Schematic with unmarked parts

Ok, so my friend gave me this schematic, and almost all the parts are unlabled, except for a few chip things and some other stuff. None of the resistors or capacitors are labled. Is there anyone who can magically figure out what value these components are? Oh, and the function of this thing is a PC Serial motor speed controller.

If you check the datasheet of each of the IC's you will find that the parts are standard values used in the manufacturers reference design. For example, the Maxim level shifter uses 10uF caps for C6, C7, C8.

The "designed" parts are given values on the schematic.

ah, thanks!

Wait are you sure they are 10 uF? The datasheet says 1.0uF. And it seems that all 5 caps it needs are 1.0uF. Is that correct?

My bad, the datasheet does say 1.0uF for a MAX232, but the MAX232A uses 0.1uF

datasheet: https://www.electro-tech-online.com/custompdfs/2006/07/MAX220-MAX249.pdf

Ok, now for the Atmel IC, on page 26 of the datasheet, it gives me about 4 different frequency ranges for the C1 and C2 in Figure 12, but I don't know what the CKSEL3..1 thing is. Which one do I use, and what would the uF of each capacitor be, within the range of 12-22 pF?

Datasheet: https://www.electro-tech-online.com/custompdfs/2006/07/doc2543.pdf

Isn't 6V alittle low for a 7805?

Honestly, I didn't design any of this, I just wrote the program that interfaces with it. The guy who did design this didn't have time to fully explain it, and this is all he gave me. He's tested it, and it works, so I guess 6V is fine. Currently, the only parts I've figured out are the ones attached to the Maxim chip, IE the 5 caps there, which are all 1.0uF.

R1-R4 look like "paranoia" resistors - just put them down as 1K, or just get rid of them (i.e. short them) if you're in a hurry.

R5, C5 aren't necessary if all the brownout reset stuff is enabled on the AVR chip.

You'd better know the frequency already, and the caps depend on the crystal, but you probably won't be too far off if you put down 15pF or so. If the caps are off, the clock will be off by some PPM, but unless this is for a clock, it won't matter.

As for the caps, C3,C4 seem odd. Typically one uses some .1uF ceramic caps or something similar for interference reduction, but these aren't even close to critical.

D2,D3, the LEDs seem to be labeled oddly - just pick your favorite color, and it'll work.

I guess that's about it.

Wow, thanks! So, where would the whole PC serial plug thing go? I'd need a D-Sub female port somewhere, but I'm not quite sure where. Is it on the 8 or so empty pins on the AVR chip? Also, unless the brownout thing is enabled by default, the assembly code for the chip I was given didn't enable anything with the brownout. What would the values of R5 and C5 be then?

The RS232 connections go to the Max232 chip. Dig up the datasheets over at www.maxim-ic.com, and follow the connections. You probably only need to hookup the TX, RX, and gnd(shell/shield) connections.

The brownout settings on AVR's are in the flags - if you use the Atmel programmer, there's an entire panel dedicated to the settings, just enable one of the brownout settings.

If you really want to dump something there, just throw another of the .1uF caps and maybe a 10K resistor. (The only reason is that some device programmers might be unhappy with a lower resistance).

BTW, Fusion's comment about the 7805 is a warranted - Typically 7805's need ~2V to function properly - a 7805 would really like to have a 7+ volt input. The easy fix is to replace it with a "LDO" (Low Drop Out) type regulator. I think the LM2940 is the "standard" replacement, but it's current limit is 1A, and it is a buck more expensive than a generic 7805. There are *piles* and *piles* of LDO's, but they usually come in different and smaller packages. I'd suggest you wander over here:

https://www.digikey.com/scripts/DkSearch/dksus.dll?KeywordSearch

and look under the "Voltage Regulators" section. - and yes, Digikey has over 19000 different regulators listed. So here's how you find what you want:

You need a 5V regulator.
You need a regulator that has an output current > 30mA
(keep in mind that regulators with higher current limits tend to be bigger, so just shift-click on some set of output currents)
and finally, you probably want a regulator with a "TO-92" style package (i.e. it looks like a normal small through-hole transistor, instead of a big TO-220 tabbed power transistor).

Incidentally, Digikey's also a pretty good place to get datasheets - do a search on MAX232, and click on the "technical/catalog information" link, and presto, instant link to a datasheet.

Digi-Key cannot seem to find any of the type I am looking for. I just selected the feature "Low Dropout Voltage" and the Case type of TO-92, and it couldn't find anything. But thanks for all the help with the other stuff, I didn't know it would be possible to identify all the missing parts here! I've been trying to figure it out for days

Also, does the voltage of the caps matter? Given the output voltages on the MAX232 of 10V and -10V, would the caps need to be 10V and -10V as well?

I use the LM2931AZ-5.0 low dropout 100mA max regulator in a small TO-92 case from Digikey for all 9V battery projects so the 5V output is still regulated when the battery voltage falls to 6V. It needs a 100uF electrolytic at its output for stability.

OnionRingOfDoom said:

Also, does the voltage of the caps matter? Given the output voltages on the MAX232 of 10V and -10V, would the caps need to be 10V and -10V as well?

Click to expand...

It's preferred to round up -it's probably easier to just get all caps rated for 16V just so you don't have to worry about anything. (And, there are no "-10" voltage rated caps. Mind you, if you plug in a polarized capacitor(i.e. any capacitor that has a polarity stripe on it) backwards, it'll be damaged and probably start leaking).

Does anyone even make 16V 1.0uF polarized caps? The lowest I see is 160V.

Electrolytic alum. capacitors ratings should go all the way down to 6V. Polarized tantalums ratings go down to 2V.

Just search for "Electrolytic" and it should give you the listing you want...

Ok, thanks! One last thing: where's the other end of the power supply go? I see it says 6V up there by the switch, but there's no other end of it. Does it go to each of the other outlined triangles that arn't specifically marked as grounds?

Well, the positive side of the power supply goes to where the +6V is marked, and the negative side of the power supply goes to ground / all the down pointing triangles.

Incidentally, if you are going into this with this level of schematic reading knowledge - I'd get your friend to double check your wiring before powering anything up. It's likely to be a hassle if you end up blowing up parts...

Each and every ground triangle thing? Someone else told me it was just the common ground one on the LM7805.

yes, every triangle... I'd suggest picking up a book called "The Art of Electronics" by Horowitz and Hill, It's a good non-technical intro, and an easy read, plus there's enough in there to cover nearly any hobby topics.