I've had an old external flash in my closet for years which had old battery crud in it. I finally decided to take it apart and clean it, and after putting it back together it works. Now I'm not really sure how to use it. It's a Vivitar 2800 "auto thyristor" and my camera is a Nikon D3100. I simply put it in the hot shoe, went through the menu and set the flash control from TTL to Manual. This was enough to allow the camera to trigger the flash. After poking around a bit, I read that this can be dangerous and the flash trigger voltage could ruin some modern DSLRs and is not recommended without checking. oops. Anyway, I checked the voltage and its 147V, where my camera's max is 250V. No sweat.
Now what? The camera menu allows control of the flash intensity, but I figure that only works with newer "smart" flashes. Since this one connects only via the central contact pin, I assume the camera can only act to trigger the flash. There are 2 auto modes and a manual mode on the flash, but I'm not really sure how they operate. There is a slider to select ISO/ASA, but this does not connect to any of the electrical components inside the unit; it only serves as a user guide telling you what aperture to use. I do understand the concept of max flash sync shutter speed, which is 1/250 on my camera, but that's about it. The user's manual isn't very clear to me. The flash auto modes simply expose a light meter through either of 2 small holes depending on which range you select.