A standard diy arc welder (transformer type - heavy) produces about 40 volts AC.
I connected this to a bridge rectifier to convert it to DC. These are availabe in most electonics stores or Ebay for about £10 or £5 if you are willing to wait for it to come from China! Make sure you get one with a heat sink as it will be working hard with 2 or 3 cells running off it.
With the welder on its lowest setting I get 36v DC from my bridge.
Connected across a single cell I got a lot off gas but also some intenal sparking (very bad with HHO gas present) and heat so did not risk using this set up.
Split across the 2 cells and a stainless coil in series provides about 12v to each just as a vehicle battery would.
The 12v per cell seems stable and runs well for long periods with little heat generation.
The construction of my cells, bubbler and 'torch' are mostly from scraps or standard diy materials.
Parts list: Welder - transformer type - about £60 but most diyers have one lying around Bridge rectifier - about £15 - had to buy two as I blew the first one which had no heat sink Double pole switch - £3 Jars - 3 for £5 in a local bargain store 8mm Copper tube - left over from an oil tank installation Thin stainless tube cut from a ball inflator KOH - drain cleaner crystals - £1 from a local hardware store T-pieces / straight connectors & plastic tubing - fish tank fittings from Ebay - about £6 Bubbler - herb jar with a rubber tube siliconed into its neck and an old broken drill chuck set on top Stainless wire taken from old rigging cables Stainless mesh plates cut from an insect screen
Already had the welder so reckon the additional parts came to less than £40
To make the cells I used cheap glass jars, stainless wire, stainless mesh, an old plastic starage container, an old chopping board, two spouting bolts, some epoxy glue and a little silicon sealant.
I cut 70mm squares out of an old chopping board (fit in the neck of the jars with a couple of mm clearance each side) and stuck them into the lids using epoxy glue.
While this set I attached the spouting bolts to the stainless wire and coiled it so it will wedge the neutral plates and spacer plates.
Once the glue was cured I drilled three 6mm holes in each lid. Two for the electode connections and one for the gas output fitting.
I then put a smear of silicon gasket sealant round the aquarium push fitting and inserted it into the center hole and also put silicon gasket sealant in the groove between the jar lid and section of chopping board. This will create a gas tight seal when the lid is held down by a cable tie.
For now I am using stainless mesh as my plates - just an old insect screen. It is easily cut with sheet metal cutters. I cut 9 of these 70mm by 120mm for each cell. Two will become active plates wedged against the stainless loops - the rest neutrals.
For spacers I used plastic sheets with holes. The red ones are an old plastic storage basket cut into sections the same size as the SS plates. The white ones in the other cell are just a plastic pipe drilled, cut and flattened.
Before final assembly it is good to put it all together and add the water / KOH. I put the loops / plates and spacer in and filled nearly to the top with just ordinary tap water. Then I applied voltage to the wire loops at each side of the cell. Not very much will happen till the KOH is added - do this slowly as heat is generated from the caustic soda as it dissolves into the water and also from the electrolysis which should get quite active after a couple of spoonfulls have been added. Be very careful with this liquid as it is very caustic and can burn skin. If your cell gets hot quickly you will need to reduce the voltage or dilute the electrolyte you have mixed.
To complete the cell I removed and washed the SS loops so I could handle them to bolt them into the lid. Use sealant around the bolts to prevent gas escaping through the threads. Putting the loops back into the jar with the lid attached is tricky enough but you can use a screwdiver to move the plates & spaces to one side then the other as you insert the loops and lower the lid. Secure with cable ties or similar.