32 Channel Relay/Solenoid Driver - MCP23017 ULN2803 

if it ends up giving problems at least it will be something fun to debug and I could use a spare board populated only with digital to control isolation.

That might be attempted in the future but not one of the near term objectives. I would need a lot more parts so with the supply chain maybe that would be half a year away at the earliest!

Adding MIDI to this board would be trivially eeasy...

There may just be some midi connectors coming up in a mailbag...

Yeah it should work because the multiplexer is just acting like a splitter cable, passing the I2C signals through when the channel is selected. Of course all other I2C devices on that same selected bus need to be operating at the same voltage, but that's the benefit of the multiplexer, I think you can use some channels at one voltage and other channels at a different voltage and then talk to the main controller at whatever other voltage it wants to see.

@@GadgetReboot Hello Gadgrt7, thank you very much for your feedback 😁 😊. I'll try it then. Have a nice week and happy new year 😄

If the 23017 pin is configured as input and is connected to a button as well as a relay driver output control pin, the relay control may go on and off as the button is pressed but if there is no relay on that driver channel, that should be fine. The real problem, which I haven't tested, is the relay driver chips have some internal resistors from the control input to ground, so combined with the 23017's weak internal pull up of 100k (if enabled when it's an input) may not allow the 23017 to actually see a logic high input with the relay driver connected to it, so if there is a button to ground on the input and a pull up enabled, the input may only go from 0v (low) to 0.5v or so (high). So maybe it would be better to not use internal pull ups and then use an external pull up of 10K or 4.7K with a button to ground and it may allow the input to reach a valid logic high with the relay driver's resistors to ground being present. Or if the input logic can be reversed, use the input button straight to VCC and then a pull down resistor, so the 23017 sees a proper logic high and then the logic low is also unaffected because it's a pull down in parallel with the relay driver chip's built in pull downs. In fact maybe an extra pull down isn't needed because the relay driver pull downs will do the job.

@@GadgetReboot I am reading the voltage at the 23017 pin using one of the Arduino analog input pins, with a momentary switch connected between the pin and ground. With 23017 pin configured as INPUT_PULLUP I get ~1.1V when the switch is open, 0.0 with the switch closed. With the pin configured as INPUT, and no external pullup, I get 0.0V whether switch is open or closed. With the pin configured as INPUT, using an external pullup of 10K, I get ~2.4V with switch open, 0.0 closed. With 4.7K; 2.8V open, 0.0 closed. 3.3K; 3.18 open, 0.0 closed. Search for Arduino logic high/low voltage levels suggests minimum of 2.9V for logic high.

I think everyone could use one of these. I hope it will be a while until I need to get a bunch of transistors diodes and resistors on a breadboard for quick experiments!

I suspect that there is a need for every enthusiast to have module boards, such as this one in their junk box. Something tested and documented that can be pulled out. "I need 32 output pins on my Arduino Nano project"

if a lot of input switches are needed along with a lot of output controls for relays and this board is used for the outputs on I2C, an additional design using the same MCP 23017 GPIO expander could be used to read as many switch inputs as there are relay outputs on this board. The extra chips were just connect to the same I2C bus. in the first video I did using this chip I had some inputs as a demo. ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-74DgM2nAeLo.html

@@GadgetReboot Hey! Thanks for responding to me. I understand what you are saying and I would be happy to maybe even the possibility of commissioning (paid of course) this project to you if that might be something you'd be interested in doing. Aside from the hardware boards themselves, I would definitely need help with any code needed. Please let me know as this is def something I would like to tackle sooner rather than later.

I currently can’t take on extra outside work but I do think it would probably be a good companion PCB project for me to do as a video project using that same chip to do switch inputs to pair with The existing board doing all relay outputs and of course then I would have sample code as a demo so if all goes well, I could have a video with a working project before the end of this month.

@@GadgetReboot Hey! I understand and really appreciate your efforts. Would you be willing to allow me to give you a better idea of exactly what we would like to accomplish and be able to do so that way maybe you could address those specifics in your endeavor, or at the very least point me in the right direction or maybe even recommend a course of action for us to take? Lmk! Thanks again!

it wouldn’t hurt to hear a description and if certain things make sense to implement on what I will be doing, I can consider that or else give any feedback I may have

What's hard about it? I think part of it is that there are a ton of options but literally the only 2 I use are the board quantity (I keep going up until my price does) and I check the little box that says they can use gold if they're already running it and I leave everything else alone.

@@AnotherMaker I was referring, rather, to their web site...took me 10 minutes to figure out how to enter my shipping address.

@@spillagonner ahhhh. That makes sense

@@spillagonner You didn't know yer address? Yer on yer own there Bud.