.
Mind you, unless the opto's are socketted (or otherwise easilly replaceable), if you kill one opto you disable the whole board regardless of weather the processor survives.
Also lost before the forum burped, my reply that I need full isolation for my EEG project
I'll try a couple of CR2032 (BIOS batteries) in series with the serial Tx line - many thanks for the great suggestion.
Andrew, the opto's are more for user protection than picaxe protection - good point though, I'll socket them, then they're perfectly welcome to blow if they feel the need to!
I'll try a couple of CR2032 (BIOS batteries) in series with the serial Tx line - many thanks for the great suggestion.
Andrew, the opto's are more for user protection than picaxe protection - good point though, I'll socket them, then they're perfectly welcome to blow if they feel the need to!