Using single LED as bipolar DC detector

Question

It would be very handy for me to have an XLR plug with an LED on the end to check for DC at the output of balanced audio gear. The challenge is two fold. First is that this gear can fail with a positive or a negative voltage on it's output pins 2 and 3 in reference to pin 1. I am using regular old red LED's. Second challenge is I would like to check pins 2 and 3 with a single LED. This brings up the issue of what happens when pin 2 is +VDC and pin 3 is -VDC, it will not light. I feel that a diode OR arrangement might get me close to the functionality I'm after but my design experience falters here.

Answer 1

^{simulate this circuit – 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.

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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.

Answer 2

Not what you asked, but I would be tempted to use one red/green bipolar LED and one series resistor from 2 to 3.

That way you get polarity indication for free and the two parts will easily fit into the plug.

^{simulate this circuit – Schematic created using CircuitLab}