With the way it works, each leaf of the shutter gets pulled open 1,2,3 (unless 3 is held open for an exposure), so I think a mechanical mod might be a little tricky.
When I was setting focus, I had to have the shutter open for long periods. To do this, I put black tape over the sensor on the front of the camera.
That does work to some extent if you want a long exposure, although it tends to eat the batteries (the solenoid needs to be powered to keep the shutter open).
I have not traced out the circuit, but I would guess it is an integrator.
The sensor is an LDR, which is high resistance when dark and low resistance when lighted.
I would guess you could mod the circuit - e.g. put a switch in series with the LDR. Opening the switch would keep the integrator off. The solenoid holding leaf 3 would then be active until you the switch is closed again and when sufficient light reaches the sensor to trip it closed.
(Of course, if you're in low light, you might want to add a low value resistor to speed up the integration (same effect as shoving the sensor in front of a lamp/torch).
How about a 3-way switch for Open (T) - Normal - Closed? Switched to Open, you could just hold the shutter down for B.
I would possibly experiment and try and lower the spring tension so that I can reduce the power required by that solenoid on long exposures, so the batteries don't get eaten up. But that's another can of worms
Last edited by Sethasaurus; 02-04-2014 at 04:06 PM. Click to view previous post history.