Hello Alec
I am at a loss still.
On your circuit, I think all is OK. If I short to 0 v the Pin 2 lead (near the Optosensor) the light flashes and the injector works. We previously saw that the brief Rise at the Gate caused a square wave Drop at the Drain, and on the Hold FET, this could be varied with Pot 1. (about 1 > 4 ms)
But ever since I put in your C7, with the two R11, R12 (12k Ohm, not 10 k), a change has occurred. The optosensor RS 307913 no longer drops to zero, so the light does not flash. This was always reliable before. I no longer have access to a Oscilloscope. But with a good multimeter, we find:
a) 12.7 v supplied to the diode side, dropping to ~1.7 v when connected up. ie, presumed continuous infra red output.
b) 12.7 v at the transistor side, but not dropping to zero when reflection surface introduced. It drops to 5 or 6 V rather slowly (a few seconds) on the multimeter. And does not go to zero. (If the transistor receptor output leads are shorted, of course the light flashes). This is true of three Optosensors.
I can't see why putting in your Pull up circuit would do this but it occurred immediately.
Should I try to drop the voltage to the transistor receptor?
Thanx
Malcolm