I want to build my own external flash/ledlight/softbox just for practice and knowledge. What pin gives signal to external flash before a photo is taken? I messured the voltage with DC multimeter A=4.77V , C=2.96V, B=C=D=G=0. No signal palm dedected (with multimeter) while a photoshooting. How to sync for example leds with photoshoot?
To trigger a flash with a basic "fire" only command, the camera does not output any voltage at all. It just closes the circuit between the ground (G in you top photo) and the X-sync pin (D in your top photo). This allows voltage supplied by the flash to flow through the circuit back into the flash and fire the strobe.
Your camera can only handle so much voltage, though. And it can vary considerably by camera. Most Nikon DSLRs can handle up to 250 volts through the hot shoe. If your self built flash uses more voltage that you'll need to take some preventative measures to prevent frying your camera's internal electronics. Please read more in this answer.
The main signal that you want to know about is at the big contact in the middle, labeled
X-Sync in your second image. To trigger the flash, the camera shorts this contact to ground (the sides of the hot shoe). It's not surprising that you didn't see this with a meter -- I believe the flash supplies the voltage, so with no flash connected there's no voltage to see. You could try setting the meter to continuity or resistance mode, but since flash duration is typically around 1/250s, the signal may be faster than what you can easily notice with a meter.
That much will let you trigger a flash in manual mode. To do more than just trigger (like using TTL metering or high speed sync), you'd need to know about the proprietary protocol that Nikon uses for controlling it's speedlights.
You can read more about basic triggering at Hot Shoe in Wikipedia, and there are some references on that page that may get you farther.
Were you able to create your own cable/cord to fire an external LED? If yes, how much voltage did the Hot Shoe generate? You think any off the shelf "off camera shoe cord" can be modified to goto my LED controller?