How to trigger multivibrator (or similar) from collector?
If you want to use BJT's to implement a reliable switch which activates when a resistance drops to 1M, then put the 1M across the base-emitter junction. Use a pair of transistors to invert the switching signal.

Something like this architecture:







simulate this circuit &ndash; Schematic created using CircuitLab

Why we use a JFET is because the resistance we are sensing is very high, so it helps us to have a device that has a very high input impedance. 

The values are very fiddly. With a 5V supply, you need a JFET that pinches off easily: a JFET is pinched off when the gate voltage is below the source voltage, which is why we need R4.

The extra Q2 stage is used to create voltage gain so that there is a sharper turn-on.

Here is the simulation (DC Sweep) which shows the load current, as the potentiometer is varied from 0 to 100%.  It is fairly sudden around the middle position. But this depends on the JFET characteristics. Experimentation with R2, R4 and R3 is necessary with different JFETs.