NAMUR TESTER
I decided to produce this tester for testing in my job ( Edwards ). It is a tester as mechanical switches and, particularly switches type NAMUR.

NAMUR switch is a switching element used mainly in hazardous environments such as ATEX Ex ia. In the case of an intrinsically safe circuit (ia) not flow through the switch charge, which could iniciate an explosion in hazardous environments. It is therefore limited current, primarily maximum voltage (typically with NAMUR 8.2VDC) and current limiting resistors - replacement scheme NAMUR switch :

As seen in the equivalent circuit during the off state, the resistance is 11kOhm, when it is closed then have only 1kOhm. Safety thus lies both in the small current so that it is possible to detect faults - open / short circuit for this purpose special barriers that have a need to relay output (already in a safe environment, in the control panel) and can recognize and failure conditions NAMUR switches .

Back to the tester, the tester thus serves to visually test NAMUR switches (2 switches at the same time), it is a comparator which operates with resistance NAMUR switch and if the current changes (closing / opening NAMUR) output is tilted. The basis of the circuit is the LM339 comparator circuit with him, I used two comparators (out of 4). Stress testing, I chose 5VDC. Since it is a testing limit switches for valves / dampers to the circuit I added a relay for the solenoid. Supply voltage is 24VDC for entire device.

Schematics :

PCB
  

PCB Layout

Tester photos :