selective amplifier

Those who may find it fun, or even use- ful in some respect, can easily extend the single lamp stereo indication of an FM-receiver to give an automatic stereo- mono indication. Experiments with the well-known IC stereo decoder MC 131 OP resulted in the circuit given here. The green LED (DI) is the normal stereo indication; lighting up of the re.d LED (D3) means that we are listening to a mono broadcast.