HHO torch.

I've built me a really tiny HHO electrolyzer torch for welding wires and stuff. Unfortunately my photo camera is dead, so no pics yet. I've made smaller scale HHO torch when I were 14... those were the days.

Browsing for similar projects, I noticed that a lot of designs posted online are more risky than it needs to be.
A couple safety facts:
1 litre of HHO mix at atmospheric pressure and room temperature stores about 8kJ of energy. 8kJ is no toy.
8kJ is a bit less than 2 grams in TNT equivalent. It's the energy of 100kg weight raised to the height of 8 meters, or 10kg weight raised to the height of 80 meters. That's about the energy of your fall onto ground if you fall off a roof of three-story house. That's about the energy of coin shrinker.
The combustion pressure is somewhere between 5 and 10 bar. For electrolyzer chamber with lid area of 'only' 216 square centimetres, the force would be 1..2 metric tons.
Electrodes and all the connections must be entirely covered with electrolyte at all times. Why: won't take a lot of spark to set off HHO mix. Cover your soldering joints with glue on anode.
I used soft-ish plastic food container (HDPE) for my electrolyzer, which seems like excellent choice - I'm pretty sure explosion would simply blow off the lid and spray the electrolyte around, but not any shards.

Calculations for energy:
2 H2 + O2 -> 2 H2O + 572 kJ (all amounts molar, i.e. 2 mol of H2 and 1 mol of O2 become 2 mols of H2O)
1 mol of gas is 24 liters at room temperature and pressure, so we have this