Here are the components for putting together a gas burner that can be turned on and off with an electrical source. The flame spreader is a modified unit from a gas hot water heater. I removed the burner gas line and welded on an extra bracket.
The added bracket has a hole just large enough for the 1/2 inch black iron pipe to fit through. When assembled, the brass cap fits between the two brackets.
The valve comes from an old gas furnace. In normal use a 24 volt AC transformer is wired to the valve through a wall mounted thermostat. When the room temperature drops below the set value a switch in the thermostat closes. This allows current to flow through the gas valve. If the thermocouple is hot, gas will then flow to the burner and the pilot flame will ignite it.
I've basically replaced the wall mounted thermostat with a switch in the controller that is controlled by my software.
These are some of the valves I've collected. They operate at low pressure. I use a regulator designed to be attached to a propane tank. Actually it came from a barbecue grill. If you're using propane don't use the regulator from the furnace. When natural gas enters your house it is fed to a large regulator that reduces the high mains pressure to a few PSI for household use. The regulators on the furnace therefore work with low pressure.
To keep propane in a liquid form it has to be stored at extremely high pressure. I don't know the exact pressure, but it's probably more than a thousand PSI! So, be sure you have the correct regulator for your gas source.
The gas pressure at the valve in my setup is around 1/2 psi.