Dude! I not only never considered building my own rebreather....I ain't never heard of no one considering building their own rebreather!
I can't imagine the electronic shit that controls the breathing mix was easy to make.
Mad respect dude.

we are using the kiss system ( gordon smith invention)
there is a valve which gets 0.8 liter of oxygen a minutein the breathing loop ( roughly the metabolization rate of the body at rest)
on the boat before jumping in the water you flush massive amount of O2 in the breathing loop getting rig of nitrogen blowing by the nose ( you know this)
when the o2 meters reads 1.00 or 0.98 you enter the water breathing pure o2 (the diluent cylinder tap is shut)
of course during the time you swim on surface to the anchor line you have to add 02 in the breathing loop as your body usus 1 liter of O2 a minute but there is nothing to control in terms of po2 , when the counter lungs are flat you can't breathe and you addo2manually no risk on hypoxia
after this we dive at 5 meter calming down for 5 minutes and doing the bubble tests ( you make sure your buddy re-breather does not leak any gaz)
also at 5 you make sure your ppo2 reads 1.5
at this stage we open the diluent cylinder tap ( generally 15% o2 heliox or trimix)
then you start your chrono and go down, when you get down the pressure reduce the breathing counter-lungs bags volume and as there is a regulator on the bag, the reg let in trimix which reduces automatically the ppo2 in the bag as the mix is poor in O2
generally we arrive at the bottom at 1.00 bars of ppo2 in the breathing loop
there we press the o2 button to add a little more o2 and get 1.5 bars of ppo2 , if you stay at the same depth the ppo2 will remain stable, it will slightly go down and it will lead you to adjust pressing the button every 5 minutes
It forces you to check coz nothing is automatic ( much better the the Eccr .. not manual), believe me you never forget.. and anyway it would take 30 minutes to get down under 1 bar even without pressing the button...
of course going up the ppo2 gets down and you have to press the button every 10 meters of ascent .. but in the IANTD deco protocols we ascent so slowly that it is not a problem, basically when you arrive at your first deep short stop you adjust and before leaving the bottom you get up tu 1.8 bars and arrive at your first stop at 1 bar .. very easy to handle

as for the electronic it is not so difficult to design, you can use 3 small lcd voltmeters with 3 potentiometer to calibrate your sensors readings in o2 before the dive but it doen not give you low and hi ppo2 alamrs
so we developed a more complicated system using a pic microchip ( long before the arduinos) and we implemented a voting logic ( to compare the 3 sensors result and track a problen with the sensors)
but it did not bring much more safety because at the bottom you often check you ppo2 and it is obvious to see if the sensors have not the same value ... if one is reading more or less then the two other you only use the 2 good ones .. and if you have a doubt you make a diluant flush and as you know the o2 fraction of the diluant and your depth you can find out which sensor is failling

but the sensors never fail if you change them often ...

it works well .. but seriously diving at 40 m with a twin isolated tank rig and a S80 nitrox spare is far enough to get fun .. re-breathers are like jet helis compared to gyros ... I still have my re-breather but I dive open circuit ... my mates all 3 still dive this rig

which is Funny is that I have used all the IANTD methodes ( focusing, adaptation of the rig to the goal of the dive, 5 step risk assessment, dive planning, mental preparation, details check lists etcetc ) in aviation where to be honest even if many thing were on books I saw nobody doing it fully right

in deep diving ( I was somethimes going deeeeeeeepppppppp ), the preflight is a sort of meditation ... a dialogue btw you and you gear ... mental preparation and visualization is something I was taught to practice in deep diving by the Americans lads of Iantd and it has always gave me a great mental comfort ... the only draw back is that I did not made a lot of friends in the hangar visitors ( except my mates) coz from the moment I start the to preflight I don"t speak to anybody .

imagine when after only 40 hours of flight experience in my life including training I had to take off in the mono-seater I had rebuild ... without a mental preparation and accurate planning of what I had to do I could not have copped with this

I was also greatly helped by a couple of soldiers I knew and who were able of preparing mentally in any conditions, they have taught me to track the fear coming from your stomach and going up in your brain and to Say NO to the fear to focus on what you have to do

I know a couple of seal from the commando hubert those men where good at sport but their mental stability was the key of their capacity to cope with nearly anything , and they had learnt to prevent their body to leave fear to by pass their brain.. all comes from the brain.

what I love in flying and diving is that we can do it right instead of doing anything and then after complaining of bad thing that are happening

That's fascinating, we just used them as a way to "get to work"!
Your tech skills far exceed mine, but I was able to follow your thought process.
Had a saying in jump school, everyone's first jump is easy....its the second one you fear!
One time I was approaching a dud piece of ordnance with an electric timer, had a lot of time during "the long walk" to discuss and assess my situation. I find having a health dose of fatalism helps!

For longer endurance on a gyro with a smaller fuel tank than others one is travelling with, you can add temporary extra tanks... I used to use the small air conditioning gas (R134a) aluminium tanks or the old water style fire extinguishers up the mast above the actual tank, so opening the solenoid let the fuel reticulate into the normal tank, so no pumps.... the extinguisher type tanks under the keel needed a small electric inline pump... low pressure high volume type.

Hi,
thanks, experience is the way better the theory
are you using quick connectors ?