Hi all. I'd like to build a flash system for photography based on this design. Modifications I'd like to make include the following:

• use a 12v car battery as opposed to 5v;
• add one more flash tube;
• add power/duration adjustment knob;
• optional: add high speed sync functionality.

I would greatly appreciate it if you could teach me how to add these components to the existing design, such as by drawing a schematic and providing parts details. I know only the basics, so I don't know if what I want to achieve is even practical. Thanks in advance.

 

The circuit is drawn wrong, the Diode and 1000uF cap are wrong way round,, the circuit should work on 9V, but will give more output voltage, or you can use a 5V regulator to drop it to 5V. To add a slave trigger / sync circuit you need to trigger the Thyristor gate terminal.

 

Welcome to AAC!
Given your admitted basic knowledge I would strongly advise against building this. The energy stored in the big capacitor is enough to kill you.
As Dave says, the circuit schematic has errors. That doesn't inspire confidence in its design.
To run from 12V instead of 5V would require a different transformer.... an XFT-1934 (or compatible).
Adding an extra flash tube by connecting it in parallel with the original, as the author suggests, might or might not work depending on the tube characteristics and on the ability of the pulse transformer to trigger both tubes.
Power/duration adjustment would require switching of multiple capacitors or a lot more circuit complexity.
High speed sync might require modification of the capacitor charging arrangement and the trigger components.