I’ve been puzzling over this for a while…
On the auto-flush toilets (the ones with the sensor that causes them to flush when you depart), I’ve never seen any sign of a power source.
Do they have batteries? Little photocells? Tiny generators that charge a capacitor when the water flows thru?
Thanks, Q.E.D., now I know about retrofitting with battery-powered units.
What about new units. I’ve never seen anything that looked like a connection to a power supply?
Still
I assume new units are also battery powered. Using mains power could be problematic, although it’s not out of the question. I’ve never seen one that looked like it was line powered, either.
methane?
A lot of them are are battery-powered, while others are wired in. The hard-wired ones are usually 24v, with only low voltage going to boxes in the walls behind the flush valves. There will be a transformer for stepping down from 120V to 24V somewhere near, hidden behind an access door or ceiling tile.
While standing in front of them, I have theorized that it would be possible to design a unit that needed neither mains power nor a battery that would have to be replaced periodically. You could use the water flow to charge either a capacitor or a rechargeable battery. Just put a little turbine in the flow, connected to a mini generator that would recharge the power source. The system could be designed such that if usage was low, it would automatically flush to recharge the battery/capacitor before it ran down. (Yes, this would waste water, but in many areas this is not a major issue, and would be preferable to having to replace batteries.)
I posted my theory in a similar thread here on the SDMB, and another poster provided a link to a system that worked exactly that way. I was pretty proud of myself for figuring it out.
This one uses 4 C batteries. (PDF)
This one is wired into household AC.
These can be AC or DC powered.
You didn’t ask for it, but here’s some commentary on self-flushing toilets with regards to ergonomics:
Haven’t found any self-powered ones yet, though. That would seem like a giant step forward. What happens with the battery-powered ones when the batteries run down and no one changes them? Are they then un-flushable?
I really hate it when the autoflusher decides to flush when I’m still doing my business. Perhaps they are powered by rage.
I hate that too. It seems to happen more often in airports and low-end department stores.
What boggles my mind is that the ones at work operate when the power is out. The battery part understand. I just don’t know how they know I’ve moved when it’s pitch dark in the room.
Infrared. To the sensor, you glow in the dark.
Slight hijack-I was using the urinal in a facility with auto-flushers, and a little kid asked his Dad how the urinal knew when to flush. Techno-Dad replied that behind the little red window was a tiny man who pushed a button to activate the flush cycle. :eek:
Toilets-R-Us: Now Hiring. Persons wanted to watch others urinate and defecate. No previous experience needed. Will train. Competitive salary and benefit package. Toilets-R-Us is an equal opportunity employer. 
Man, what a shitty job.
I believe they are in the same union with the little men who watch your food spoil and turn on the light inside your refrigerator.
Seems like they should be able to run one with a combination of abmient power and water power. The water pressure can do the work to move the parts around and reset things after the flush, and the electric eye simply power a release catch. With all the public johns in the world, somebody will make a mint off this idea, so you electronics wizards should go rush home to your work benches.
Good idea. That should make the battery life not much different from shelf life. 5 years? 10 years?