Hi.
I want to buy this relay board : sainsmart 16 relay board .
It is stated to be compatible with 3.3v board, but I have question:
I ask for the datasheet, and now I have the schematic.
The fist level of connection is an optocoupler, but it is hook to 5V supply:
5V --> resistor --> led --> GPIO
The optocouple have a typical foward voltage of 1.24V. That leave 0.46V over the 3.3v... Is that not good for the GPIO ??
Thanks
Nitrof
Be warned that 16-channel boards have a design fault.
Opto couplers, but NO user selectable opto isolation.
Only ok if you don't need opto isolation, otherwise better to buy two 8-channel boards.
Opto VCC must be powered from 5volt.
Opto LED and indicator LED drop 1.2+1.8 = 3volt volt together, so the Arduino pin never sees more than 5-3 = 2volt.
16 relay 2.pdf (55.1 KB)
Hum, I'm confuse.
If I look into the shematic, the indicator led is on the 12V side...
For the opto isolation, I plan to use the same dc supply for both the relay board and the arduino... should have been better but...
You're right. The 16-ch version has it's indicator LEDs on the ULN output.
2,4,8 channel boards have it as I said.
Sainsmart also falsely states opto isolation, despite me telling them it's not.
Opto LED 'hot' is permanently connected to the 5volt output of the built-in buck converter.
That puts 5-1.2 = 3.8volt on the IN pins. The optos could be partially on when they have to be off.
Just give it a try. You can always add a 1N4148 diode in series with the IN pin if needed.
Leo..
I think I will go with 2 time 8 relay instead... they should review there design...
Thanks a lot.
Nitrof
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