Without wanting to get into an argument, you have still missed the point. I introduced the dimmer to explain the operation - bear with me here.
Fair enough. I won't argue at this point, but will bear with you. I think we are probably talking at cross purposes!
Consider we've got a dimmer, and it's set to "almost" full brightness, so we have a short period 100 times a a second where we can collect a bit of power. With me so far ?
Yep.
Now remove the "variable" bit from the dimmer - so we have a "switch" which is either "off (so we have the full mains voltage to play with), or it's "on" which corresponds to the situation described (we get a short period to collect power 100 times a second). Now operate that "switch" with a PIR sensor and timer - hey presto we have a two wire PIR switch which needs no separate power supply.
Yep. Bearing in mind that most PIRs are going to spend the vast majority (often virtually all) of their time with their triac 'off', virtually all of the power will be derived during the 'off period' - which, as I've been saying all along, in fine provided one accepts the concept of a very small current flowing through the load when 'off'.
Delaying the turn on of the triac needn't be complicated - all we have to do is wait until the voltage across the device reaches a certain level (which will have charged our supply) before triggering it.
True - although, as above, in the case of a PIR it's nearly always going to be during the triac's totally untriggered time that power will be derived for the electronics.
Also, bear in mind the level of power consumption the boffins have got electronics down to these days. I've just fitted an electronic thermostat for a friend - it's a 2 wire device and uses batteries for it's own supply. The destructions say the batteries should last for at least a year - that's 2x AA cells, over a year. That's some fraction of a mA average maximum power usage at 3V.
Sure, as I said at the start, a battery is one obvious solution. However, if it were only charged during the 'on' period of the PIR (e.g. with a small series resistor),one has to bear in mind that some PIRs may go for years without being triggered.
An alternative would be to have a larger reserve of power - there are "supercaps" which can store phenomenal amounts these days.
Sure, but that's just a variation on the theme of a battery - but, I suspect, probably more bulky and more expensive.
I suspect with the right electronics (see above about power consumption) these could provide power for several hours (or longer) of device "on" operation. Should you need longer, then a very brief "off" period (could be just 1/2 a cycle or less with a triac switch) would recharge it - probably without the user detecting it.
As above, PIRs are virtually never 'on' for hours (if ever 'on' at all!) - it would therefore make sense to major of the 'off-time power sourcing'
And finally, have we completely discounted the possibility of the electronics being powered by the sensor itself ? Back when I were a lad, I recall my dad had a light meter to go with his camera - no batteries, just a meter powered by a small "solar cell" of some sort. The more light, the more power from the cell, and the more the meter moved.
You may have 'discounted it', but solar charging of a battery was one of the options I mentioned in my first post on this topic. There would,of course, have to be a battery (or 'supercap'), since power would be needed at night. In passing, I still have at least a couple selenium cell light meters which I've had since the 60s, and they still work fine!
So, I think we're probably saying essentially the same, even if we are 'arguing'
Kind Regards, John
