After a few months of work, I have gotten a project doing exactly what I want it to do. It's pretty simple, it's an Uno with a sketch, plus a sensor, a RS485 board and a couple other components.
Without saying exactly what it does, it's for a specialized application, and I'm part of a community of people who are enthusiasts in this area, and others have expressed interest in using my application too. I'd be happy to make my design available to them at my cost. I estimate there are about 100 people who would want one.
What I'm not excited about is teaching people how to use Arduino starting from zero, nor am I excited about hand wiring a bunch of boards, nor debugging them when the handwired connections prove unreliable. What I'd rather do is get a service to put my circuit onto a single board, and give my friends a black box that's all set up for them.
At this point don't order inventory for 100 units.
Garage Shop it. Buy enough for 10, get firm commitments from your 'customers' and see how that goes.
Laying out a PCB and sending that off to oshpark will be the easiest part.
I looked at the Oshpark website, it seems like they produce PCB's, which means I'd still have to assemble the circuit myself. If going that route it seems like I'd want to make a shield to snap onto an Arduino, which is something I could make a prototype of using a prototype shield.
What I'd prefer is someone who can also handle assembly. Ideally, I'd like to be able to download a PCB design for an Arduino circuit, then modify it to include maybe a half dozen additional components and send the whole thing to someone who could make me an assembled PCB. Is that possible? Would it be feasible for small numbers?
I've already offered the code, but this isn't a technical group. One member took me up on it, followed my instructions and couldn't get it to run. I don't want to do that level of hand-holding.
There are Chinese PCB companies (PCBWay, JLCPCB, etc) that will do assembly as well as making the PCB. People have said good things about them; I think it will get really complicated if you use any parts that aren't in their stock...
I think they're supposed to be "reasonably affordable" in the 50+ boards range. (I've never made anything that "successful", so I wouldn't know. They make nice PCBs. Hopefully they'll still be accessible after the next election. ) At 100 units, you may need to deal with import tariffs and such.
I have used them extensively and have had no problems. They will also assemble your product, many times free at the start. Cross your BOM (Bill Of Materials) with what they have. If possible go to there parts. Shipping can be selected where you can judge time to cost. Everything I have done with them was with Gerber files generated by KiCad. You upload and in a few seconds get a view of your potential board.
OK, I looked at the JLCPCB website, they seem to be what I'm looking for. It's kind of unbelievable to me, the pricing: $2 per board and $8 for assembly? Sounds great.
Now I'm thinking the way to go may be to use a Nano and just get a board made with a socket for it and the other components I need. IE, rather than trying to replicate the Arduino circuit treating the Nano as a component.
You could design and order the pcb with an IC socket, then you can just plug the arduino nano on it, without soldering.
And the rest of external connections with headers or pcb connectors. So mounting it would be easy, just plug in each the component in the right place. They can even do the plastic housing, I think.
The quality for me has always been top notch. They give a tour on there web site of part of their manufacturing process, it is unbelievable. About 2 months ago I forgot to put the drill file in the sip upload, they notified me and I uploaded them the next day. Delivery was not impacted. Probably just luck.
I've done pretty much that for one of my projects. I did up a simple 'carrier' board. Header sockets for the Nano and a MAX485 board, and header pins for plugging in several optional add-ons - keypad, I2C for LCD and MCP23017 expanders, screw terms for power in, a spot for a buck converter to power the board if I want. Worked very nicely for me, in about a 3"x3" footprint.
You could also incorporate the MAX485 circuit completely on the board, so you'd only have the Nano plug-in. If you do that, remember to add a jumper to disconnect the 120 ohm terminator for the RS485 loop. I've had to desolder the termination resistor from most of my MAX485 boards, because they come with the resistor in place and no disconnect jumper. Doesn't matter if you're only putting one or two nodes on the loop, but the more you add the more you affect the loop performance otherwise. My next version of the carrier will incorporate the MAX485 design onboard, for that reason.
This sounds doable. My board needs to fit into a 2"x2.75" space and the Nano is 43mm long by 18mm wide which is 1.7" x 0.7 so I could mount the Nano sideways and have about 2"x2" for whatever I want to do.
I have a similarly low-volume product for a niche market and that's the approach I took. Reduces inventory cost and easy to assemble. I built them by hand, but I'm used to assembly so it wasn't a big deal. Doing it again, I'd probably use a contract assembler and save myself the time.
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An alternate approach that I take when I have to do very low-volume (under 20 units) is to look for something similar off the shelf that I can reprogram to do what I want. There's a huge variety of arduino boards and arduino-based products and I can often find something that's "close enough" so I just need to e.g., add a sensor or a relay or something and I don't need to lay out a PCB, buy parts and assemble a full board, etc.
Good point, @cedarlakeinstruments
I have several of these: https://www.amazon.ca/gp/product/B08198MSJ2/
repurposed for N-servo control. What one needs to do is use a 5V power supply, and add a wire from the barrel Jack center pin direct to the 5V on the board, bypassing the input diode, and the Vin - 5V regulator on the Nano, due to it's limited current output. It's a hack, but quite useful.
There is a part I used a several years back called Massduino It is about the same as a UNO but it mounts sideways and plugs into a connector. It is basically a daughter board. You can find them on line.
They make nice PCBs. Hopefully they'll still be accessible after the next election. 