Level Switch - 
As per our supplier if the Zener barriers you are using compliances the voltage & current specification mentioned below then the level switch can be directly used. 

Specification.

1) I/P Voltage ( Vi) - 24VDC   

2) O/P Voltage (Vo) - 24VDC   

3) I/P Current ( Io) - 110MA.   

Kindly the level switch catlouge which is also uploded for your referance.  