Thanks for the head banging kit, I have a feeling I will got a lot of use out of it while trying to get up to speed.

If you guys don't mind I have another probably very fundamental question, but my basic knowledge of electronics and programming is probably dangerous

For my first Pic project I want to sense the current state of an existing piece of electronics and then be able to control it based on the current state and other inputs that are not part of the existing circuit.

Below is the basics of the existing circuit.



I've measured the current (flowing between 3 and 4) when the switch is closed and it is only a few mA so I think I can happily pull point 4 to ground to emulate the switch being pressed.

For the sensing of the existing circuit and whether the LED is lit, can I just attach point 2 to an input and read a low (using internal pull up resistors)? I didn't know if that would interfere with the existing circuit. For the switch side of things I know I could use a relay or something to completely isolate the two circuits (mine from the existing), but wasn't sure about reading the state of the LED.

The existing hardware isn't cheap, so I don't want to experiment too much.

Any help or advice appreciated.