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
