Using single LED as bipolar DC detector
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Single and dual-channel checkers.

Figure 1a shows how to use a bridge rectifier for a single-channel.

Figure 1b extends the design to include a second channel. 

If the 1b circuit could be exposed to both a positive and negative simultaneously then size R2 to suit the maximum voltage difference.



How it works







simulate this circuit

Figure 2. Fault condition: XLR 2 faulted to V+; XLR 3 faulted to V-.

Figure 2 shows the circuit with a certain fault condition and the reverse-biased diodes removed for clarity. R2 has to be selected to limit the current to a safe value when the maximum difference in fault voltages appears on 2 and 3.