Theory of Operation:
When the photo-transistor's voltage goes low (at night), it allows current to flow to the PIR (motion detector) and to the 555 Timer. Motion triggers the 555 and causes the LED to come on for 30 seconds.
I'm having trouble following.
When T2 turns off (in the dark), T3 will turn off. When T3 turns off the base of T4 will be pulled high, turning it on. Which will drive the dark detected signal level low. Do you have the logic inverted?
Quite possibly, my assumption was that Dark_Detected would also go high when T4 was high.
My original bread-boarded (working) design had the PIR, 555 and photo-transistor all powered. Movement detected by the PIR caused the timer to fire and if it was dark, send T3 low to light the LED (or activate a relay) .
Which seem inefficient, as the PIR and timer weren't needed unless it was dark. So I rearranged it to where the photo-transistor was acting as a master switch, to only enable the PIR and 555 after dark.
Here's my original bread-boarded schematic. I'll plug in a inverter and mess with that 5K resistor to see if I can get it to work.
Thanks everyone, I'll post results tomorrow. I probably won't sleep a wink with this problem frying my brain.