Stereo Indicator Detects L-R Signal Difference

This true stereo indicator is different from what we usually find on FM radio receiver, which is usually a pilot tone detector. A stereo broadcast from FM radio station contain pilot tone, but a presence of pilot tone doesn’t necessarily a stereo broadcast signal since a mono FM transmitter ca broadcast pilot tone as well.  Since this circuit detect the difference between left and right channel, this circuit can detect a real stereophonic programs. 

When there is no difference between R and L input signals, the output A1 and output A2 is at the same potential. That will make a a virtual ground rail at half the supply voltage. Here is the schematic diagram of the circuit. The A1 will supply a negative or positive voltage when A1 detects a difference between R and L input signals with respect to the virtual ground rail. 

The C4 will be charged via D2 an C3 via D1. The LED is turned on by the comparator A3/A4 via OR circuit D3/D4. The input signal level should be greater than 100mV to compensate for the drop across D2 or D1. P1 is used to adjust the sensitivity of stereo indicator.

 Stereo Indicator Detects L-R Signal Difference Circuit Diagram