The hot-shoe contact on older cameras was just a mechanical switch contact. These are designed to trigger a flash gun. The contact in its normal use would switch a fraction of a milliamp from the flash trigger. It wouldn't be a good idea to use it to switch more than that.
"All I could find is "be careful not to let the circuit voltage over x amount ..."
This refers to replacement of the mechanical switches with electronic switches. The xenon flash guns required a high-voltage to ignite the tube and on some devices a high voltage appeared across the flash contacts. This would be enough to destroy the electronic hot-shoe switch on a newer camera. You have to mix and match flash and cameras carefully.
Figure 1. An exploded Nikon F3. The flash contact is in there somewhere. Source: PetaPixel
Notice that there is no big battery amongst the electronics and that many of these older mechanical cameras could work in manual mode without even the meter battery. The flash contact was purely passive.
simulate this circuit – Schematic created using CircuitLab
Figure 2. A circuit for your flash.
- Select a battery voltage > 3 V.
- Set R1 to limit the current to about 20 mA in your LED.
- Set R2 to limit the current through your flash contact to < 0.5 mA.
- Keep the battery ground isolated from your camera to prevent any short circuits.
This topic has been discussed on Photography.StackExchange. See https://photo.stackexchange.com/questions/27283/how-to-use-old-flash-units-with-digital-slr, for example. It references the Wein Safe-Synch Hot Shoe which will trigger a high voltage flash from a low-voltage DSLR.
The Wein Safe-Sync Hot Shoe to Hot Shoe regulates and reduces the flash sync voltage of the flash from up to 400V to less than 6V. This is especially important for current automated SLRs or digital cameras when used with older flashes or lighting systems. [Emphasis mine.]
This model mounts directly to a camera's hot shoe and provides a hot shoe on top and a PC female flash connection on the side. You can have a flash connected to the hot shoe and a flash being triggered by the PC female connection-and unlike so many other offerings of this type, both will fire simultaneously from the same signal. The exception to this rule pertains when using 1 or 2 flashes that are already under 5V sync voltage. In this case, the flash or flashes will not fire.