6 ms·

How to Read a Schematic

113 points by iuguy 9y ago

netvarun 9y ago
One trick I learned at university while analyzing a circuit is to identify the various 'design patterns' in it - just like software engineering, circuit design also features a lot of recurring common patterns. Here is a fantastic post on it: http://www.arachnidlabs.com/blog/2013/10/17/electronics-patterns/ http://www.arachnidlabs.com/blog/2013/10/17/electronics-patt...
- cjsuk 9y ago
  Exactly this. The first step for me is usually to redraw out the messed up crap the last person drafted as things that look like the design patterns. Even Tektronix and HP can produce some dire schematics so when you've got your head in some old kit I'd probably spend more time deciphering the service manual schematics than actually fixing it (something I do as a leisure activity)
- iuguy 9y ago
  Oh wow, that's a really good find, thanks!
Animats 9y ago
That's a breakout board - mostly connectors, little functionality. Not too interesting to study. A simple blinky board with a 555 timer would be a better place to start. Here's one of my schematics[1] with a detailed explanation of how it works [2] This is a small board which is used to power old Teletype machines. It's a mixed analog/digital board, with a custom switching power supply onboard to provide the high output voltage needed using only power from the USB port. This gives some insight into why modern power supplies have so many parts. They work by creating big spikes, and they're always a few microseconds from a short circuit. So they need bypass capacitors and ferrite beads in the right places, and protection circuitry in case something fails. (MOSFETs tend to fail into the ON state.) If you really want to learn this stuff, get "The Art of Electronics", by Horowitz and Hill. [1] https://raw.githubusercontent.com/John-Nagle/ttyloopdriver/master/board/ttydriver01.png https://raw.githubusercontent.com/John-Nagle/ttyloopdriver/m... [2] https://github.com/John-Nagle/ttyloopdriver/blob/master/README.md https://github.com/John-Nagle/ttyloopdriver/blob/master/READ...
- sleavey 9y ago
  +1 for using KiCad to draw the schematic - it's important to use open source software when you're developing open source hardware. The licences for Altium, Eagle and co can cost many hundreds or thousands of dollars/euros/pounds a year, which bars entry to a lot of hobbyists. Improving what is already a fantastic piece of software (KiCad) is a big priority in my view. It's good that places like CERN are getting behind it.
  - Nanite 9y ago
    Eagle has a free licence version, If you're ok with 2 layer pcb's and fairly small boards, which would suit quite a lot of hobbyists. KiCad was in need of some UX love back when I looked at it 2 years ago, the feature set is definitely impressive though, maybe time to revisit it in the near future.
    - hugs 9y ago
      This is why I used Fritzing, not KiCad to design my first board. KiCad's interface was hard to get used to, even though I understand that KiCad is more powerful. It would be awesome, however, if either KiCad became more beginner user-friendly or if Fritzing became more powerful.
      - wanderingjew 9y ago
        You should not use Fritzing. It is a bad tool. This isn't a vi vs. emacs argument, either: Fritzing is like using a crescent wrench to drive a nail. Yeah, you can do it, but it's the wrong tool for the job. Schematic capture in Fritzing is, in short, terrible. You can only draw straight lines between nodes in a schematic. Furthermore, the design philosophy of Fritzing schematics is, 'wires between pins' instead of nets between pins. That is, you can't 'tap into' a wire, like you could with a net. It's truly idiotic if you've ever used any other EDA suite. You can't name connections in a Fritzing schematic. You cannot rotate a part by right clicking, or through keyboard commands. Rotations must be done through a contextual menu. For PCB design: the color for the top layer of copper is yellow, and the color for the bottom layer of copper is a slightly darker yellow. Compare that with something sensible, like Red and Green for KiCad, or Red and Blue for Eagle. Despite conventional wisdom, you can create custom parts in Fritzing. No, I don't mean re-labeling pins on packages, like what the Fritzing tutorials tell you to do. You can create custom parts in Fritzing, but you have to use Inkscape, Illustrator, or another vector design program. Every other EDA suite can create custom parts without an external tool. Regarding the last point: until very recently, the official Fritzing FAQ stated creating custom parts was impossible. That part of the FAQ has changed, but there are still no links official tutorials for -- or even links to third-party tutorials available elsewhere on the Internet -- on the official Fritzing site. I will assume from this the efforts for support and documentation are effectively dead. There are several dozen other shortcomings in Fritzing, but these are the worst faults. Fritzing is a good tool for what it is designed to do: create graphics of breadboard layouts. Unfortunately, that's the only thing it can do well. Anyone looking to design a PCB should look at literally any other software besides Fritzing. Do not use Fritzing. I believe the authors of Fritzing have never used another EDA suite, and it shows. Recommending Fritzing to someone 'to make their first PCB' or 'to learn circuits' does them a disservice. Fritzing is a piece of software that should be expunged from the Internet.
deleted 9y ago
[deleted]
metaphor 9y ago
A few pro-tips on drawing schematics: 1. ASME Y14.100 and its references, in particular, Y14.1, Y14.2, and Y14.44. 2. IEEE Std 315 for symbols and reference designators. 3. Zen feng shui a la Tufte. 4. For multi-sheet schematics, provide a list of every reference designator and applicable sheet(s). Use sheet/zone references for all labels that span multiple sheets. 5. In general, functional left-to-right flow tends to be optimal for interpretation. Keep the application intent in mind. 6. Sideways text is never acceptable. 7. Q0O5S2Z1IX...if these characters aren't distinct, your typeface is busted. 8. Minimize net crossings. If you feel cross-eyed looking at any sheet, your schematics are likely busted. 9. Assume black/white printing. If you need colors to disambiguate, your schematics are certainly busted. 10. Electrical schematics convey eletrical intent, mechanical drawings convey mechanical intent. Sometimes it's useful to convey elements of the latter in the former.

113 points by iuguy 9y ago

One trick I learned at university while analyzing a circuit is to identify the various 'design patterns' in it - just like software engineering, circuit design also features a lot of recurring common patterns. Here is a fantastic post on it: http://www.arachnidlabs.com/blog/2013/10/17/electronics-patterns/ http://www.arachnidlabs.com/blog/2013/10/17/electronics-patt...

cjsuk 9y ago
Exactly this. The first step for me is usually to redraw out the messed up crap the last person drafted as things that look like the design patterns. Even Tektronix and HP can produce some dire schematics so when you've got your head in some old kit I'd probably spend more time deciphering the service manual schematics than actually fixing it (something I do as a leisure activity)
iuguy 9y ago
Oh wow, that's a really good find, thanks!

Animats 9y ago

That's a breakout board - mostly connectors, little functionality. Not too interesting to study. A simple blinky board with a 555 timer would be a better place to start. Here's one of my schematics[1] with a detailed explanation of how it works [2] This is a small board which is used to power old Teletype machines. It's a mixed analog/digital board, with a custom switching power supply onboard to provide the high output voltage needed using only power from the USB port. This gives some insight into why modern power supplies have so many parts. They work by creating big spikes, and they're always a few microseconds from a short circuit. So they need bypass capacitors and ferrite beads in the right places, and protection circuitry in case something fails. (MOSFETs tend to fail into the ON state.) If you really want to learn this stuff, get "The Art of Electronics", by Horowitz and Hill. [1] https://raw.githubusercontent.com/John-Nagle/ttyloopdriver/master/board/ttydriver01.png https://raw.githubusercontent.com/John-Nagle/ttyloopdriver/m... [2] https://github.com/John-Nagle/ttyloopdriver/blob/master/README.md https://github.com/John-Nagle/ttyloopdriver/blob/master/READ...

sleavey 9y ago
+1 for using KiCad to draw the schematic - it's important to use open source software when you're developing open source hardware. The licences for Altium, Eagle and co can cost many hundreds or thousands of dollars/euros/pounds a year, which bars entry to a lot of hobbyists. Improving what is already a fantastic piece of software (KiCad) is a big priority in my view. It's good that places like CERN are getting behind it.
- Nanite 9y ago
  Eagle has a free licence version, If you're ok with 2 layer pcb's and fairly small boards, which would suit quite a lot of hobbyists. KiCad was in need of some UX love back when I looked at it 2 years ago, the feature set is definitely impressive though, maybe time to revisit it in the near future.
  - hugs 9y ago
    This is why I used Fritzing, not KiCad to design my first board. KiCad's interface was hard to get used to, even though I understand that KiCad is more powerful. It would be awesome, however, if either KiCad became more beginner user-friendly or if Fritzing became more powerful.
    - wanderingjew 9y ago
      You should not use Fritzing. It is a bad tool. This isn't a vi vs. emacs argument, either: Fritzing is like using a crescent wrench to drive a nail. Yeah, you can do it, but it's the wrong tool for the job. Schematic capture in Fritzing is, in short, terrible. You can only draw straight lines between nodes in a schematic. Furthermore, the design philosophy of Fritzing schematics is, 'wires between pins' instead of nets between pins. That is, you can't 'tap into' a wire, like you could with a net. It's truly idiotic if you've ever used any other EDA suite. You can't name connections in a Fritzing schematic. You cannot rotate a part by right clicking, or through keyboard commands. Rotations must be done through a contextual menu. For PCB design: the color for the top layer of copper is yellow, and the color for the bottom layer of copper is a slightly darker yellow. Compare that with something sensible, like Red and Green for KiCad, or Red and Blue for Eagle. Despite conventional wisdom, you can create custom parts in Fritzing. No, I don't mean re-labeling pins on packages, like what the Fritzing tutorials tell you to do. You can create custom parts in Fritzing, but you have to use Inkscape, Illustrator, or another vector design program. Every other EDA suite can create custom parts without an external tool. Regarding the last point: until very recently, the official Fritzing FAQ stated creating custom parts was impossible. That part of the FAQ has changed, but there are still no links official tutorials for -- or even links to third-party tutorials available elsewhere on the Internet -- on the official Fritzing site. I will assume from this the efforts for support and documentation are effectively dead. There are several dozen other shortcomings in Fritzing, but these are the worst faults. Fritzing is a good tool for what it is designed to do: create graphics of breadboard layouts. Unfortunately, that's the only thing it can do well. Anyone looking to design a PCB should look at literally any other software besides Fritzing. Do not use Fritzing. I believe the authors of Fritzing have never used another EDA suite, and it shows. Recommending Fritzing to someone 'to make their first PCB' or 'to learn circuits' does them a disservice. Fritzing is a piece of software that should be expunged from the Internet.

deleted 9y ago

[deleted]

metaphor 9y ago

A few pro-tips on drawing schematics: 1. ASME Y14.100 and its references, in particular, Y14.1, Y14.2, and Y14.44. 2. IEEE Std 315 for symbols and reference designators. 3. Zen feng shui a la Tufte. 4. For multi-sheet schematics, provide a list of every reference designator and applicable sheet(s). Use sheet/zone references for all labels that span multiple sheets. 5. In general, functional left-to-right flow tends to be optimal for interpretation. Keep the application intent in mind. 6. Sideways text is never acceptable. 7. Q0O5S2Z1IX...if these characters aren't distinct, your typeface is busted. 8. Minimize net crossings. If you feel cross-eyed looking at any sheet, your schematics are likely busted. 9. Assume black/white printing. If you need colors to disambiguate, your schematics are certainly busted. 10. Electrical schematics convey eletrical intent, mechanical drawings convey mechanical intent. Sometimes it's useful to convey elements of the latter in the former.