Make one circuit current track with another circuit current

[Alec_t]

with very finicky values, the tolerances can be a real problem.

The switching threshold in the above circuit would be somewhat temperature dependent.
To make things less finicky you could incorporate a comparator and trim-pot, like this:-

Note that current sensor R3 is now in the ground line.

 

[pratto]

ok. It's time to learn about comparators. looks like a zener in there too. I'll get to modeling first.