For basic flash sync, a Nikon DSLR typically does not output a trigger voltage on the hot shoe. The standard firing contact is the center pin (X-sync), and the camera triggers the flash by briefly shorting that center contact to ground (the metal rails/sides of the hot shoe).

So if you measured voltages with no compatible flash connected, that may not tell you much—the flash usually supplies the trigger voltage/current, and the camera just closes the circuit.

A multimeter usually won’t show the firing event well because it is very brief. For a simple manual flash trigger, you only need the center pin and ground.

If you want TTL, power control, or high-speed sync, those use Nikon’s proprietary communication pins/protocol, which is much more complex.

Important: be careful with trigger voltage. Many Nikon DSLRs can tolerate relatively high trigger voltages, but exceeding the camera’s limit can damage it. If you’re building your own device, use a low-voltage isolated trigger circuit to protect the camera.

For LEDs, note that the hot shoe only gives you a sync event, not continuous power or advanced timing control.