Well this is a bit sad, and definitely a beginners question...
I decided to put Mk 1 circuit in the shed as it is basically complete... (I will do Mk 2 in due course, promise!) it works fine (although out of spec at 6.5V) except.. the timer interface does not work at all... it senses that the timer is always open circuit indicating that the door should be open... this is a problem!
When prototyping I had assumed the timer was a voltage free contact and this worked well on the bench but in the shed with the real timer I get no joy.
I do know that when the timer wants the door open, it is open circuit, and when it wants the door closed it conducts (measurements given earlier)
I don't really know the standard approach for interfacing with this timer but my assumption is that the internal pull-up isn't strong enough however due to the convoluted design that is Mk1 I also have a diode to contend with.
I'm planning to try putting a 10k pull-up to (switched VCC) on the RB1 side of the diode but I would be interested to know why it doesn't work in this form.
Also... on reflection I don't like this interface at all, it looks inefficient... is there a better way to interface that would take less current when in steady state? e.g. charging a capacitor and then sensing voltage rather than current?
You can tell I do mostly software nowadays