What's considered the maximum number of parts on a single page?
Depends on the size of the page. You can fit more on a D-sized plotter sheet than a B-sized (roughly A4) sheet. Don't crowd things to the point it gets difficult to read.
What to consider when making a schematic multiple pages?
Almost all my designs end up as multiple sheets. Sometimes the manufacturing guys cut them all up and paste them together in one big plotter sheet to make it easier to follow the signal flow. But normally I don't print out bigger than 11x17 so I work at that size.
Something you didn't ask: I tend to make the first sheet be the critical input and output connections of my circuit, and work up towards more complex circuits on later pages. Other people like to put the critical signal path parts on the first page, and the input and output connections end up deep in the stack of schematics. I'm not sure which is really better.
When should I consider putting multiple tracks into a buss?
I rarely do this, but its a matter of style (and convention in your workgroup).
How should I name busses, netlists, and the references to other pages?
I tend toward all-caps net names, but otherwise I don't have fixed rules. More disciplined organizations might have more detailed rules.
How should I place components to minimize the number of nets?
I prefer to place components to make the signal flow clear. I don't worry about the number of named nets.
What kind of comments should I include on a schematic?
Anything important for the layout guy to know (matched length traces, place bypass caps near ICs, etc.) Anything a future engineer might need to know if they're looking to replace an obsolete part. Non-obvious critical specs like higher-than-normal resistor power requirements or tight tolerances. Anything that has to be tuned in production (Like "tune pot to achieve 50% duty cycle" or whatever).
Where should I place the designation and value for horizontal and vertical components? Does it matter as long as I stay consistent?
I use vertical text for vertical components to allow more parts to fit cleanly on a sheet. Others (apparently) consider this a grave sin. Be consistent and be consistent with others in your organization.
Should I note component packaging & rating on the schematic? Meaning discrete vs SMD or if a specific resistor is high powered?
Specifying the package type for each part visibly on the schematic would be clutter. But obviously that information has to be in the design to get transferred to layout. As mentioned above mention nonobvious specs that might trip someone up if they have to replace an obsolete part or find an alternate vendor due to a shortage.
Your BOM (Bill of Materials) will need to specify an exact manufacturers part number (or a list of acceptable alternates called an AVL "approved vendor list") for each part.
Should I customize nets in different colors or widths?
I don't recommend this. I'd prefer to get schematics that make sense if printed out in black & white.
How should I version control schematics?
I store datecoded backups (like "mydesign_20120205.zip" on my own pc and a remote share drive. Definitely store a backup whenever you release a design (either to layout or to manufacturing).
Edit: There are better ways to do this (see comments) but a simple process like dated zip files is also perfectly workable.
What workflow should a single person use to keep designs organized?
Keep backups. Use all the tools you have available. If you aren't doing your own layout, keep good communication with the layout guy.
Kudos for using a refdes (reference designator) for (most) components. Especially if you want to discuss a schematic they're needed for decent communication.
The power supply
- you use the refdes "L1" and "L2" for the LEDs. Don't. "L" is the standard designator for inductors. Use "LD" or "LED" or, as I do, "D" for diode.
- the value of R1 is too low. It will give the LED 45 mA which is too much for an indicator LED. Increase the value to 560 Ω and you'll have a safe 18 mA; they're usually rated at 20 mA. Check the datasheet. By the way, do you really need that LED? It will always consume power.
- C1 and C2 are indicated as "10 mF", where I presume they should be "10 µF", that's a factor 1000 difference. They'll most likely be electrolytic capacitors, which are polarized. Use a symbol which indicates the polarization and clearly indicates which is the positive side. Also for electrolytics it's good practice to mention the voltage in the schematic as well. C1 should be at least 20 V, C2 10 V.
- Place a 100 nF parallel to C1 and C2
- draw C2 closer to the regulator's output than the LED. Electrically it makes no difference, but that's how you should place them on the PCB. The 100 nF should be closest to the output.
The microcontroller
- the ATmega328 doesn't have a VREF pin. That should probably be Vcc. Add a 100 nF decoupling capacitor between Vcc and ground, as close as possible to the pins. Always decouple an IC's power supply.
- Reset is connected to ground. That's OK if you use the internal reset circuitry, but don't forget to program the the RSTDISBL bit to "1".
- you can't drive a speaker directly from an I/O pin. You'll need a transistor there.
- you can save a resistor if you use the internal pull-up of PC0 and connect the switch to ground. R4 won't be needed then. Remember that the logic will be inverted.
- same for PB2 to PB5 and switches S2 and S4: internal pull-ups and switches to ground instead of +5 V.
- switches S2 and S4 are confusing. You have 2 contacts on the lower side, and 5 on the high side. Are they supposed to be change-over contacts? If so, you won't need that: one input will always be complementary to the other, so you'll only need one. In any case the lowest of the pull-down resistors serves no function.
- I would use more descriptive names for the nets on Port D, like "Digit1", "Digit2", etc.
The display
- Again, decouple the power supply with a 100 nF capacitor.
- the resistor values for R4 are way too high. Swap them for 150 Ω types.
- the 5 R5 resistors can be dropped. They serve no function.
- the microcontroller can't drive the display common cathodes directly: with all the LEDs on you'll have 7 \$\times\$ 20 mA = 140 mA, that's much more than an I/O can sink. You'll need 5 NPN transistors here, or a transistor array like the ULN2803.
Conclusion
This is a long list, but I think you did a fine job, considering it's your first project. I've seen much worse schematics. Success!
edit Re the update of the question
Your circuit around Q1 and D3 is not quite OK: the battery will feed the LED, but not the rest of the circuit. I'm not sure the LED as battery indicator is a good idea: especially with battery power you have to be economical, and not waste power on a LED.
How about this: keep the diodes like in your first version, but control the LED from the microcontroller. Use one of the free pins to detect the presence of the 12 V through a 5 V zener diode and a series resistor. You can then blink the LED when you're running on battery power. A short flash once every second is much more economical.
Best Answer