check resistance is bigger then a constant?
If you want a logical pulse, you might want to consider some type of circuit that changes state across a relatively fine boundary, so you can be as precise as possible.  For instance, you could create a simple voltage divider that is connected in series with your load, and put a BJT with the base connected between the voltage divider such that it turns on when the load is 50K Ohms.  Since a silicon BJT generally turns on at 0.7 volts, just select the values for your circuit to accomplish this based on your voltage input.  If your probe has a 5V input, you could stick a 300K Ohm in series and when the load rises to 50K, it will create a ratio of 0.14 (50/350) of the input voltage, or 0.714 volts at the divider, enough to turn on the BJT.  Of course this isn't extremely precise, but this is just one way.  This could also be done with zener diodes.

Granted this solution is not a pulse, it is a level output.  

It could easily be made a pulse with a timer or a uC.  An ATTiny85 is only $2.