Two light level ballast

1. A ballast for powering at least one gas discharge lamp from an alternating current (AC) voltage source, the ballast comprising:

a first hot input connection adapted for coupling to a hot wire of the AC voltage source via a first switch, the first switch having an on state and an off state;

a second hot input connection adapted for coupling to the hot wire of the AC voltage source via a second switch, the second switch having an on state and an off state;

a neutral input connection adapted for coupling to a neutral wire of the AC voltage source;

a sensing transformer coupled to the first and second hot input connections; and

a detector circuit coupled to the sensing transformer, the detector circuit being operable to provide an output voltage having a magnitude that is dependent on the states of the first and second switches.

2. The ballast of claim 1 , wherein the detector circuit is further operable such that:

(i) in response to both the first and second switches being in the on state, the magnitude of the output voltage is at a first level; and

(ii) in response to only one of the first and second switches being in the on state, the magnitude of the output voltage is at a second level.

3. The ballast of claim 2 , wherein the first level is approximately zero volts and the second level is approximately 15 volts.

4. The ballast of claim 1 , wherein the sensing transformer comprises:

a first primary winding electrically coupled to the first hot input connection, the first primary winding have a first polarity;

a second primary winding electrically coupled to the second hot input connection, wherein the second primary winding is magnetically coupled to the first winding and has a second polarity that is opposite that of the first polarity; and

a secondary winding electrically coupled to the detector circuit, wherein the secondary winding is magnetically coupled to the first and second primary windings.

5. The ballast of claim 4 , wherein the detector circuit comprises:

first and second input terminals coupled to the secondary winding of the sensing transformer, wherein the first input terminal is coupled to circuit ground;

first and second output terminals, wherein the second output terminal is coupled to circuit ground; and

a comparator having a non-inverting input, an inverting input, and a comparator output, wherein the non-inverting input is coupled to the second input terminal, the inverting input is coupled to a reference voltage, and the comparator output is coupled to the first output terminal.

6. The ballast of claim 5 , wherein the detector circuit further comprises:

a diode coupled between the second input terminal and the non-inverting input of the comparator;

a first resistor coupled between the non-inverting input of the comparator and circuit ground;

a capacitor coupled between the non-inverting input of the comparator and circuit ground;

a second resistor coupled between a direct current (DC) voltage source and the inverting input of the comparator;

a third resistor coupled between the inverting input of the comparator and circuit ground; and

a fourth resistor coupled between the comparator output and circuit ground.

7. A ballast for powering at least one gas discharge lamp from an alternating current (AC) voltage source, the ballast comprising:

a plurality of input connections, comprising:

a first hot input connection adapted for coupling to a hot wire of the AC voltage source via a first switch, the first switch having an on state and an off state;

a second hot input connection adapted for coupling to the hot wire of the AC voltage source via a second switch, the second switch having an on state and an off state; and

a neutral input connection adapted for coupling to a neutral wire of the AC voltage source;

first and second output connections adapted for coupling to the at least one gas discharge lamp;

a sensing transformer coupled to the first and second hot input connections;

an electromagnetic interference (EMI) filter coupled to the sensing transformer and to the neutral input connection;

a full-wave rectifier circuit coupled to the EMI filter;

power factor correction (PFC) and inverter circuits coupled between the full-wave rectifier circuit and the first and second output connection; and

a detector circuit coupled to the sensing transformer and to the PFC and inverter circuits, the detector circuit being operable to provide an output voltage having a magnitude that is dependent on the states of the first and second switches.

8. The ballast of claim 7 , wherein the detector circuit is further operable such that:

(i) in response to both the first and second switches being in the on state, the magnitude of the output voltage is at a first level; and

(ii) in response to only one of the first and second switches being in the on state, the magnitude of the output voltage is at a second level.

9. The ballast of claim 8 , wherein the first level is approximately zero volts and the second level is approximately 15 volts.

10. The ballast of claim 7 , wherein the sensing transformer comprises:

a first primary winding electrically coupled to the first hot input connection, the first primary winding have a first polarity;

a second primary winding electrically coupled to the second hot input connection, wherein the second primary winding is magnetically coupled to the first winding and has a second polarity that is opposite that of the first polarity; and

a secondary winding electrically coupled to the detector circuit, wherein the secondary winding is magnetically coupled to the first and second primary windings.

11. The ballast of claim 10 , wherein the detector circuit comprises:

first and second input terminals coupled to the secondary winding of the sensing transformer, wherein the first input terminal is coupled to circuit ground;

first and second output terminals coupled to the PFC and inverter circuits, wherein the second output terminal is coupled to circuit ground; and

a comparator having a non-inverting input, an inverting input, and a comparator output, wherein the non-inverting input is coupled to the second input terminal, the inverting input is coupled to a reference voltage, and the comparator output is coupled to the first output terminal.

12. The ballast of claim 11 , wherein the detector circuit further comprises:

a diode coupled between the second input terminal and the non-inverting input of the comparator;

a first resistor coupled between the non-inverting input of the comparator and circuit ground;

a capacitor coupled between the non-inverting input of the comparator and circuit ground;

a second resistor coupled between a direct current (DC) voltage source and the inverting input of the comparator;

a third resistor coupled between the inverting input of the comparator and circuit ground; and

a fourth resistor coupled between the comparator output and circuit ground.

13. A ballast for powering at least one gas discharge lamp from an alternating current (AC) voltage source, the ballast comprising:

a plurality of input connections, comprising:

a first hot input connection adapted for coupling to a hot wire of the AC voltage source via a first switch, the first switch having an on state and an off state;

a second hot input connection adapted for coupling to the hot wire of the AC voltage source via a second switch, the second switch having an on state and an off state; and

a neutral input connection adapted for coupling to a neutral wire of the AC voltage source;

first and second output connections adapted for coupling to the at least one gas discharge lamp;

a sensing transformer, comprising:

a first primary winding electrically coupled to the first hot input connection, the first primary winding have a first polarity;

a second primary winding electrically coupled to the second hot input connection, wherein the second primary winding is magnetically coupled to the first winding and has a second polarity that is opposite that of the first polarity; and

a secondary winding electrically coupled to the detector circuit, wherein the secondary winding is magnetically coupled to the first and second primary windings;

an electromagnetic interference (EMI) filter coupled to the sensing transformer and to the neutral input connection;

a full-wave rectifier circuit coupled to the EMI filter;

power factor correction (PFC) and inverter circuits coupled to the full-wave rectifier circuit; and

a detector circuit coupled to the sensing transformer and to the PFC and inverter circuits, the detector circuit comprising:

first and second input terminals coupled to the secondary winding of the sensing transformer, wherein the first input terminal is coupled to circuit ground;

first and second output terminals coupled to the PFC and inverter circuits, wherein the second output terminal is coupled to circuit ground;

a comparator having a non-inverting input, an inverting input, and a comparator output, wherein the comparator output is coupled to the first output terminal;

a diode coupled between the second input terminal and the non-inverting input of the comparator;

a first resistor coupled between the non-inverting input of the comparator and circuit ground;

a capacitor coupled between the non-inverting input of the comparator and circuit ground;

a second resistor coupled between a direct current (DC) voltage source and the inverting input of the comparator;

a third resistor coupled between the inverting input of the comparator and circuit ground; and

a fourth resistor coupled between the comparator output and circuit ground.

14. The ballast of claim 13 , wherein the detector circuit is operable such that:

(i) in response to both the first and second switches being in the on state, the magnitude of the output voltage is at a first level; and

(ii) in response to only one of the first and second switches being in the on state, the magnitude of the output voltage is at a second level.

15. The ballast of claim 14 , wherein the first level is approximately zero volts and the second level is approximately 15 volts.