Trigger Low to Pin 2: I don't have an oscilloscope but I am pretty sure the answer is yes. I will try to make sure.
4.5 v: no of course not, it used to be OK (no questions asked) on 15 v, but when I wondered what was wrong, I tested to see if the 555 would only give a short signal (yellow light flash), I used a low test voltage in case something went wrong & at a higher voltage, something might be damaged. But the light showed, implying 555 output, till I turned the reflector away.
I was surprised that the light being on, ie the HOLD to Gate being live, was not determined by the (?) reset circuitry around the 555. Surely if the Pin 2 is kept low, the output to HOLD should not stay active?
Thank you. Malc