The AC power line to a street luminaire carries a 200 KHz carrier signal for operating a switching relay that connects a dimming reactor in series with the discharge lamp. A pick-off transformer has a primary winding connected in series with the ballasting means for the lamp, and a secondary winding connected to a carrier signal detector operating the switching relay. A capacitor connected across the primary has a value resonating the primary magnetizing reactance of the transformer at the carrier frequency. A resistor substantially higher in value than the characteristic impedance of the line but not several orders of magnitude higher, is preferably connected across the primary winding to serve as a terminating impedance.