Interfacing hardware with a Nano

Hi everyone,

This is my first experience with the arduino, so please go easy on me. My background is mostly electrical, working on relay control systems and PLCs. I did a bit of C programming at college but that was a long time ago..... I've got some questions regarding my first arduino project, but so far they mainly concern the external hardware. If I've posted this in the wrong section, please accept my apologies.

I am building a rig to test some equipment at work. Originally I designed it with simple relay logic as that's what I know. However project creep has complicated things to the point where relays are no longer practical. That's where the arduino comes in. I'll be using a number of digital ins and outs, and I've chosen a Nano for its small footprint. I am going to use a ladder logic interface (e.g Soapbox) as that's what I'm familiar with. I appreciate that it's not as efficient as C++, but it will allow me to get it running now. I can always reprogram it later once I've learnt a bit of C++.

On with the questions....

I intend to power the Nano (and some other components) with either a 12Vdc or 9Vdc PSU. I've read here that 9v is recommended because it's not as taxing on the Nano's regulator. Is this true?

The equipment to be tested includes a small 110Vac motor of around 2W or less. I intend to switch this motor with a double-pole solid state relay (appropriately rated, of course) driven by the Nano; any potential problems with this from the arduino's perspective?

I understand that the arduino's inputs are most reliable when used with the internal pull-up resistor. I want one of my external input switches to also illuminate an LED; can I use the LED and its associated current-limiting resistor as an external pull-up? I don't think I've explained that too well so I'll post up a diagram later if necessary.

I need to use 2 outputs to provide sink inputs / switching on the equipment being tested. This can of course be achieved using an NPN transistor driven by the arduino output. Is a transistor the easiest way of doing this, or does the Nano have some kind of built-in facility for this? Nothing I've read suggests that there is, I just wanted to confirm.

Thanks in advance!

9V vs 12V - 9V is better, 5V regulator will run cooler.

Nano probably needs to drive a transistor to sink the relay coil current. Solid state, need to see a spec first.

LED/current limit resistor to input pin, pin to Gnd to read low when switch is closed to turn on LED, should be fine.

Outputs controlling transistors to sink current is safest, then don't need to rely on Arduino's 5V and 20-25mA current limit per IO pin.

Thanks CrossRoads, that's put my mind at ease.

With regards to the relay, I'm having difficulty finding exactly what I'm after in a solid-state. I might just go with a mechanical relay driven by a sink transistor, at least for now. Thanks

You can buy relay modules with the transistor included.