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This question is inspired by Connectorless USB on a PCB. I saw a cool USB LED on AliExpress that can be inserted in either direction:

enter image description here

I'd like to build a board with a similar symmetric connectorless USB plug, but I am pretty sure the unused side of the connector will short out against the two tabs that I see on the inside of the metal sleeve of most USB Type-A sockets:

enter image description here

Since the AliExpress board works, I assume that a portion of its circuit handles the case where the +5V lead on the connector touches the sleeve. I bet that responsibility lies with the two diodes I see on the board near the connector.

If these assumptions are correct, would you please draw a partial schematic that shows the relationship between the diodes and the two sides of the connector? If nobody answers, I'll use a continuity tester on one of these USB LED boards to deduce the circuit myself, and then I'll post an answer. But I am a novice with electronics design, and I'd appreciate the expertise of someone who really gets what's happening in this circuit, rather than my own blundering observations with a multimeter.

And an extra bonus question: what is the function of the two tiny angled sections on the AliExpress board's connector that are absent on almost all other male USB plugs?

enter image description here

Do they do anything? Are they racing stripes?

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  • \$\begingroup\$ Does that SOIC8 have a part number? I'm curious as to what it is. One would hope a constant current regulator, but I'm not convinced. \$\endgroup\$ – uint128_t Jan 7 '16 at 6:21
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    \$\begingroup\$ The image appears to have been altered to delete package markings and traces so a schematic likely can't be sketched. The data pins (inner) are pushed back so that power/gnd is connected first before the data pins (look inside a regular usb connector and you will see this there too). I suspect the extra bit of copper at the end is there to "push" the contacts on the receptacle so that the edge of the data pad is not as liable to be scratched or divoted by a particularly stiff contact or someone trying to stuff it into an incorrect receptacle \$\endgroup\$ – crasic Jan 7 '16 at 7:06
  • \$\begingroup\$ @crasic no, all these chinese penny items tend to have ICs with no silk screen markings and these boards in particular have a heavy solder mask, so the traces are barely visible even in person. \$\endgroup\$ – Passerby Jan 7 '16 at 7:23
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    \$\begingroup\$ I'm pretty sure the SOIC8 is a small microcontroller. On the back side you can barely make out a large pad that acts as a capacitive touch sensor that lets you switch the light on/off or PWM-dim it. \$\endgroup\$ – sowbug Jan 7 '16 at 15:01
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    \$\begingroup\$ @sowbug it's likely not a mcu, but a dedicated touch sensor. There are ones that will allow automatic dimming and such, without needing to be programmed. \$\endgroup\$ – Passerby Jan 8 '16 at 3:42
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A better picture, black diodes on white board. enter image description here

Big Clive on youtube did a video on these very same boards about 2 years ago, including a schematic starting at 1m50s.

It is a simple diode OR circuit. The diode simply prevents the grounded VUSB pin from shorting out, via reverse protection. General idea:

schematic

Important Design Consideration: The Diodes will induce a Voltage Drop equal to their Forward Voltage. Typical Silicon Diodes have a 0.7V Vf, while a Schottky Diode or Germanium Diode will have a lower Vf of 0.2V~0.4V. Plan according to your needs.


As for the extra copper on the data lines, that looks it's just for style. Like flames on a hot rod. It makes it go faster. The same style is used on most of the black boards.

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    \$\begingroup\$ Looks like that gang of resistors robbed those poor diodes of all their solder. \$\endgroup\$ – crasic Jan 7 '16 at 7:21
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    \$\begingroup\$ Grr, the lack of video embedding on most Stacks is very annoying. M&TV.se has spoiled me. \$\endgroup\$ – Passerby Jan 7 '16 at 7:45
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    \$\begingroup\$ @sowbug don't forget to take the diodes f forward voltage drop into account \$\endgroup\$ – Passerby Jan 7 '16 at 22:02
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    \$\begingroup\$ @sowbug there are other diodes which have a lower drop. Schottky Diodes, or Germanium Diodes. 0.2~0.4 at full current. That said, why bother with a double sided usb plug? Unless your device has a specific reason for it, it's just a design inconvenience. \$\endgroup\$ – Passerby Jan 7 '16 at 23:07
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    \$\begingroup\$ That's a fair point. The reason is some people might want the light pointing in the other direction. It's going to be a general-purpose monitor light for people at work, and I've found that desktop/monitor USB ports face all sorts of different directions. Adding a USB extension cord gives more flexibility, but it is also bulky. \$\endgroup\$ – sowbug Jan 8 '16 at 3:32

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