Quick questions about a schematic

Other Hardware General Electronics
1

Hi,

So I have a device I'm trying to reverse-engineer and part of it is a small I/O board consisting of a 74165 and 595 with some LEDs and connectors for buttons. Earlier today I poked around with a multimeter and made this schematic out of it:

(edit: This is much smaller than I assumed it would be, imgur link here: Imgur: The magic of the Internet)

J3 and 4 are input headers for three buttons each, J5 connects this board to the main microcontroller board. I have a few questions because it involves features I've not seen before.

  1. R14, 15 and 16 (bottom middle-ish) - Why are these outputs pulled down like this? I've only ever seen outputs left floating. The PCB has absolutely no room for more LEDs so I'd be surprised if they're a placeholder for future redesigns, since the board would need to be completely redesigned. In the software these outputs will never go high, so I'm puzzled by this.

  2. R18 - I understand this is a pull down, but is it necessary to pull an output down like that? Surely a pull down near the microcontroller's input pin would make more sense for noise immunity, etc.? Is it possible to have a pull down at both ends of the line? I haven't checked the main board yet.

  3. C13 - Given that this is to decouple AC noise from the DC supply to prevent resets I just kind of assumed a ballpark figure of 100n (since SMD caps are hardly ever marked). Is there any way I can determine the value of this cap without desoldering it? Is there a sort of rule of thumb about input decouplers such as this or do they follow the same rules as power supply decouplers?

  4. It's been a couple of years since I've done any transistor theory, but: R21 and 22 struck me as being a bit high for base resistors, at 500uA I guess they're not but I'm sure I've seen lower values. Assuming an hFE of 100, does that mean Ic would be limited to 50mA? With two LEDs in series, each dropping between 1.8 and 2V, would the calculation for the current through either LED (for D1-8) be (Vcc - (Vf1+Vf2))/R? i.e. (5-4)/75 = 13.3mA?

  5. D1, 2, 3 and 4 - Any idea why these LEDs are connected to Vcc and switched on the cathode while the other six are connected to ground and switched on the anode? I can't work that one out. D1-4 are green, D5-8 are yellow and D9 and 10 are red, if that makes any difference - that's why R13 is a higher value too, I assume.

I think that's all for now, everything else makes sense. Thanks!

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  1. Weird. Could those pins ever be inputs? People sometimes avoid having floating inputs.

  2. Unsure what you're commenting on here.

  3. Oddly high, but should be fine, if those are normal indicator LEDs. Probably trying to limit the number of resistor values they use to make it cheaper to build.
    Typical LEDs are 5~20mA. Red LEDs have a lower drop than green or blue. Green leds have lower drop than blue.

  4. Odd. How well would the chip source 10-20 mA at a high enough voltage to light both LEDs? Maybe that's why? Really, what stands out to me is that they used a transistor for some of them, ran others directly....

Strange board - lots of things done oddly.

What is the board supposed to do?

3

  1. Odd. I agree. Unless they are trying to keep the load on the device stable as these outputs MAY get values shifted into them.

  2. No. You could want the pull downs local to the Shift register as these are the inputs that could float. Not an issue.

  3. I agree with previous comment

  4. Maybe to control total package dissipation, which has a hard limit that gets exceeded if all were on... but does not mandate using a transistor for all. Saving on parts count always wins you points. Different values only mean that different LED's have different current needs for similar brightness. RED usually needs much less current than other colors

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  1. C13 - Given that this is to decouple AC noise from the DC supply to prevent resets I just kind of assumed a ballpark figure of 100n (since SMD caps are hardly ever marked). Is there any way I can determine the value of this cap without desoldering it? Is there a sort of rule of thumb about input decouplers such as this or do they follow the same rules as power supply decouplers?

No, its not to decouple noise. Pin 10 is the master reset, which is active when low. The R17/C13 is 10K/0.1µF which is the RC time constant for POR (Power-On-Reset).

Maybe there's another page to this schematic because no decoupling capacitors are shown.

Q8 and Q9 are very weakly driven ... R21 and R22 should probably be around 1K.

The 10K pulldown resistors on unused CMOS outputs serve no purpose.

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DrAzzy:

  1. Weird. Could those pins ever be inputs? People sometimes avoid having floating inputs.

No, its a 595 shift register and those are Q5, Q6 and Q7 - outputs only. I'm glad I'm not the only one foxed by this!

DrAzzy:
Strange board - lots of things done oddly.

What is the board supposed to do?

It's taken from a Logitech G27 racing wheel controller, specifically it's the board for the eight buttons and the ten tachometer LEDs in the steering wheel itself. The 165 converts the parallel button states to serial while the 595 converts a serial byte into parallel LED states.

pwillard:

  1. Odd. I agree. Unless they are trying to keep the load on the device stable as these outputs MAY get values shifted into them.

Perhaps, but I would've thought having them float would be better in any case? And if not, 10k resistors mean 500uA which is significantly less than the load on the other five pins. Also, having spent a fair bit of time analysing the USB data that controls these LEDs I've never seen a value that would cause them to go high (the byte that controls the LEDs is strictly only ever 0x01, 03, 07, 0f or 1f).

pwillard:
4. Maybe to control total package dissipation, which has a hard limit that gets exceeded if all were on... but does not mandate using a transistor for all. Saving on parts count always wins you points. Different values only mean that different LED's have different current needs for similar brightness. RED usually needs much less current than other colors

True, I hadn't thought of that. I'll read up on the 595's datasheet and see how things add up. It might be worth noting that given the sequence in which the LEDs illuminate (outer to inner), the outer pair (the two sunk by the transistor) are on longer than any of the others since they come on first and stay on until the 'engine speed' drops after changing gear, at which point all the LEDs turn off again. Perhaps it could've been done for reliability purposes?

dlloyd:
No, its not to decouple noise. Pin 10 is the master reset, which is active when low. The R17/C13 is 10K/0.1µF which is the RC time constant for POR (Power-On-Reset).

Maybe there's another page to this schematic because no decoupling capacitors are shown.

Q8 and Q9 are very weakly driven ... R21 and R22 should probably be around 1K.

The 10K pulldown resistors on unused CMOS outputs serve no purpose.

I drew this schematic from my observations of an existing PCB, there are two caps not shown because the software I used doesn't include supply pins on ICs. Of course I should've caught the RC network there, I kind of figured it was local decoupling just for that pin but a reset when powering on makes sense - I seem to recall that 595s are volatile but also full of nonsense when you first power them up, I think? That's going back seven years now, though...

Q8 and Q9 - is it possible they're being driven just enough to function to improve reliability/lower the risk of failure?

Is it possible this board was designed by the work experience kid? It's marked REV.005 though...

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Q8 and Q9 - is it possible they're being driven just enough to function to improve reliability/lower the risk of failure?

... possibly to drive the transistors just enough to adjust LED brightness, but not for reliability reasons. Probably not for brightness either because 10K is a common value they've used.

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did you try to reproduce this schematic in practice? I have the g27 and I don't have the original board because I use a personalized steering wheel, I was trying to find a schematic to reproduce the pcb in another format for my steering wheel, I'll try to reproduce this pcb with your schematic and return if successful, thank you very much by sharing this schematic!

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