## Friday Free Board Quiz – PCB routing – Price is electric door lock

DOOR-LOCK is electromagnetic door lock working with 12V DC.

To participate in the Quiz is enough to re-tweet the Twitter Quiz announcement message.

To double your chances you have to answer the Quiz question

The question is simple: Which PCB trace routing is best for high speed digital signals? A – B – C and why?

You have time to re-tweet and/or answer until Monday 2nd of November.

In Monday we will post the correct answer and ask random.org to generate random number in range then announce the winner and ship the board by post/airmail.

Good Luck!

### 19 Comments (+add yours?)

1. Radu DAN
Oct 30, 2015 @ 16:44:48

B !!! 🙂

• C I guess cause its twisty

2. Georgi Krachmarov
Oct 30, 2015 @ 22:05:00

B, because electrons easy can manage the corner (<90) with the speed of light in wide area than A and C.

• LinuxUser
Oct 31, 2015 @ 19:09:52

There is some correction: electrons do NOT reach speed of light, ever. They have non-zero positive mass, and therefore unable to reach speed of light at all, unless you have infinite amount of energy to accelerate them. Welcome to strange world of relativistic things.

It is interaction (“field”) what actually spreads at speed of “light” (light is just a “pinch” of electromagnetic waves, as well as many other things).

But electrons on their own have very low motion speed. Exact computations are rather complicated, but if I remember, even for reasonable DC current it would be something like millineters per minute or so, if we take a look on how particular electron travels between metal atoms. Needless to say it hardly looks like speed of light. It gets even more interesting in case of AC or bidirectional data bus, because current can flow either way. In this case whole notion of “speed” becomes interesting and would take some differential and integral math to describe it, depending on what you want: “instant” or “average”.

3. Lahcen
Oct 30, 2015 @ 22:56:07

C To minimize any impedance change

4. C since the variation in trace width along the track length is minimised such hat stray capacitances associated with sharp transitions are minimised resulting in uniform characteristic impedance.

5. Um, B?

6. To put this question into perspective:

“shortest path” is in most cases probably more important.

7. Brendan Simon
Oct 31, 2015 @ 13:02:59

C — less discontinuities will cause less reflections. Also easier to match trace lengths for trace pairs and busses.

• Agreed. Reflections can make the signal allmost unrecognizable. Rounded corners are considered “old looking design” but create no reflections and are perfect for fast digital signals. Your signal will reach the endpoint without distortion.

8. bremenpl
Oct 31, 2015 @ 16:27:56

C. Easier to match lenghts and impedances!

9. Assuming an already impedance matched transmission line, the C trance minimizes impedance change which in turn results in less reflections.

10. LinuxUser
Oct 31, 2015 @ 18:28:28

A is definitely worst, because it would cause more EMI and there will be strong “ringing” due to signal reflections from corner of the track and it really matters for high-speed digital transmisson lines. B is better in this regard, and C is probably the best. If we take a look around, both B and C are used on actual PCBs. B is rather typical on PCBs, and something similar to C could often be seen on circuitry involving DDR memory (where it used to equalize track length, new KiCad can do it, now, he-he)

11. B least chance of loop interference

12. Gianni B.
Nov 02, 2015 @ 05:18:25

B

13. B – to avoid reflections

14. oheryus
Nov 02, 2015 @ 13:33:31

B – cause you do it all the time

15. Changing the subject completely, have a look at that door lock. It looks wrong to me.
Normally I’d expect to screw the mounting plate onto the door, so that the base is next to the door and closing the door would have the wedge-shaped bolt pushed in as it passes the latch.. But mounted as shown in the photo, the bolt is at 90° to the door, so one of the straight edges would hit the latch on the door frame that is supposed to keep it closed.
One would assume that the body of the lock could be rotated, but why show the piece assembled in this way?

• the push rod can be rotated at any angle, so you can mount it on the frame and on the door whatever is more convenient for you